The following sections describe the standard types that are built into the interpreter.
Note
Historically (until release 2.2), Python’s built-in types have differed from user-defined types because it was not possible to use the built-in types as the basis for object-oriented inheritance. This limitation no longer exists.
The principal built-in types are numerics, sequences, mappings, files, classes, instances and exceptions.
Some operations are supported by several object types; in particular, practically all objects can be compared, tested for truth value, and converted to a string (with the repr() function or the slightly different str() function). The latter function is implicitly used when an object is written by the print() function.
Any object can be tested for truth value, for use in an if or while condition or as operand of the Boolean operations below. The following values are considered false:
None
False
zero of any numeric type, for example, 0, 0L, 0.0, 0j.
any empty sequence, for example, '', (), [].
any empty mapping, for example, {}.
instances of user-defined classes, if the class defines a __nonzero__() or __len__() method, when that method returns the integer zero or bool value False. [1]
All other values are considered true — so objects of many types are always true.
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.)
These are the Boolean operations, ordered by ascending priority:
| Operation | Result | Notes |
|---|---|---|
| x or y | if x is false, then y, else x | (1) |
| x and y | if x is false, then x, else y | (2) |
| not x | if x is false, then True, else False | (3) |
Notes:
Comparison operations are supported by all objects. They all have the same priority (which is higher than that of the Boolean operations). Comparisons can be chained arbitrarily; 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:
| Operation | Meaning | Notes |
|---|---|---|
| < | strictly less than | |
| <= | less than or equal | |
| > | strictly greater than | |
| >= | greater than or equal | |
| == | equal | |
| != | not equal | (1) |
| is | object identity | |
| is not | negated object identity |
Notes:
Objects of different types, except different numeric types and different string types, never compare equal; such objects are ordered consistently but arbitrarily (so that sorting a heterogeneous array yields a consistent result). Furthermore, some types (for example, file objects) support only a degenerate notion of comparison where any two objects of that type are unequal. Again, such objects are ordered arbitrarily but consistently. The <, <=, > and >= operators will raise a TypeError exception when any operand is a complex number.
Instances of a class normally compare as non-equal unless the class defines the __cmp__() method. Refer to Basic customization) for information on the use of this method to effect object comparisons.
Implementation note: Objects of different types except numbers are ordered by their type names; objects of the same types that don’t support proper comparison are ordered by their address.
Two more operations with the same syntactic priority, in and not in, are supported only by sequence types (below).
There are four distinct numeric types: plain integers, long integers, floating point numbers, and complex numbers. In addition, Booleans are a subtype of plain integers. Plain integers (also just called integers) are implemented using long in C, which gives them at least 32 bits of precision (sys.maxint is always set to the maximum plain integer value for the current platform, the minimum value is -sys.maxint - 1). Long integers have unlimited precision. Floating point numbers are implemented using double in C. All bets on their precision are off unless you happen to know the machine you are working with.
Complex numbers have a real and imaginary part, which are each implemented using double in C. To extract these parts from a complex number z, use z.real and z.imag.
Numbers are created by numeric literals or as the result of built-in functions and operators. Unadorned integer literals (including hex and octal numbers) yield plain integers unless the value they denote is too large to be represented as a plain integer, in which case they yield a long integer. Integer literals with an 'L' or 'l' suffix yield long integers ('L' is preferred because 1l looks too much like eleven!). Numeric literals containing a decimal point or an exponent sign yield floating point numbers. Appending 'j' or 'J' to a numeric literal yields a complex number with a zero real part. A complex numeric literal is the sum of a real and an imaginary part.
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 plain integer is narrower than long integer is narrower than floating point is narrower than complex. Comparisons between numbers of mixed type use the same rule. [2] The constructors int(), long(), float(), and complex() can be used to produce numbers of a specific type.
All builtin numeric types support the following operations. See The power operator and later sections for the operators’ priorities.
| Operation | Result | Notes |
|---|---|---|
| x + y | sum of x and y | |
| x - y | difference of x and y | |
| x * y | product of x and y | |
| x / y | quotient of x and y | (1) |
| x // y | (floored) quotient of x and y | (4)(5) |
| x % y | remainder of x / y | (4) |
| -x | x negated | |
| +x | x unchanged | |
| abs(x) | absolute value or magnitude of x | (3) |
| int(x) | x converted to integer | (2) |
| long(x) | x converted to long integer | (2) |
| float(x) | x converted to floating point | (6) |
| complex(re,im) | a complex number with real part re, imaginary part im. im defaults to zero. | |
| c.conjugate() | conjugate of the complex number c. (Identity on real numbers) | |
| divmod(x, y) | the pair (x // y, x % y) | (3)(4) |
| pow(x, y) | x to the power y | (3)(7) |
| x ** y | x to the power y | (7) |
Notes:
For (plain or long) integer division, the result is an integer. 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. Note that the result is a long integer if either operand is a long integer, regardless of the numeric value.
Conversion from floating point to (long or plain) integer may round or truncate as in C; see functions math.floor() and math.ceil() for well-defined conversions.
Deprecated in version 2.6: Instead, convert floats to long explicitly with trunc().
See Built-in Functions for a full description.
Complex floor division operator, modulo operator, and divmod().
Deprecated in version 2.3: Instead convert to float using abs() if appropriate.
Also referred to as integer division. The resultant value is a whole integer, though the result’s type is not necessarily int.
float also accepts the strings “nan” and “inf” with an optional prefix “+” or “-” for Not a Number (NaN) and positive or negative infinity.
New in version 2.6.
Python defines pow(0, 0) and 0 ** 0 to be 1, as is common for programming languages.
All numbers.Real types (int, long, and float) also include the following operations:
| Operation | Result | Notes |
|---|---|---|
| trunc(x) | x truncated to Integral | |
| round(x[, n]) | x rounded to n digits, rounding half to even. If n is omitted, it defaults to 0. | |
| math.floor(x) | the greatest integral float <= x | |
| math.ceil(x) | the least integral float >= x |
Plain and long integer types support additional operations that make sense only for bit-strings. Negative numbers are treated as their 2’s complement value (for long integers, this assumes a sufficiently large number of bits that no overflow occurs during the operation).
The priorities of the binary bitwise operations are all lower than the numeric operations and higher than the comparisons; the unary operation ~ has the same priority as the other unary numeric operations (+ and -).
This table lists the bit-string operations sorted in ascending priority:
| Operation | Result | Notes |
|---|---|---|
| x | y | bitwise or of x and y | |
| x ^ y | bitwise exclusive or of x and y | |
| x & y | bitwise and of x and y | |
| x << n | x shifted left by n bits | (1)(2) |
| x >> n | x shifted right by n bits | (1)(3) |
| ~x | the bits of x inverted |
Notes:
The float type has some additional methods to 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. In contrast, hexadecimal strings allow exact representation and specification of floating-point numbers. This can be useful when debugging, and in numerical work.
Return a representation of a floating-point number as a hexadecimal string. For finite floating-point numbers, this representation will always include a leading 0x and a trailing p and exponent.
New in version 2.6.
Class method to return the float represented by a hexadecimal string s. The string s may have leading and trailing whitespace.
New in version 2.6.
Note that float.hex() is an instance method, while float.fromhex() 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 float.hex() 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 float.fromhex().
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 3740.0:
>>> float.fromhex('0x3.a7p10')
3740.0
Applying the reverse conversion to 3740.0 gives a different hexadecimal string representing the same number:
>>> float.hex(3740.0)
'0x1.d380000000000p+11'
New in version 2.2.
Python supports a concept of iteration over containers. This is implemented using two distinct methods; 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 support:
The iterator objects themselves are required to support the following two methods, which together form the iterator protocol:
Python defines several iterator objects to support iteration over general and specific sequence types, dictionaries, and other more specialized forms. The specific types are not important beyond their implementation of the iterator protocol.
The intention of the protocol is that once an iterator’s next() method raises StopIteration, it will continue to do so on subsequent calls. Implementations that do not obey this property are deemed broken. (This constraint was added in Python 2.3; in Python 2.2, various iterators are broken according to this rule.)
Python’s generators provide a convenient way to implement the iterator protocol. If a container object’s __iter__() method is implemented as a generator, it will automatically return an iterator object (technically, a generator object) supplying the __iter__() and next() methods.
There are six sequence types: strings, Unicode strings, lists, tuples, buffers, and xrange objects. (For other containers see the built in dict, list, set, and tuple classes, and the collections module.)
String literals are written in single or double quotes: 'xyzzy', "frobozz". See String literals for more about string literals. Unicode strings are much like strings, but are specified in the syntax using a preceding 'u' character: u'abc', u"def". In addition to the functionality described here, there are also string-specific methods described in the String Methods section. Lists are constructed with square brackets, separating items with commas: [a, b, c]. Tuples are constructed by the comma operator (not within square brackets), with or without enclosing parentheses, but an empty tuple must have the enclosing parentheses, such as a, b, c or (). A single item tuple must have a trailing comma, such as (d,).
Buffer objects are not directly supported by Python syntax, but can be created by calling the builtin function buffer(). They don’t support concatenation or repetition.
Objects of type xrange are similar to buffers in that there is no specific syntax to create them, but they are created using the xrange() function. They don’t support slicing, concatenation or repetition, and using in, not in, min() or max() on them is inefficient.
Most sequence types support the following operations. The in and not in operations have the same priorities as the comparison operations. The + and * operations have the same priority as the corresponding numeric operations. [3] Additional methods are provided for Mutable Sequence Types.
This table lists the sequence operations sorted in ascending priority (operations in the same box have the same priority). In the table, s and t are sequences of the same type; n, i and j are integers:
| Operation | Result | Notes |
|---|---|---|
| x in s | True if an item of s is equal to x, else False | (1) |
| x not in s | False if an item of s is equal to x, else True | (1) |
| s + t | the concatenation of s and t | (6) |
| s * n, n * s | n shallow copies of s concatenated | (2) |
| s[i] | i‘th item of s, origin 0 | (3) |
| s[i:j] | slice of s from i to j | (3)(4) |
| s[i:j:k] | slice of s from i to j with step k | (3)(5) |
| len(s) | length of s | |
| min(s) | smallest item of s | |
| max(s) | largest item of s |
Sequence types 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. (For full details see Comparisons in the language reference.)
Notes:
When s is a string or Unicode string object the in and not in operations act like a substring test. In Python versions before 2.3, x had to be a string of length 1. In Python 2.3 and beyond, x may be a string of any length.
Values of n less than 0 are treated as 0 (which yields an empty sequence of the same type as s). Note also that the copies are shallow; nested structures are not copied. This often haunts new Python programmers; consider:
>>> lists = [[]] * 3
>>> lists
[[], [], []]
>>> lists[0].append(3)
>>> lists
[[3], [3], [3]]
What has happened is that [[]] is a one-element list containing an empty list, so all three elements of [[]] * 3 are (pointers to) this single empty list. 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)
>>> lists
[[3], [5], [7]]
If i or j is negative, the index is relative to the end of the string: len(s) + i or len(s) + j is substituted. But note that -0 is still 0.
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 len(s). 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 empty.
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). If i or j is greater than len(s), use len(s). 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.
If s and t are both strings, some Python implementations such as CPython can usually perform an in-place optimization for assignments of the form s=s+t or s+=t. When applicable, this optimization makes quadratic run-time much less likely. This optimization is both version and implementation dependent. For performance sensitive code, it is preferable to use the str.join() method which assures consistent linear concatenation performance across versions and implementations.
Changed in version 2.4: Formerly, string concatenation never occurred in-place.
Below are listed the string methods which both 8-bit strings and Unicode objects support. Note that none of these methods take keyword arguments.
In addition, Python’s strings support the sequence type methods described in the Sequence Types — str, unicode, list, tuple, buffer, xrange section. To output formatted strings use template strings or the % operator described in the String Formatting Operations section. Also, see the re module for string functions based on regular expressions.
Return a copy of the string with only its first character capitalized.
For 8-bit strings, this method is locale-dependent.
Return centered in a string of length width. Padding is done using the specified fillchar (default is a space).
Changed in version 2.4: Support for the fillchar argument.
Decodes the string using the codec registered for encoding. encoding defaults to the default string encoding. errors may be given to set a different error handling scheme. The default is 'strict', meaning that encoding errors raise UnicodeError. Other possible values are 'ignore', 'replace' and any other name registered via codecs.register_error(), see section Codec Base Classes.
New in version 2.2.
Changed in version 2.3: Support for other error handling schemes added.
Return an encoded version of the string. Default encoding is the current default string encoding. errors may be given to set a different error handling scheme. The default for errors is 'strict', meaning that encoding errors raise a UnicodeError. Other possible values are 'ignore', 'replace', 'xmlcharrefreplace', 'backslashreplace' and any other name registered via codecs.register_error(), see section Codec Base Classes. For a list of possible encodings, see section Standard Encodings.
New in version 2.0.
Changed in version 2.3: Support for 'xmlcharrefreplace' and 'backslashreplace' and other error handling schemes added.
Return True if the string ends with the specified suffix, otherwise return False. 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.
Changed in version 2.5: Accept tuples as suffix.
Perform a string formatting operation. The format_string argument 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 format_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'
See Format String Syntax for a description of the various formatting options that can be specified in format strings.
This method of string formatting is the new standard in Python 3.0, and should be preferred to the % formatting described in String Formatting Operations in new code.
New in version 2.6.
Return true if all characters in the string are alphanumeric and there is at least one character, false otherwise.
For 8-bit strings, this method is locale-dependent.
Return true if all characters in the string are alphabetic and there is at least one character, false otherwise.
For 8-bit strings, this method is locale-dependent.
Return true if all characters in the string are digits and there is at least one character, false otherwise.
For 8-bit strings, this method is locale-dependent.
Return true if all cased characters in the string are lowercase and there is at least one cased character, false otherwise.
For 8-bit strings, this method is locale-dependent.
Return true if there are only whitespace characters in the string and there is at least one character, false otherwise.
For 8-bit strings, this method is locale-dependent.
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.
For 8-bit strings, this method is locale-dependent.
Return true if all cased characters in the string are uppercase and there is at least one cased character, false otherwise.
For 8-bit strings, this method is locale-dependent.
Return the string left justified in a string of length width. Padding is done using the specified fillchar (default is a space). The original string is returned if width is less than len(s).
Changed in version 2.4: Support for the fillchar argument.
Return a copy of the string converted to lowercase.
For 8-bit strings, this method is locale-dependent.
Return a copy of the string with leading 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 not a prefix; rather, all combinations of its values are stripped:
>>> ' spacious '.lstrip()
'spacious '
>>> 'www.example.com'.lstrip('cmowz.')
'example.com'
Changed in version 2.2.2: Support for the chars argument.
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.
New in version 2.5.
Return the string right justified in a string of length width. Padding is done using the specified fillchar (default is a space). The original string is returned if width is less than len(s).
Changed in version 2.4: Support for the fillchar argument.
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.
New in version 2.5.
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 ones. If sep is not specified or None, any whitespace string is a separator. Except for splitting from the right, rsplit() behaves like split() which is described in detail below.
New in version 2.4.
Return a copy of the string with 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 not a suffix; rather, all combinations of its values are stripped:
>>> ' spacious '.rstrip()
' spacious'
>>> 'mississippi'.rstrip('ipz')
'mississ'
Changed in version 2.2.2: Support for the chars argument.
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 (thus, the list will have at most maxsplit+1 elements). If maxsplit is not specified, 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 [''].
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() returns ['1', '2', '3'], and ' 1 2 3 '.split(None, 1) returns ['1', '2 3 '].
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.
Changed in version 2.5: Accept tuples as prefix.
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 not a prefix or suffix; rather, all combinations of its values are stripped:
>>> ' spacious '.strip()
'spacious'
>>> 'www.example.com'.strip('cmowz.')
'example'
Changed in version 2.2.2: Support for the chars argument.
Return a copy of the string with uppercase characters converted to lowercase and vice versa.
For 8-bit strings, this method is locale-dependent.
Return a titlecased version of the string: words start with uppercase characters, all remaining cased characters are lowercase.
For 8-bit strings, this method is locale-dependent.
Return a copy of the string where all characters occurring in the optional argument deletechars are removed, and the remaining characters have been mapped through the given translation table, which must be a string of length 256.
You can use the maketrans() helper function in the string module to create a translation table. For string objects, set the table argument to None for translations that only delete characters:
>>> 'read this short text'.translate(None, 'aeiou')
'rd ths shrt txt'
New in version 2.6: Support for a None table argument.
For Unicode objects, the translate() method does not accept the optional deletechars argument. Instead, it returns a copy of the s where all characters have been mapped through the given translation table which must be a mapping of Unicode ordinals to Unicode ordinals, Unicode strings or None. Unmapped characters are left untouched. Characters mapped to None are deleted. Note, a more flexible approach is to create a custom character mapping codec using the codecs module (see encodings.cp1251 for an example).
Return a copy of the string converted to uppercase.
For 8-bit strings, this method is locale-dependent.
Return the numeric string left filled with zeros in a string of length width. A sign prefix is handled correctly. The original string is returned if width is less than len(s).
New in version 2.2.2.
The following methods are present only on unicode objects:
String and Unicode 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 or Unicode object), % conversion specifications in format are replaced with zero or more elements of values. The effect is similar to the using sprintf in the C language. If format is a Unicode object, or if any of the objects being converted using the %s conversion are Unicode objects, the result will also be a Unicode object.
If format requires a single argument, values may be a single non-tuple object. [4] 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:
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 %(#)03d quote types.' % \
... {'language': "Python", "#": 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:
| Flag | Meaning |
|---|---|
| '#' | The value conversion will use the “alternate form” (where defined below). |
| '0' | 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:
| Conversion | Meaning | Notes |
|---|---|---|
| 'd' | Signed integer decimal. | |
| 'i' | Signed integer decimal. | |
| 'o' | Signed octal value. | (1) |
| 'u' | Obselete type – it is identical to 'd'. | (7) |
| 'x' | Signed hexadecimal (lowercase). | (2) |
| 'X' | Signed hexadecimal (uppercase). | (2) |
| 'e' | Floating point exponential format (lowercase). | (3) |
| 'E' | Floating point exponential format (uppercase). | (3) |
| 'f' | Floating point decimal format. | (3) |
| 'F' | Floating point decimal format. | (3) |
| 'g' | Floating point format. Uses lowercase exponential format if exponent is less than -4 or not less than precision, decimal format otherwise. | (4) |
| 'G' | Floating point format. Uses uppercase exponential format if exponent is less than -4 or not less than precision, decimal format otherwise. | (4) |
| 'c' | Single character (accepts integer or single character string). | |
| 'r' | String (converts any python object using repr()). | (5) |
| 's' | String (converts any python object using str()). | (6) |
| '%' | No argument is converted, results in a '%' character in the result. |
Notes:
The alternate form causes a leading zero ('0') to be inserted between left-hand padding and the formatting of the number if the leading character of the result is not already a zero.
The alternate form causes a leading '0x' or '0X' (depending on whether the 'x' or 'X' format was used) to be inserted between left-hand padding and the formatting of the number if the leading character of the result is not already a zero.
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.
The %r conversion was added in Python 2.0.
The precision determines the maximal number of characters used.
If the object or format provided is a unicode string, the resulting string will also be unicode.
The precision determines the maximal number of characters used.
See PEP 237.
Since Python strings have an explicit length, %s conversions do not assume that '\0' is the end of the string.
For safety reasons, floating point precisions are clipped to 50; %f conversions for numbers whose absolute value is over 1e25 are replaced by %g conversions. [5] All other errors raise exceptions.
Additional string operations are defined in standard modules string and re.
The xrange type is an immutable sequence which is commonly used for looping. The advantage of the xrange type is that an xrange object will always take the same amount of memory, no matter the size of the range it represents. There are no consistent performance advantages.
XRange objects have very little behavior: they only support indexing, iteration, and the len() function.
List objects support additional operations that allow in-place modification of the object. Other mutable sequence types (when added to the language) should also support these operations. Strings and tuples are immutable sequence types: such objects cannot be modified once created. The following operations are defined on mutable sequence types (where x is an arbitrary object):
| Operation | Result | Notes |
|---|---|---|
| s[i] = x | item i of s is replaced by x | |
| 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 | (1) |
| del s[i:j:k] | removes the elements of s[i:j:k] from the list | |
| s.append(x) | same as s[len(s):len(s)] = [x] | (2) |
| s.extend(x) | same as s[len(s):len(s)] = x | (3) |
| s.count(x) | return number of i‘s for which s[i] == x | |
| s.index(x[, i[, j]]) | return smallest k such that s[k] == x and i <= k < j | (4) |
| s.insert(i, x) | same as s[i:i] = [x] | (5) |
| s.pop([i]) | same as x = s[i]; del s[i]; return x | (6) |
| s.remove(x) | same as del s[s.index(x)] | (4) |
| s.reverse() | reverses the items of s in place | (7) |
| s.sort([cmp[, key[, reverse]]]) | sort the items of s in place | (7)(8)(9)(10) |
Notes:
t must have the same length as the slice it is replacing.
The C implementation of Python has historically accepted multiple parameters and implicitly joined them into a tuple; this no longer works in Python 2.0. Use of this misfeature has been deprecated since Python 1.4.
x can be any iterable object.
Raises ValueError when x is not found in s. When a negative index is passed as the second or third parameter to the index() method, the list length is added, as for slice indices. If it is still negative, it is truncated to zero, as for slice indices.
Changed in version 2.3: Previously, index() didn’t have arguments for specifying start and stop positions.
When a negative index is passed as the first parameter to the insert() method, the list length is added, as for slice indices. If it is still negative, it is truncated to zero, as for slice indices.
Changed in version 2.3: Previously, all negative indices were truncated to zero.
