5. Built-in Types

5.4.1. Bitwise Operations on Integer Types

Bitwise operations only make sense for integers. Negative numbers are treated as their 2’s complement value (this assumes 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 bitwise operations sorted in ascending priority:

Notes:

1. Negative shift counts are illegal and cause a ValueError to be raised.

2. A left shift by n bits is equivalent to multiplication by pow(2, n). A long integer is returned if the result exceeds the range of plain integers.

3. A right shift by n bits is equivalent to division by pow(2, n).

5.4.2. Additional Methods on Integer Types

The integer types implement the numbers.Integral abstract base class. In addition, they provide one more method:

int. bit_length ( )

long. bit_length ( )

Return the number of bits necessary to represent an integer in binary, excluding the sign and leading zeros:

>>> n = -37
>>> bin(n)
'-0b100101'
>>> n.bit_length()
6

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) --> '-0b100101'
    s = s.lstrip('-0b') # remove leading zeros and minus sign
    return len(s)       # len('100101') --> 6

New in version 2.7.

5.4.3. Additional Methods on Float

The float type implements the numbers.Real abstract base class. 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. Raises OverflowError on infinities and a ValueError on NaNs.

New in version 2.6.

float. is_integer ( )

Return True if the float instance is finite with integral value, and False otherwise:

>>> (-2.0).is_integer()
True
>>> (3.2).is_integer()
False

New in version 2.6.

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. In 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 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.

float. fromhex ( s )

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'

5.5.1. Generator Types

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. More information about generators can be found in the documentation for the yield expression.

5.6.1. String Methods

Below are listed the string methods which both 8-bit strings and Unicode objects support. Some of them are also available on bytearray objects.

In addition, Python’s strings support the sequence type methods described in the Sequence Types — str, unicode, list, tuple, bytearray, 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.

str. capitalize ( )

Return a copy of the string with its first character capitalized and the rest lowercased.

For 8-bit strings, this method is locale-dependent.

str. center ( width [, fillchar ] )

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.

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. decode ( [ encoding [, errors ] ] )

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.

Changed in version 2.7: Support for keyword arguments added.

str. encode ( [ encoding [, errors ] ] )

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.

Changed in version 2.7: Support for keyword arguments added.

str. endswith ( suffix [, start [, end ] ] )

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.

str. expandtabs ( [ tabsize ] )

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. Tab 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. If 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 zero. Any other character is copied unchanged and the current column is incremented by one regardless of how the character is represented when printed.

>>> '01\t012\t0123\t01234'.expandtabs()
'01      012     0123    01234'
>>> '01\t012\t0123\t01234'.expandtabs(4)
'01  012 0123    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.

Note

The find() 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 in operator:

>>> 'Py' in 'Python'
True

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'

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, and should be preferred to the % formatting described in String Formatting Operations in new code.

New in version 2.6.

str. index ( sub [, start [, end ] ] )

Like find(), but raise ValueError 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.

For 8-bit strings, this method is locale-dependent.

str. isalpha ( )

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.

str. isdigit ( )

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.

str. islower ( )

Return true if all cased characters 4 in the string are lowercase and there is at least one cased character, false otherwise.

For 8-bit strings, this method is locale-dependent.

str. isspace ( )

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.

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.

For 8-bit strings, this method is locale-dependent.

str. isupper ( )

Return true if all cased characters 4 in the string are uppercase and there is at least one cased character, false otherwise.

For 8-bit strings, this method is locale-dependent.

str. join ( iterable )

Return a string which is the concatenation of the strings in iterable. If there is any Unicode object in iterable, return a Unicode instead. A TypeError will be raised if there are any non-string or non Unicode object values in iterable. 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 a space). The original string is returned if width is less than or equal to len(s).

Changed in version 2.4: Support for the fillchar argument.

str. lower ( )

Return a copy of the string with all the cased characters 4 converted to lowercase.

For 8-bit strings, this method is locale-dependent.

str. lstrip ( [ chars ] )

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.

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.

New in version 2.5.

str. replace ( old, new [, count ] )

Return a copy of the string with all occurrences of substring old replaced by new. 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 ] ] )

Like rfind() but raises ValueError when the substring sub is not found.

str. rjust ( width [, fillchar ] )

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 or equal to len(s).

