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(最多只允许输入30个字)MySQL :: MySQL 5.6 Reference Manual :: 12.5 String Functions4. Built-in Types & Python 3.5.5 documentation
4. Built-in Types
The following sections describe the standard types that are built into the
interpreter.
The principal built-in types are numerics, sequences, mappings, classes,
instances and exceptions.
Some collection classes are mutable.
The methods that add, subtract, or
rearrange their members in place, and don’t return a specific item, never return
the collection instance itself but None.
Some operations are supported by
in particular,
practically all objects can be compared, tested for truth value, and converted
to a string (with the
function or the slightly different
function).
The latter function is implicitly used when an object is
written by the
4.1. Truth Value Testing
Any object can be tested for truth value, for use in an
condition or as operand of the Boolean operations below. The
following values are considered false:
zero of any numeric type, for example, 0, 0.0, 0j.
any empty sequence, for example, '', (), [].
any empty mapping, for example, {}.
instances of user-defined classes, if the class defines a
method, when that method returns the integer zero or
value False.
All other values are considered true — so objects of many types are always
Operations and built-in functions that have a Boolean result always return 0
or False for false and 1 or True for true, unless otherwise stated.
(Important exception: the Boolean operations or and and always return
one of their operands.)
4.2. Boolean Operations — , ,
These are the Boolean operations, ordered by ascending priority:
if x is false, then y, else
if x is false, then x, else
if x is false, then True,
else False
This is a short-circuit operator, so it only evaluates the second
argument if the first one is false.
This is a short-circuit operator, so it only evaluates the second
argument if the first one is true.
not has a lower priority than non-Boolean operators, so not a == b is
interpreted as not (a == b), and a == not b is a syntax error.
4.3. Comparisons
There are eight comparison operations in Python.
They all have the same
priority (which is higher than that of the Boolean operations).
Comparisons can
be for example, x & y &= z is equivalent to x & y and
y &= z, except that y is evaluated only once (but in both cases z is not
evaluated at all when x & y is found to be false).
This table summarizes the comparison operations:
strictly less than
less than or equal
strictly greater than
greater than or equal
object identity
negated object identity
Objects of different types, except different numeric types, never compare equal.
Furthermore, some types (for example, function objects) support only a degenerate
notion of comparison where any two objects of that type are unequal.
&=, & and &= operators will raise a
exception when
comparing a complex number with another built-in numeric type, when the objects
are of different types that cannot be compared, or in other cases where there is
no defined ordering.
Non-identical instances of a class normally compare as non-equal unless the
class defines the
Instances of a class cannot be ordered with respect to other instances of the
same class, or other types of object, unless the class defines enough of the
methods , , , and
are sufficient, if you want the
conventional meanings of the comparison operators).
The behavior of the
operators cannot be
also they can be applied to any two objects and never raise an
exception.
Two more operations with the same syntactic priority,
, are supported only by sequence types (below).
4.4. Numeric Types — , ,
There are three distinct numeric types: integers, floating
point numbers, and complex numbers.
In addition, Booleans are a
subtype of integers.
Integers have unlimited precision.
Floating point
numbers are usually implemented using double in C; information
about the precision and internal representation of floating point
numbers for the machine on which your program is running is available
Complex numbers have a real and imaginary
part, which are each a floating point number.
To extract these parts
from a complex number z, use z.real and z.imag. (The standard
library includes additional numeric types,
rationals, and
that hold floating-point numbers with
user-definable precision.)
Numbers are created by numeric literals or as the result of built-in functions
and operators.
Unadorned integer literals (including hex, octal and binary
numbers) yield integers.
Numeric literals containing a decimal point or an
exponent sign yield floating point numbers.
Appending 'j' or 'J' to a
numeric literal yields an imaginary number (a complex number with a zero real
part) which you can add to an integer or float to get a complex number with real
and imaginary parts.
Python fully supports mixed arithmetic: when a binary arithmetic operator has
operands of different numeric types, the operand with the “narrower” type is
widened to that of the other, where integer is narrower than floating point,
which is narrower than complex.
Comparisons between numbers of mixed type use
the same rule.
The constructors , , and
can be used to produce numbers of a specific type.
All numeric types (except complex) support the following operations, sorted by
ascending priority (all numeric operations have a higher priority than
comparison operations):
sum of x and y
difference of x and y
product of x and y
quotient of x and y
floored quotient of x and
remainder of x / y
x unchanged
absolute value or magnitude of
x converted to integer
x converted to floating point
complex(re, im)
a complex number with real part
re, imaginary part im.
im defaults to zero.
c.conjugate()
conjugate of the complex number
divmod(x, y)
the pair (x // y, x % y)
x to the power y
x to the power y
Also referred to as integer division.
The resultant value is a whole
integer, though the result’s type is not necessarily int.
The result is
always rounded towards minus infinity: 1//2 is 0, (-1)//2 is
-1, 1//(-2) is -1, and (-1)//(-2) is 0.
Not for complex numbers.
Instead convert to floats using
appropriate.
Conversion from floating point to integer may round or truncate
as in C; see functions
well-defined conversions.
float also accepts the strings “nan” and “inf” with an optional prefix “+”
or “-” for Not a Number (NaN) and positive or negative infinity.
Python defines pow(0, 0) and 0 ** 0 to be 1, as is common for
programming languages.
The numeric literals accepted include the digits 0 to 9 or any
Unicode equivalent (code points with the Nd property).
for a complete list of code points with the Nd property.
types ( and ) also include
the following operations:
x truncated to
x rounded to n digits,
rounding half to even. If n is
omitted, it defaults to 0.
the greatest
For additional numeric operations see the
4.4.1. Bitwise Operations on Integer Types
Bitwise operations only make sense for integers.
Negative numbers are treated
as their 2’s complement value (this assumes that there are enough bits so that
no overflow occurs during the operation).
The priorities of the binary bitwise operations are all lower than the numeric
operations and higher
the unary operation ~ has the
same priority as the other unary numeric operations (+ and -).
This table lists the bitwise operations sorted in ascending priority:
bitwise or of x and
bitwise exclusive or of
bitwise and of x and
x shifted left by n bits
x shifted right by n bits
the bits of x inverted
Negative shift counts are illegal and cause a
to be raised.
A left shift by n bits is equivalent to multiplication by pow(2, n)
without overflow check.
A right shift by n bits is equivalent to division by pow(2, n) without
overflow check.
4.4.2. Additional Methods on Integer Types
The int type implements the
. In addition, it provides a few more methods:
int.bit_length()
Return the number of bits necessary to represent an integer in binary,
excluding the sign and leading zeros:
&&& n = -37
&&& bin(n)
&&& n.bit_length()
More precisely, if x is nonzero, then x.bit_length() is the
unique positive integer k such that 2**(k-1) &= abs(x) & 2**k.
Equivalently, when abs(x) is small enough to have a correctly
rounded logarithm, then k = 1 + int(log(abs(x), 2)).
If x is zero, then x.bit_length() returns 0.
Equivalent to:
def bit_length(self):
s = bin(self)
# binary representation:
bin(-37) --& '-0b;
s = s.lstrip('-0b') # remove leading zeros and minus sign
return len(s)
# len(';) --& 6
New in version 3.1.
int.to_bytes(length, byteorder, *, signed=False)
Return an array of bytes representing an integer.
&&& (1024).to_bytes(2, byteorder='big')
b'\x04\x00'
&&& (1024).to_bytes(10, byteorder='big')
b'\x00\x00\x00\x00\x00\x00\x00\x00\x04\x00'
&&& (-1024).to_bytes(10, byteorder='big', signed=True)
b'\xff\xff\xff\xff\xff\xff\xff\xff\xfc\x00'
&&& x = 1000
&&& x.to_bytes((x.bit_length() + 7) // 8, byteorder='little')
b'\xe8\x03'
The integer is represented using length bytes.
is raised if the integer is not representable with the given number of
The byteorder argument determines the byte order used to represent the
If byteorder is &big&, the most significant byte is at the
beginning of the byte array.
If byteorder is &little&, the most
significant byte is at the end of the byte array.
To request the native
byte order of the host system, use
as the byte order
The signed argument determines whether two’s complement is used to
represent the integer.
If signed is False and a negative integer is
is raised. The default value for signed
New in version 3.2.
classmethod int.from_bytes(bytes, byteorder, *, signed=False)
Return the integer represented by the given array of bytes.
&&& int.from_bytes(b'\x00\x10', byteorder='big')
&&& int.from_bytes(b'\x00\x10', byteorder='little')
&&& int.from_bytes(b'\xfc\x00', byteorder='big', signed=True)
&&& int.from_bytes(b'\xfc\x00', byteorder='big', signed=False)
&&& int.from_bytes([255, 0, 0], byteorder='big')
The argument bytes must either be a
iterable producing bytes.
The byteorder argument determines the byte order used to represent the
If byteorder is &big&, the most significant byte is at the
beginning of the byte array.
If byteorder is &little&, the most
significant byte is at the end of the byte array.
To request the native
byte order of the host system, use
as the byte order
The signed argument indicates whether two’s complement is used to
represent the integer.
New in version 3.2.
4.4.3. Additional Methods on Float
The float type implements the
. float also has the following additional methods.
float.as_integer_ratio()
Return a pair of integers whose ratio is exactly equal to the
original float and with a positive denominator.
on infinities and a
float.is_integer()
Return True if the float instance is finite with integral
value, and False otherwise:
&&& (-2.0).is_integer()
&&& (3.2).is_integer()
Two methods support conversion to
and from hexadecimal strings.
Since Python’s floats are stored
internally as binary numbers, converting a float to or from a
decimal string usually involves a small rounding error.
contrast, hexadecimal strings allow exact representation and
specification of floating-point numbers.
This can be useful when
debugging, and in numerical work.
float.hex()
Return a representation of a floating-point number as a hexadecimal
For finite floating-point numbers, this representation
will always include a leading 0x and a trailing p and
classmethod float.fromhex(s)
Class method to return the float represented by a hexadecimal
The string s may have leading and trailing
whitespace.
is an instance method, while
is a class method.
A hexadecimal string takes the form:
[sign] ['0x'] integer ['.' fraction] ['p' exponent]
where the optional sign may by either + or -, integer
and fraction are strings of hexadecimal digits, and exponent
is a decimal integer with an optional leading sign.
Case is not
significant, and there must be at least one hexadecimal digit in
either the integer or the fraction.
This syntax is similar to the
syntax specified in section 6.4.4.2 of the C99 standard, and also to
the syntax used in Java 1.5 onwards.
In particular, the output of
is usable as a hexadecimal floating-point literal in
C or Java code, and hexadecimal strings produced by C’s %a format
character or Java’s Double.toHexString are accepted by
Note that the exponent is written in decimal rather than hexadecimal,
and that it gives the power of 2 by which to multiply the coefficient.
For example, the hexadecimal string 0x3.a7p10 represents the
floating-point number (3 + 10./16 + 7./16**2) * 2.0**10, or
&&& float.fromhex('0x3.a7p10')
Applying the reverse conversion to 3740.0 gives a different
hexadecimal string representing the same number:
&&& float.hex(3740.0)
'0x1.dp+11'
4.4.4. Hashing of numeric types
For numbers x and y, possibly of different types, it’s a requirement
that hash(x) == hash(y) whenever x == y (see the
method documentation for more details).
For ease of implementation and
efficiency across a variety of numeric types (including ,
Python’s hash for numeric types is based on a single mathematical function
that’s defined for any rational number, and hence applies to all instances of
and , and all finite instances of
Essentially, this function is
given by reduction modulo P for a fixed prime P.
The value of P is
made available to Python as the modulus attribute of
CPython implementation detail: Currently, the prime used is P = 2**31 - 1 on machines with 32-bit C
longs and P = 2**61 - 1 on machines with 64-bit C longs.
Here are the rules in detail:
If x = m / n is a nonnegative rational number and n is not divisible
by P, define hash(x) as m * invmod(n, P) % P, where invmod(n,
P) gives the inverse of n modulo P.
If x = m / n is a nonnegative rational number and n is
divisible by P (but m is not) then n has no inverse
modulo P and the rule above doesn’ in this case define
hash(x) to be the constant value sys.hash_info.inf.
If x = m / n is a negative rational number define hash(x)
as -hash(-x).
If the resulting hash is -1, replace it with
The particular values sys.hash_info.inf, -sys.hash_info.inf
and sys.hash_info.nan are used as hash values for positive
infinity, negative infinity, or nans (respectively).
(All hashable
nans have the same hash value.)
number z, the hash values of the real
and imaginary parts are combined by computing hash(z.real) +
sys.hash_info.imag * hash(z.imag), reduced modulo
2**sys.hash_info.width so that it lies in
range(-2**(sys.hash_info.width - 1), 2**(sys.hash_info.width -
Again, if the result is -1, it’s replaced with -2.
To clarify the above rules, here’s some example Python code,
equivalent to the built-in hash, for computing the hash of a rational
number, , or :
import sys, math
def hash_fraction(m, n):
&&&Compute the hash of a rational number m / n.
Assumes m and n are integers, with n positive.
Equivalent to hash(fractions.Fraction(m, n)).
P = sys.hash_info.modulus
# Remove common factors of P.
(Unnecessary if m and n already coprime.)
while m % P == n % P == 0:
m, n = m // P, n // P
if n % P == 0:
hash_value = sys.hash_info.inf
# Fermat's Little Theorem: pow(n, P-1, P) is 1, so
# pow(n, P-2, P) gives the inverse of n modulo P.
hash_value = (abs(m) % P) * pow(n, P - 2, P) % P
hash_value = -hash_value
if hash_value == -1:
hash_value = -2
return hash_value
def hash_float(x):
&&&Compute the hash of a float x.&&&
if math.isnan(x):
return sys.hash_info.nan
elif math.isinf(x):
return sys.hash_info.inf if x & 0 else -sys.hash_info.inf
return hash_fraction(*x.as_integer_ratio())
def hash_complex(z):
&&&Compute the hash of a complex number z.&&&
hash_value = hash_float(z.real) + sys.hash_info.imag * hash_float(z.imag)
# do a signed reduction modulo 2**sys.hash_info.width
M = 2**(sys.hash_info.width - 1)
hash_value = (hash_value & (M - 1)) - (hash_value & M)
if hash_value == -1:
hash_value = -2
return hash_value
4.5. Iterator Types
Python supports a concept of iteration over containers.
This is implemented
these are used to allow user-defined classes to
support iteration.
Sequences, described below in more detail, always support
the iteration methods.
One method needs to be defined for container objects to provide iteration
container.__iter__()
Return an iterator object.
The object is required to support the iterator
protocol described below.
If a container supports different types of
iteration, additional methods can be provided to specifically request
iterators for those iteration types.
(An example of an object supporting
multiple forms of iteration would be a tree structure which supports both
breadth-first and depth-first traversal.)
This method corresponds to the
slot of the type structure for Python objects in the Python/C
The iterator objects themselves are required to support the following two
methods, which together form the iterator protocol:
iterator.__iter__()
Return the iterator object itself.
This is required to allow both containers
and iterators to be used with the
statements.
This method corresponds to the
slot of the type structure for
Python objects in the Python/C API.
iterator.__next__()
Return the next item from the container.
If there are no further items, raise
exception.
This method corresponds to the
slot of the type structure for Python objects in the
Python/C API.
Python defines several iterator objects to support iteration over general and
specific sequence types, dictionaries, and other more specialized forms.
specific types are not important beyond their implementation of the iterator
Once an iterator’s
method raises
, it must continue to do so on subsequent calls.
Implementations that do not obey this property are deemed broken.
4.5.1. Generator Types
Python’s s provide a convenient way to implement the iterator
If a container object’s
method is implemented as a
generator, it will automatically return an iterator object (technically, a
generator object) supplying the
More information about generators can be found in .
4.6. Sequence Types — , ,
There are three basic sequence types: lists, tuples, and range objects.
Additional sequence types tailored for processing of
described in dedicated sections.
4.6.1. Common Sequence Operations
The operations in the following table are supported by most sequence types,
both mutable and immutable. The
provided to make it easier to correctly implement these operations on
custom sequence types.
This table lists the sequence operations sorted in ascending priority.
table, s and t are sequences of the same type, n, i, j and k are
integers and x is an arbitrary object that meets any type and value
restrictions imposed by s.
The in and not in operations have the same priorities as the
comparison operations. The + (concatenation) and * (repetition)
operations have the same priority as the corresponding numeric operations.
True if an item of s is
equal to x, else False
x not in s
False if an item of s is
equal to x, else True
the concatenation of s and
equivalent to adding s to
itself n times
ith item of s, origin 0
slice of s from i to j
slice of s from i to j
with step k
length of s
smallest item of s
largest item of s
s.index(x[, i[, j]])
index of the first occurrence
of x in s (at or after
index i and before index j)
s.count(x)
total number of occurrences of
Sequences of the same type also support comparisons.
In particular, tuples
and lists are compared lexicographically by comparing corresponding elements.
This means that to compare equal, every element must compare equal and the
two sequences must be of the same type and have the same length.
details see
in the language reference.)
While the in and not in operations are used only for simple
containment testing in the general case, some specialised sequences
(such as ,
and ) also use
them for subsequence testing:
&&& &gg& in &eggs&
Values of n less than 0 are treated as 0 (which yields an empty
sequence of the same type as s).
Note that items in the sequence s
they are referenced multiple times.
This often haunts
new P consider:
&&& lists = [[]] * 3
[[], [], []]
&&& lists[0].append(3)
[[3], [3], [3]]
What has happened is that [[]] is a one-element list containing an empty
list, so all three elements of [[]] * 3 are references to this single empty
Modifying any of the elements of lists modifies this single list.
You can create a list of different lists this way:
&&& lists = [[] for i in range(3)]
&&& lists[0].append(3)
&&& lists[1].append(5)
&&& lists[2].append(7)
[[3], [5], [7]]
Further explanation is available in the FAQ entry
If i or j is negative, the index is relative to the end of sequence s:
len(s) + i or len(s) + j is substituted.
But note that -0 is
The slice of s from i to j is defined as the sequence of items with index
k such that i &= k & j.
If i or j is greater than len(s), use
If i is omitted or None, use 0.
If j is omitted or
None, use len(s).
If i is greater than or equal to j, the slice is
The slice of s from i to j with step k is defined as the sequence of
items with index
x = i + n*k such that 0 &= n & (j-i)/k.
In other words,
the indices are i, i+k, i+2*k, i+3*k and so on, stopping when
j is reached (but never including j).
When k is positive,
i and j are reduced to len(s) if they are greater.
When k is negative, i and j are reduced to len(s) - 1 if
they are greater.
If i or j are omitted or None, they become
“end” values (which end depends on the sign of k).
Note, k cannot be zero.
If k is None, it is treated like 1.
Concatenating immutable sequences always results in a new object.
means that building up a sequence by repeated concatenation will have a
quadratic runtime cost in the total sequence length.
To get a linear
runtime cost, you must switch to one of the alternatives below:
if concatenating
objects, you can build a list and use
at the end or else write to an
instance and retrieve its value when complete
if concatenating
objects, you can similarly use
or , or you can do in-place
concatenation with a
objects are mutable and have an efficient overallocation mechanism
if concatenating
objects, extend a
for other types, investigate the relevant class documentation
Some sequence types (such as ) only support item sequences
that follow specific patterns, and hence don’t support sequence
concatenation or repetition.
index raises
when x is not found in s.
When supported, the additional arguments to the index method allow
efficient searching of subsections of the sequence. Passing the extra
arguments is roughly equivalent to using s[i:j].index(x), only
without copying any data and with the returned index being relative to
the start of the sequence rather than the start of the slice.
4.6.2. Immutable Sequence Types
The only operation that immutable sequence types generally implement that is
not also implemented by mutable sequence types is support for the
This support allows immutable sequences, such as
instances, to
be used as
keys and stored in
instances.
Attempting to hash an immutable sequence that contains unhashable values will
result in .
4.6.3. Mutable Sequence Types
The operations in the following table are defined on mutable sequence types.
ABC is provided to make it
easier to correctly implement these operations on custom sequence types.
In the table s is an instance of a mutable sequence type, t is any
iterable object and x is an arbitrary object that meets any type
and value restrictions imposed by s (for example,
accepts integers that meet the value restriction 0 &= x &= 255).
item i of s is replaced by
s[i:j] = t
slice of s from i to j
is replaced by the contents of
the iterable t
del s[i:j]
same as s[i:j] = []
s[i:j:k] = t
the elements of s[i:j:k]
are replaced by those of t
del s[i:j:k]
removes the elements of
s[i:j:k] from the list
s.append(x)
appends x to the end of the
sequence (same as
s[len(s):len(s)] = [x])
removes all items from s
(same as del s[:])
creates a shallow copy of s
(same as s[:])
s.extend(t) or
extends s with the
contents of t (for the
most part the same as
s[len(s):len(s)] = t)
updates s with its contents
repeated n times
s.insert(i, x)
inserts x into s at the
index given by i
(same as s[i:i] = [x])
s.pop([i])
retrieves the item at i and
also removes it from s
s.remove(x)
remove the first item from s
where s[i] == x
s.reverse()
reverses the items of s in
t must have the same length as the slice it is replacing.
The optional argument i defaults to -1, so that by default the last
item is removed and returned.
remove raises
when x is not found in s.
The reverse() method modifies the sequence in place for economy of
space when reversing a large sequence.
To remind users that it operates by
side effect, it does not return the reversed sequence.
clear() and copy() are included for consistency with the
interfaces of mutable containers that don’t support slicing operations
New in version 3.3: clear() and copy() methods.
The value n is an integer, or an object implementing
Zero and negative values of n clear
the sequence.
Items in the seq they are referenced
multiple times, as explained for s * n under .
4.6.4. Lists
Lists are mutable sequences, typically used to store collections of
homogeneous items (where the precise degree of similarity will vary by
application).
class list([iterable])
Lists may be constructed in several ways:
Using a pair of square brackets to denote the empty list: []
Using square brackets, separating items with commas: [a], [a, b, c]
Using a list comprehension: [x for x in iterable]
Using the type constructor: list() or list(iterable)
The constructor builds a list whose items are the same and in the same
order as iterable‘s items.
iterable may be either a sequence, a
container that supports iteration, or an iterator object.
If iterable
is already a list, a copy is made and returned, similar to iterable[:].
For example, list('abc') returns ['a', 'b', 'c'] and
list( (1, 2, 3) ) returns [1, 2, 3].
If no argument is given, the constructor creates a new empty list, [].
Many other operations also produce lists, including the
Lists implement all of the
sequence operations. Lists also provide the
following additional method:
sort(*, key=None, reverse=None)
This method sorts the list in place, using only & comparisons
between items. Exceptions are not suppressed - if any comparison operations
fail, the entire sort operation will fail (and the list will likely be left
in a partially modified state).
accepts two arguments that can only be passed by keyword
key specifies a function of one argument that is used to extract a
comparison key from each list element (for example, key=str.lower).
The key corresponding to each item in the list is calculated once and
then used for the entire sorting process. The default value of None
means that list items are sorted directly without calculating a separate
key value.
utility is available to convert a 2.x
style cmp function to a key function.
reverse is a boolean value.
If set to True, then the list elements
are sorted as if each comparison were reversed.
This method modifies the sequence in place for economy of space when
sorting a large sequence.
To remind users that it operates by side
effect, it does not return the sorted sequence (use
explicitly request a new sorted list instance).
method is guaranteed to be stable.
A sort is stable if it
guarantees not to change the relative order of elements that compare equal
— this is helpful for sorting in multiple passes (for example, sort by
department, then by salary grade).
CPython implementation detail: While a list is being sorted, the effect of attempting to mutate, or even
inspect, the list is undefined.
The C implementation of Python makes the
list appear empty for the duration, and raises
detect that the list has been mutated during a sort.
4.6.5. Tuples
Tuples are immutable sequences, typically used to store collections of
heterogeneous data (such as the 2-tuples produced by the
built-in). Tuples are also used for cases where an immutable sequence of
homogeneous data is needed (such as allowing storage in a
instance).
class tuple([iterable])
Tuples may be constructed in a number of ways:
Using a pair of parentheses to denote the empty tuple: ()
Using a trailing comma for a singleton tuple: a, or (a,)
Separating items with commas: a, b, c or (a, b, c)
built-in: tuple() or tuple(iterable)
The constructor builds a tuple whose items are the same and in the same
order as iterable‘s items.
iterable may be either a sequence, a
container that supports iteration, or an iterator object.
If iterable
is already a tuple, it is returned unchanged. For example,
tuple('abc') returns ('a', 'b', 'c') and
tuple( [1, 2, 3] ) returns (1, 2, 3).
If no argument is given, the constructor creates a new empty tuple, ().
Note that it is actually the comma which makes a tuple, not the parentheses.
The parentheses are optional, except in the empty tuple case, or
when they are needed to avoid syntactic ambiguity. For example,
f(a, b, c) is a function call with three arguments, while
f((a, b, c)) is a function call with a 3-tuple as the sole argument.
Tuples implement all of the
operations.
For heterogeneous collections of data where access by name is clearer than
access by index,
may be a more appropriate
choice than a simple tuple object.
4.6.6. Ranges
type represents an immutable sequence of numbers and is
commonly used for looping a specific number of times in
class range(stop)
class range(start, stop[, step])
The arguments to the range constructor must be integers (either built-in
or any object that implements the __index__ special
If the step argument is omitted, it defaults to 1.
If the start argument is omitted, it defaults to 0.
If step is zero,
is raised.
For a positive step, the contents of a range r are determined by the
formula r[i] = start + step*i where i &= 0 and
r[i] & stop.
For a negative step, the contents of the range are still determined by
the formula r[i] = start + step*i, but the constraints are i &= 0
and r[i] & stop.
A range object will be empty if r[0] does not meet the value
constraint. Ranges do support negative indices, but these are interpreted
as indexing from the end of the sequence determined by the positive
Ranges containing absolute values larger than
permitted but some features (such as ) may raise
Range examples:
&&& list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
&&& list(range(1, 11))
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
&&& list(range(0, 30, 5))
[0, 5, 10, 15, 20, 25]
&&& list(range(0, 10, 3))
[0, 3, 6, 9]
&&& list(range(0, -10, -1))
[0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
&&& list(range(0))
&&& list(range(1, 0))
Ranges implement all of the
sequence operations
except concatenation and repetition (due to the fact that range objects can
only represent sequences that follow a strict pattern and repetition and
concatenation will usually violate that pattern).
The value of the start parameter (or 0 if the parameter was
not supplied)
The value of the stop parameter
The value of the step parameter (or 1 if the parameter was
not supplied)
The advantage of the
type over a regular
object will always take the same
(small) amount of memory, no matter the size of the range it represents (as it
only stores the start, stop and step values, calculating individual
items and subranges as needed).
Range objects implement the
ABC, and provide
features such as containment tests, element index lookup, slicing and
support for negative indices (see ):
&&& r = range(0, 20, 2)
range(0, 20, 2)
&&& 11 in r
&&& 10 in r
&&& r.index(10)
range(0, 10, 2)
Testing range objects for equality with == and != compares
them as sequences.
That is, two range objects are considered equal if
they represent the same sequence of values.
(Note that two range
objects that compare equal might have different ,
attributes, for example
range(0) == range(2, 1, 3) or range(0, 3, 2) == range(0, 4, 2).)
Changed in version 3.2: Implement the Sequence ABC.
Support slicing and negative indices.
objects for membership in constant time instead of
iterating through all items.
Changed in version 3.3: Define ‘==’ and ‘!=’ to compare range objects based on the
sequence of values they define (instead of comparing based on
object identity).
New in version 3.3: The ,
attributes.
4.7. Text Sequence Type —
Textual data in Python is handled with
objects, or strings.
Strings are immutable
of Unicode code points.
String literals are
written in a variety of ways:
Single quotes: 'allows embedded &double& quotes'
Double quotes: &allows embedded 'single' quotes&.
Triple quoted: '''Three single quotes''', &&&Three double quotes&&&
Triple quoted strings may span multiple lines - all associated whitespace will
be included in the string literal.
String literals that are part of a single expression and have only whitespace
between them will be implicitly converted to a single string literal. That
is, (&spam & &eggs&) == &spam eggs&.
for more about the various forms of string literal,
including supported escape sequences, and the r (“raw”) prefix that
disables most escape sequence processing.
Strings may also be created from other objects using the
constructor.
Since there is no separate “character” type, indexing a string produces
strings of length 1. That is, for a non-empty string s, s[0] == s[0:1].
There is also no mutable string type, but
can be used to efficiently construct strings from
multiple fragments.
Changed in version 3.3: For backwards compatibility with the Python 2 series, the u prefix is
once again permitted on string literals. It has no effect on the meaning
of string literals and cannot be combined with the r prefix.
class str(object='')
class str(object=b'', encoding='utf-8', errors='strict')
version of object.
If object is not
provided, returns the empty string.
Otherwise, the behavior of str()
depends on whether encoding or errors is given, as follows.
If neither encoding nor errors is given, str(object) returns
, which is the “informal” or nicely
printable string representation of object.
For string objects, this is
the string itself.
If object does not have a
method, then
falls back to returning
If at least one of encoding or errors is given, object should be a
this case, if object is a
(or ) object,
then str(bytes, encoding, errors) is equivalent to
Otherwise, the bytes
object underlying the buffer object is obtained before calling
for information on buffer objects.
without the encoding
or errors arguments falls under the first case of returning the informal
string representation (see also the
command-line option to
For example:
&&& str(b'Zoot!')
&b'Zoot!'&
For more information on the str class and its methods, see
section below.
formatted strings, see the
In addition,
4.7.1. String Methods
Strings implement all of the
operations, along with the additional methods described below.
Strings also support two styles of string formatting, one providing a large
degree of flexibility and customization (see ,
and ) and the other based on C
printf style formatting that handles a narrower range of types and is
slightly harder to use correctly, but is often faster for the cases it can
handle ().
section of the standard library covers a number of
other modules that provide various text related utilities (including regular
expression support in the
str.capitalize()
Return a copy of the string with its first character capitalized and the
rest lowercased.
str.casefold()
Return a casefolded copy of the string. Casefolded strings may be used for
caseless matching.
Casefolding is similar to lowercasing but more aggressive because it is
intended to remove all case distinctions in a string. For example, the German
lowercase letter 'ss' is equivalent to &ss&. Since it is already
lowercase,
would do nothing to 'ss';
converts it to &ss&.
The casefolding algorithm is described in section 3.13 of the Unicode
New in version 3.3.
str.center(width[, fillchar])
Return centered in a string of length width. Padding is done using the
specified fillchar (default is an ASCII space). The original string is
returned if width is less than or equal to len(s).
str.count(sub[, start[, end]])
Return the number of non-overlapping occurrences of substring sub in the
range [start, end].
Optional arguments start and end are
interpreted as in slice notation.
str.encode(encoding=&utf-8&, errors=&strict&)
Return an encoded version of the string as a bytes object. Default encoding
is 'utf-8'. errors may be given to set a different error handling scheme.
The default for errors is 'strict', meaning that encoding errors raise
a . Other possible
values are 'ignore', 'replace', 'xmlcharrefreplace',
'backslashreplace' and any other name registered via
, see section . For a
list of possible encodings, see section .
Changed in version 3.1: Support for keyword arguments added.
str.endswith(suffix[, start[, end]])
Return True if the string ends with the specified suffix, otherwise return
suffix can also be a tuple of suffixes to look for.
With optional
start, test beginning at that position.
With optional end, stop comparing
at that position.
str.expandtabs(tabsize=8)
Return a copy of the string where all tab characters are replaced by one or
more spaces, depending on the current column and the given tab size.
positions occur every tabsize characters (default is 8, giving tab
positions at columns 0, 8, 16 and so on).
To expand the string, the current
column is set to zero and the string is examined character by character.
the character is a tab (\t), one or more space characters are inserted
in the result until the current column is equal to the next tab position.
(The tab character itself is not copied.)
If the character is a newline
(\n) or return (\r), it is copied and the current column is reset to
Any other character is copied unchanged and the current column is
incremented by one regardless of how the character is represented when
&&& '01\t012\t0123\t01234'.expandtabs()
01234'
&&& '01\t012\t0123\t01234'.expandtabs(4)
01234'
str.find(sub[, start[, end]])
Return the lowest index in the string where substring sub is found within
the slice s[start:end].
Optional arguments start and end are
interpreted as in slice notation.
Return -1 if sub is not found.
method should be used only if you need to know the
position of sub.
To check if sub is a substring or not, use the
&&& 'Py' in 'Python'
str.format(*args, **kwargs)
Perform a string formatting operation.
The string on which this method is
called can contain literal text or replacement fields delimited by braces
Each replacement field contains either the numeric index of a
positional argument, or the name of a keyword argument.
Returns a copy of
the string where each replacement field is replaced with the string value of
the corresponding argument.
&&& &The sum of 1 + 2 is {0}&.format(1+2)
'The sum of 1 + 2 is 3'
for a description of the various formatting options
that can be specified in format strings.
str.format_map(mapping)
Similar to str.format(**mapping), except that mapping is
used directly and not copied to a .
This is useful
if for example mapping is a dict subclass:
&&& class Default(dict):
def __missing__(self, key):
return key
&&& '{name} was born in {country}'.format_map(Default(name='Guido'))
'Guido was born in country'
New in version 3.2.
str.index(sub[, start[, end]])
Like , but raise
when the substring is
not found.
str.isalnum()
Return true if all characters in the string are alphanumeric and there is at
least one character, false otherwise.
A character c is alphanumeric if one
of the following returns True: c.isalpha(), c.isdecimal(),
c.isdigit(), or c.isnumeric().
str.isalpha()
Return true if all characters in the string are alphabetic and there is at least
one character, false otherwise.
Alphabetic characters are those characters defined
in the Unicode character database as “Letter”, i.e., those with general category
property being one of “Lm”, “Lt”, “Lu”, “Ll”, or “Lo”.
Note that this is different
from the “Alphabetic” property defined in the Unicode Standard.
str.isdecimal()
Return true if all characters in the string are decimal
characters and there is at least one character, false
otherwise. Decimal characters are those that can be used to form
numbers in base 10, e.g. U+0660, ARABIC-INDIC DIGIT
Formally a decimal character is a character in the Unicode
General Category “Nd”.
str.isdigit()
Return true if all characters in the string are digits and there is at least one
character, false otherwise.
Digits include decimal characters and digits that need
special handling, such as the compatibility superscript digits.
This covers digits which cannot be used to form numbers in base 10,
like the Kharosthi numbers.
Formally, a digit is a character that has the
property value Numeric_Type=Digit or Numeric_Type=Decimal.
Return true if the string is a valid identifier according to the language
definition, section .
to test for reserved identifiers such as
str.islower()
Return true if all cased characters
in the string are lowercase and
there is at least one cased character, false otherwise.
str.isnumeric()
Return true if all characters in the string are numeric
characters, and there is at least one character, false
otherwise. Numeric characters include digit characters, and all characters
that have the Unicode numeric value property, e.g. U+2155,
VULGAR FRACTION ONE FIFTH.
Formally, numeric characters are those with the property
value Numeric_Type=Digit, Numeric_Type=Decimal or Numeric_Type=Numeric.
str.isprintable()
Return true if all characters in the string are printable or the string is
empty, false otherwise.
Nonprintable characters are those characters defined
in the Unicode character database as “Other” or “Separator”, excepting the
ASCII space (0x20) which is considered printable.
(Note that printable
characters in this context are those which should not be escaped when
is invoked on a string.
It has no bearing on the handling of
strings written to
str.isspace()
Return true if there are only whitespace characters in the string and there is
at least one character, false otherwise.
Whitespace characters
characters defined in the Unicode character database as “Other” or “Separator”
and those with bidirectional property being one of “WS”, “B”, or “S”.
str.istitle()
Return true if the string is a titlecased string and there is at least one
character, for example uppercase characters may only follow uncased characters
and lowercase characters only cased ones.
Return false otherwise.
str.isupper()
Return true if all cased characters
in the string are uppercase and
there is at least one cased character, false otherwise.
str.join(iterable)
Return a string which is the concatenation of the strings in iterable.
will be raised if there are any non-string values in
iterable, including
The separator between
elements is the string providing this method.
str.ljust(width[, fillchar])
Return the string left justified in a string of length width. Padding is
done using the specified fillchar (default is an ASCII space). The
original string is returned if width is less than or equal to len(s).
str.lower()
Return a copy of the string with all the cased characters
converted to
lowercase.
The lowercasing algorithm used is described in section 3.13 of the Unicode
str.lstrip([chars])
Return a copy of the string with leading characters removed.
argument is a string specifying the set of characters to be removed.
If omitted
or None, the chars argument defaults to removing whitespace.
argu rather, all combinations of its values are stripped:
'.lstrip()
'spacious
&&& 'www.example.com'.lstrip('cmowz.')
'example.com'
static str.maketrans(x[, y[, z]])
This static method returns a translation table usable for .
If there is only one argument, it must be a dictionary mapping Unicode
ordinals (integers) or characters (strings of length 1) to Unicode ordinals,
strings (of arbitrary lengths) or None.
Character keys will then be
converted to ordinals.
If there are two arguments, they must be strings of equal length, and in the
resulting dictionary, each character in x will be mapped to the character at
the same position in y.
If there is a third argument, it must be a string,
whose characters will be mapped to None in the result.
str.partition(sep)
Split the string at the first occurrence of sep, and return a 3-tuple
containing the part before the separator, the separator itself, and the part
after the separator.
If the separator is not found, return a 3-tuple containing
the string itself, followed by two empty strings.
str.replace(old, new[, count])
Return a copy of the string with all occurrences of substring old replaced by
If the optional argument count is given, only the first count
occurrences are replaced.
str.rfind(sub[, start[, end]])
Return the highest index in the string where substring sub is found, such
that sub is contained within s[start:end].
Optional arguments start
and end are interpreted as in slice notation.
Return -1 on failure.
str.rindex(sub[, start[, end]])
but raises
when the substring sub is not
str.rjust(width[, fillchar])
Return the string right justified in a string of length width. Padding is
done using the specified fillchar (default is an ASCII space). The
original string is returned if width is less than or equal to len(s).
str.rpartition(sep)
Split the string at the last occurrence of sep, and return a 3-tuple
containing the part before the separator, the separator itself, and the part
after the separator.
If the separator is not found, return a 3-tuple containing
two empty strings, followed by the string itself.
str.rsplit(sep=None, maxsplit=-1)
Return a list of the words in the string, using sep as the delimiter string.
If maxsplit is given, at most maxsplit splits are done, the rightmost
If sep is not specified or None, any whitespace string is a
separator.
Except for splitting from the right,
behaves like
which is described in detail below.
str.rstrip([chars])
Return a copy of the string with trailing characters removed.
argument is a string specifying the set of characters to be removed.
If omitted
or None, the chars argument defaults to removing whitespace.
argu rather, all combinations of its values are stripped:
'.rstrip()
spacious'
&&& 'mississippi'.rstrip('ipz')
'mississ'
str.split(sep=None, maxsplit=-1)
Return a list of the words in the string, using sep as the delimiter
If maxsplit is given, at most maxsplit splits are done (thus,
the list will have at most maxsplit+1 elements).
If maxsplit is not
specified or -1, then there is no limit on the number of splits
(all possible splits are made).
If sep is given, consecutive delimiters are not grouped together and are
deemed to delimit empty strings (for example, '1,,2'.split(',') returns
['1', '', '2']).
The sep argument may consist of multiple characters
(for example, '1&&2&&3'.split('&&') returns ['1', '2', '3']).
Splitting an empty string with a specified separator returns [''].
For example:
&&& '1,2,3'.split(',')
['1', '2', '3']
&&& '1,2,3'.split(',', maxsplit=1)
['1', '2,3']
&&& '1,2,,3,'.split(',')
['1', '2', '', '3', '']
If sep is not specified or is None, a different splitting algorithm is
applied: runs of consecutive whitespace are regarded as a single separator,
and the result will contain no empty strings at the start or end if the
string has leading or trailing whitespace.
Consequently, splitting an empty
string or a string consisting of just whitespace with a None separator
returns [].
For example:
&&& '1 2 3'.split()
['1', '2', '3']
&&& '1 2 3'.split(maxsplit=1)
['1', '2 3']
'.split()
['1', '2', '3']
str.splitlines([keepends])
Return a list of the lines in the string, breaking at line boundaries.
breaks are not included in the resulting list unless keepends is given and
This method splits on the following line boundaries.
In particular, the
boundaries are a superset of .
Carriage Return
Carriage Return + Line Feed
\v or \x0b
Line Tabulation
\f or \x0c
File Separator
Group Separator
Record Separator
Next Line (C1 Control Code)
Line Separator
Paragraph Separator
Changed in version 3.2: \v and \f added to list of line boundaries.
For example:
&&& 'ab c\n\nde fg\rkl\r\n'.splitlines()
['ab c', '', 'de fg', 'kl']
&&& 'ab c\n\nde fg\rkl\r\n'.splitlines(keepends=True)
['ab c\n', '\n', 'de fg\r', 'kl\r\n']
when a delimiter string sep is given, this
method returns an empty list for the empty string, and a terminal line
break does not result in an extra line:
&&& &&.splitlines()
&&& &One line\n&.splitlines()
['One line']
For comparison, split('\n') gives:
&&& ''.split('\n')
['']
&&& 'Two lines\n'.split('\n')
['Two lines', '']
str.startswith(prefix[, start[, end]])
Return True if string starts with the prefix, otherwise return False.
prefix can also be a tuple of prefixes to look for.
With optional start,
test string beginning at that position.
With optional end, stop comparing
string at that position.
str.strip([chars])
Return a copy of the string with the leading and trailing characters removed.
The chars argument is a string specifying the set of characters to be removed.
If omitted or None, the chars argument defaults to removing whitespace.
The chars argument is no rather, all combinations of its
values are stripped:
'.strip()
'spacious'
&&& 'www.example.com'.strip('cmowz.')
'example'
The outermost leading and trailing chars argument values are stripped
from the string. Characters are removed from the leading end until
reaching a string character that is not contained in the set of
characters in chars. A similar action takes place on the trailing end.
For example:
&&& comment_string = '#....... Section 3.2.1 Issue #32 .......'
&&& comment_string.strip('.#! ')
'Section 3.2.1 Issue #32'
str.swapcase()
Return a copy of the string with uppercase characters converted to lowercase and
vice versa. Note that it is not necessarily true that
s.swapcase().swapcase() == s.
str.title()
Return a titlecased version of the string where words start with an uppercase
character and the remaining characters are lowercase.
For example:
&&& 'Hello world'.title()
'Hello World'
The algorithm uses a simple language-independent definition of a word as
groups of consecutive letters.
The definition works in many contexts but
it means that apostrophes in contractions and possessives form word
boundaries, which may not be the desired result:
&&& &they're bill's friends from the UK&.title()
&They'Re Bill'S Friends From The Uk&
A workaround for apostrophes can be constructed using regular expressions:
&&& import re
&&& def titlecase(s):
return re.sub(r&[A-Za-z]+('[A-Za-z]+)?&,
lambda mo: mo.group(0)[0].upper() +
mo.group(0)[1:].lower(),
&&& titlecase(&they're bill's friends.&)
&They're Bill's Friends.&
str.translate(table)
Return a copy of the string in which each character has been mapped through
the given translation table.
The table must be an object that implements
indexing via , typically a
When indexed by a Unicode ordinal (an integer), the
table object can do any of the following: return a Unicode ordinal or a
string, to map the character to one or m return
None, to delete the character fr or raise a
exception, to map the character to itself.
You can use
to create a translation map from
character-to-character mappings in different formats.
See also the
module for a more flexible approach to custom
character mappings.
str.upper()
Return a copy of the string with all the cased characters
converted to
uppercase.
Note that str.upper().isupper() might be False if s
contains uncased characters or if the Unicode category of the resulting
character(s) is not “Lu” (Letter, uppercase), but e.g. “Lt” (Letter,
titlecase).
The uppercasing algorithm used is described in section 3.13 of the Unicode
str.zfill(width)
Return a copy of the string left filled with ASCII '0' digits to
make a string of length width. A leading sign prefix ('+'/'-')
is handled by inserting the padding after the sign character rather
than before. The original string is returned if width is less than
or equal to len(s).
For example:
&&& &42&.zfill(5)
'00042'
&&& &-42&.zfill(5)
'-0042'
4.7.2. printf-style String Formatting
The formatting operations described here exhibit a variety of quirks that
lead to a number of common errors (such as failing to display tuples and
dictionaries correctly).
Using the newer
helps avoid these errors, and also provides a generally more powerful,
flexible and extensible approach to formatting text.
String objects have one unique built-in operation: the % operator (modulo).
This is also known as the string formatting or interpolation operator.
Given format % values (where format is a string), % conversion
specifications in format are replaced with zero or more elements of values.
The effect is similar to using the sprintf() in the C language.
If format requires a single argument, values may be a single non-tuple
Otherwise, values must be a tuple with exactly the number of
items specified by the format string, or a single mapping object (for example, a
dictionary).
A conversion specifier contains two or more characters and has the following
components, which must occur in this order:
The '%' character, which marks the start of the specifier.
Mapping key (optional), consisting of a parenthesised sequence of characters
(for example, (somename)).
Conversion flags (optional), which affect the result of some conversion
Minimum field width (optional).
If specified as an '*' (asterisk), the
actual width is read from the next element of the tuple in values, and the
object to convert comes after the minimum field width and optional precision.
Precision (optional), given as a '.' (dot) followed by the precision.
specified as '*' (an asterisk), the actual precision is read from the next
element of the tuple in values, and the value to convert comes after the
precision.
Length modifier (optional).
Conversion type.
When the right argument is a dictionary (or other mapping type), then the
formats in the string must include a parenthesised mapping key into that
dictionary inserted immediately after the '%' character. The mapping key
selects the value to be formatted from the mapping.
For example:
&&& print('%(language)s has %(number)03d quote types.' %
{'language': &Python&, &number&: 2})
Python has 002 quote types.
In this case no * specifiers may occur in a format (since they require a
sequential parameter list).
The conversion flag characters are:
The value conversion will use the “alternate form” (where defined
The conversion will be zero padded for numeric values.
The converted value is left adjusted (overrides the '0'
conversion if both are given).
(a space) A blank should be left before a positive number (or empty
string) produced by a signed conversion.
A sign character ('+' or '-') will precede the conversion
(overrides a “space” flag).
A length modifier (h, l, or L) may be present, but is ignored as it
is not necessary for Python – so e.g. %ld is identical to %d.
The conversion types are:
Signed integer decimal.
Signed integer decimal.
Signed octal value.
Obsolete type – it is identical to 'd'.
Signed hexadecimal (lowercase).
Signed hexadecimal (uppercase).
Floating point exponential format (lowercase).
Floating point exponential format (uppercase).
Floating point decimal format.
Floating point decimal format.
Floating point format. Uses lowercase exponential
format if exponent is less than -4 or not less than
precision, decimal format otherwise.
Floating point format. Uses uppercase exponential
format if exponent is less than -4 or not less than
precision, decimal format otherwise.
Single character (accepts integer or single
character string).
String (converts any Python object using
String (converts any Python object using
String (converts any Python object using
No argument is converted, results in a '%'
character in the result.
The alternate form causes a leading octal specifier ('0o') to be
inserted before the first digit.
The alternate form causes a leading '0x' or '0X' (depending on whether
the 'x' or 'X' format was used) to be inserted before the first digit.
The alternate form causes the result to always contain a decimal point, even if
no digits follow it.
The precision determines the number of digits after the decimal point and
defaults to 6.
The alternate form causes the result to always contain a decimal point, and
trailing zeroes are not removed as they would otherwise be.
The precision determines the number of significant digits before and after the
decimal point and defaults to 6.
If precision is N, the output is truncated to N characters.
Since Python strings have an explicit length, %s conversions do not assume
that '\0' is the end of the string.
Changed in version 3.1: %f conversions for numbers whose absolute value is over 1e50 are no
longer replaced by %g conversions.
4.8. Binary Sequence Types — , ,
The core built-in types for manipulating binary data are
. They are supported by
which uses
to access the memory of other
binary objects without needing to make a copy.
module supports efficient storage of basic data types like
32-bit integers and IEEE754 double-precision floating values.
4.8.1. Bytes
Bytes objects are immutable sequences of single bytes. Since many major
binary protocols are based on the ASCII text encoding, bytes objects offer
several methods that are only valid when working with ASCII compatible
data and are closely related to string objects in a variety of other ways.
Firstly, the syntax for bytes literals is largely the same as that for string
literals, except that a b prefix is added:
Single quotes: b'still allows embedded &double& quotes'
Double quotes: b&still allows embedded 'single' quotes&.
Triple quoted: b'''3 single quotes''', b&&&3 double quotes&&&
Only ASCII characters are permitted in bytes literals (regardless of the
declared source code encoding). Any binary values over 127 must be entered
into bytes literals using the appropriate escape sequence.
As with string literals, bytes literals may also use a r prefix to disable
processing of escape sequences. See
for more about the various
forms of bytes literal, including supported escape sequences.
While bytes literals and representations are based on ASCII text, bytes
objects actually behave like immutable sequences of integers, with each
value in the sequence restricted such that 0 &= x & 256 (attempts to
violate this restriction will trigger . This is done
deliberately to emphasise that while many binary formats include ASCII based
elements and can be usefully manipulated with some text-oriented algorithms,
this is not generally the case for arbitrary binary data (blindly applying
text processing algorithms to binary data formats that are not ASCII
compatible will usually lead to data corruption).
In addition to the literal forms, bytes objects can be created in a number of
other ways:
A zero-filled bytes object of a specified length: bytes(10)
From an iterable of integers: bytes(range(20))
Copying existing binary data via the buffer protocol:
bytes(obj)
Also see the
Since 2 hexadecimal digits correspond precisely to a single byte, hexadecimal
numbers are a commonly used format for describing binary data. Accordingly,
the bytes type has an additional class method to read data in that format:
classmethod bytes.fromhex(string)
class method returns a bytes object, decoding the
given string object.
The string must contain two hexadecimal digits per
byte, with ASCII spaces being ignored.
&&& bytes.fromhex('2Ef0 F1f2
b'.\xf0\xf1\xf2'
A reverse conversion function exists to transform a bytes object into its
hexadecimal representation.
bytes.hex()
Return a string object containing two hexadecimal digits for each
byte in the instance.
&&& b'\xf0\xf1\xf2'.hex()
'f0f1f2'
New in version 3.5.
Since bytes objects are sequences of integers (akin to a tuple), for a bytes
object b, b[0] will be an integer, while b[0:1] will be a bytes
object of length 1.
(This contrasts with text strings, where both indexing
and slicing will produce a string of length 1)
The representation of bytes objects uses the literal format (b'...')
since it is often more useful than e.g. bytes([46, 46, 46]).
always convert a bytes object into a list of integers using list(b).
For Python 2.x users: In the Python 2.x series, a variety of implicit
conversions between 8-bit strings (the closest thing 2.x offers to a
built-in binary data type) and Unicode strings were permitted. This was a
backwards compatibility workaround to account for the fact that Python
originally only supported 8-bit text, and Unicode text was a later
addition. In Python 3.x, those implicit conversions are gone - conversions
between 8-bit binary data and Unicode text must be explicit, and bytes and
string objects will always compare unequal.
4.8.2. Bytearray Objects
objects are a mutable counterpart to
objects. There is no dedicated literal syntax for bytearray objects, instead
they are always created by calling the constructor:
Creating an empty instance: bytearray()
Creating a zero-filled instance with a given length: bytearray(10)
From an iterable of integers: bytearray(range(20))
Copying existing binary data via the buffer protocol:
bytearray(b'Hi!')
As bytearray objects are mutable, they support the
sequence operations in addition to the
common bytes and bytearray operations described in .
Also see the
Since 2 hexadecimal digits correspond precisely to a single byte, hexadecimal
numbers are a commonly used format for describing binary data. Accordingly,
the bytearray type has an additional class method to read data in that format:
classmethod bytearray.fromhex(string)
class method returns bytearray object, decoding
the given string object.
The string must contain two hexadecimal digits
per byte, with ASCII spaces being ignored.
&&& bytearray.fromhex('2Ef0 F1f2
bytearray(b'.\xf0\xf1\xf2')
A reverse conversion function exists to transform a bytearray object into its
hexadecimal representation.
bytearray.hex()
Return a string object containing two hexadecimal digits for each
byte in the instance.
&&& bytearray(b'\xf0\xf1\xf2').hex()
'f0f1f2'
New in version 3.5.
Since bytearray objects are sequences of integers (akin to a list), for a
bytearray object b, b[0] will be an integer, while b[0:1] will be
a bytearray object of length 1.
(This contrasts with text strings, where
both indexing and slicing will produce a string of length 1)
The representation of bytearray objects uses the bytes literal format
(bytearray(b'...')) since it is often more useful than e.g.
bytearray([46, 46, 46]).
You can always convert a bytearray object into
a list of integers using list(b).
4.8.3. Bytes and Bytearray Operations
Both bytes and bytearray objects support the
sequence operations. They interoperate not just with operands of the same
type, but with any . Due to this flexibility, they can be
freely mixed in operations without causing errors. However, the return type
of the result may depend on the order of operands.
The methods on bytes and bytearray objects don’t accept strings as their
arguments, just as the methods on strings don’t accept bytes as their
arguments.
For example, you have to write:
b = a.replace(&a&, &f&)
a = b&abc&
b = a.replace(b&a&, b&f&)
Some bytes and bytearray operations assume the use of ASCII compatible
binary formats, and hence should be avoided when working with arbitrary
binary data. These restrictions are covered below.
Using these ASCII based operations to manipulate binary data that is not
stored in an ASCII based format may lead to data corruption.
The following methods on bytes and bytearray objects can be used with
arbitrary binary data.
bytes.count(sub[, start[, end]])
bytearray.count(sub[, start[, end]])
Return the number of non-overlapping occurrences of subsequence sub in
the range [start, end].
Optional arguments start and end are
interpreted as in slice notation.
The subsequence to search for may be any