The pop() method is only supported by the list and array types. The optional argument i defaults to -1, so that by default the last item is removed and returned.
The sort() and reverse() methods modify the list in place for economy of space when sorting or reversing a large list. To remind you that they operate by side effect, they don’t return the sorted or reversed list.
The sort() method takes optional arguments for controlling the comparisons.
cmp specifies a custom comparison function of two arguments (list items) which should return a negative, zero or positive number depending on whether the first argument is considered smaller than, equal to, or larger than the second argument: cmp=lambda x,y: cmp(x.lower(), y.lower()). The default value is None.
key specifies a function of one argument that is used to extract a comparison key from each list element: key=str.lower. The default value is None.
reverse is a boolean value. If set to True, then the list elements are sorted as if each comparison were reversed.
In general, the key and reverse conversion processes are much faster than specifying an equivalent cmp function. This is because cmp is called multiple times for each list element while key and reverse touch each element only once.
Changed in version 2.3: Support for None as an equivalent to omitting cmp was added.
Changed in version 2.4: Support for key and reverse was added.
Starting with Python 2.3, the sort() 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).
While a list is being sorted, the effect of attempting to mutate, or even inspect, the list is undefined. The C implementation of Python 2.3 and newer makes the list appear empty for the duration, and raises ValueError if it can detect that the list has been mutated during a sort.
A set object is an unordered collection of distinct hashable objects. Common uses include membership testing, removing duplicates from a sequence, and computing mathematical operations such as intersection, union, difference, and symmetric difference. (For other containers see the built in dict, list, and tuple classes, and the collections module.)
New in version 2.4.
Like other collections, sets support x in set, len(set), and for x in set. Being an unordered collection, sets do not record element position or order of insertion. Accordingly, sets do not support indexing, slicing, or other sequence-like behavior.
There are currently two builtin set types, set and frozenset. The set type is mutable — the contents can be changed using methods like add() and remove(). Since it is mutable, it has no hash value and cannot be used as either a dictionary key or as an element of another set. The frozenset type is immutable and hashable — its contents cannot be altered after it is created; it can therefore be used as a dictionary key or as an element of another set.
The constructors for both classes work the same:
Return a new set or frozenset object whose elements are taken from iterable. The elements of a set must be hashable. To represent sets of sets, the inner sets must be frozenset objects. If iterable is not specified, a new empty set is returned.
Instances of set and frozenset provide the following operations:
Return True if the set has no elements in common with other. Sets are disjoint if and only if their intersection is the empty set.
New in version 2.6.
Return a new set with elements from both sets.
Changed in version 2.6: Accepts multiple input iterables.
Return a new set with elements common to both sets.
Changed in version 2.6: Accepts multiple input iterables.
Return a new set with elements in the set that are not in the others.
Changed in version 2.6: Accepts multiple input iterables.
Note, the non-operator versions of union(), intersection(), difference(), and symmetric_difference(), issubset(), and issuperset() methods will accept any iterable as an argument. In contrast, their operator based counterparts require their arguments to be sets. This precludes error-prone constructions like set('abc') & 'cbs' in favor of the more readable set('abc').intersection('cbs').
Both set and frozenset support set to set comparisons. Two sets are equal if and only if every element of each set is contained in the other (each is a subset of the other). A set is less than another set if and only if the first set is a proper subset of the second set (is a subset, but is not equal). A set is greater than another set if and only if the first set is a proper superset of the second set (is a superset, but is not equal).
Instances of set are compared to instances of frozenset based on their members. For example, set('abc') == frozenset('abc') returns True and so does set('abc') in set([frozenset('abc')]).
The subset and equality comparisons do not generalize to a complete ordering function. For example, any two disjoint sets are not equal and are not subsets of each other, so all of the following return False: a<b, a==b, or a>b. Accordingly, sets do not implement the __cmp__() method.
Since sets only define partial ordering (subset relationships), the output of the list.sort() method is undefined for lists of sets.
Set elements, like dictionary keys, must be hashable.
Binary operations that mix set instances with frozenset return the type of the first operand. For example: frozenset('ab') | set('bc') returns an instance of frozenset.
The following table lists operations available for set that do not apply to immutable instances of frozenset:
Update the set, adding elements from other.
Changed in version 2.6: Accepts multiple input iterables.
Update the set, keeping only elements found in it and other.
Changed in version 2.6: Accepts multiple input iterables.
Update the set, removing elements found in others.
Changed in version 2.6: Accepts multiple input iterables.