Changed in version 2.4: Support for the fillchar argument.

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.

New in version 2.5.

str. rsplit ( [ sep [, maxsplit ] ] )

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.

str. rstrip ( [ chars ] )

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.

str. split ( [ sep [, maxsplit ] ] )

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 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 [''].

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  '].

str. splitlines ( [ keepends ] )

Return a list of the lines in the string, breaking at line boundaries. This method uses the universal newlines approach to splitting lines. Line breaks are not included in the resulting list unless keepends is given and true.

Python recognizes "\r", "\n", and "\r\n" as line boundaries for 8-bit strings.

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(True)
['ab c\n', '\n', 'de fg\r', 'kl\r\n']

Unlike split() 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', '']

unicode. splitlines ( [ keepends ] )

Return a list of the lines in the string, like str.splitlines(). However, the Unicode method splits on the following line boundaries, which are a superset of the universal newlines recognized for 8-bit strings.

Representation	Description
\n	Line Feed
\r	Carriage Return
\r\n	Carriage Return + Line Feed
\v or \x0b	Line Tabulation
\f or \x0c	Form Feed
\x1c	File Separator
\x1d	Group Separator
\x1e	Record Separator
\x85	Next Line (C1 Control Code)
\u2028	Line Separator
\u2029	Paragraph Separator

Changed in version 2.7: \v and \f added to list of line boundaries.

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.

Changed in version 2.5: Accept tuples as prefix.

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 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.

str. swapcase ( )

Return a copy of the string with uppercase characters converted to lowercase and vice versa.

For 8-bit strings, this method is locale-dependent.

str. title ( )

Return a titlecased version of the string where words start with an uppercase character and the remaining characters are lowercase.

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(),
...                   s)
...
>>> titlecase("they're bill's friends.")
"They're Bill's Friends."

For 8-bit strings, this method is locale-dependent.

str. translate ( table [, deletechars ] )

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).

str. upper ( )

Return a copy of the string with all the cased characters 4 converted to uppercase. Note that s.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).

For 8-bit strings, this method is locale-dependent.

str. zfill ( width )

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 or equal to len(s).

New in version 2.2.2.

The following methods are present only on unicode objects:

unicode. isnumeric ( )

Return True if there are only numeric characters in S, 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.

unicode. isdecimal ( )

Return True if there are only decimal characters in S, False otherwise. Decimal characters include digit characters, and all characters that can be used to form decimal-radix numbers, e.g. U+0660, ARABIC-INDIC DIGIT ZERO.

5.6.2. String Formatting Operations

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. 5 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:

1. The '%' character, which marks the start of the specifier.

2. Mapping key (optional), consisting of a parenthesised sequence of characters (for example, (somename)).

3. Conversion flags (optional), which affect the result of some conversion types.

4. 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.

5. Precision (optional), given as a '.' (dot) followed by the precision. If specified as '*' (an asterisk), the actual width is read from the next element of the tuple in values, and the value to convert comes after the precision.

6. Length modifier (optional).

7. 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:

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:

Notes:

1. 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.

2. 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.

3. 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.

4. 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.

5. The %r conversion was added in Python 2.0.

   The precision determines the maximal number of characters used.

6. 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.

7. See PEP 237 .

Since Python strings have an explicit length, %s conversions do not assume that '\0' is the end of the string.

Additional string operations are defined in standard modules string and re.

5.6.3. XRange Type

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.

5.6.4. Mutable Sequence Types

List and bytearray 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):

Notes:

1. t must have the same length as the slice it is replacing.

2. 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.

3. t can be any iterable object.

4. 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.

5. 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.

6. The pop() method’s optional argument i defaults to -1, so that by default the last item is removed and returned.

7. 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.

8. 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. Use functools.cmp_to_key() to convert an old-style cmp function to a key function.

   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.

9. 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).

10. 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 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.

11. The value n is an integer, or an object implementing __index__(). Zero and negative values of n clear the sequence. Items in the sequence are not copied; they are referenced multiple times, as explained for s * n under Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange.