# 4.1 `string` -- Common string operations.

This module defines some constants useful for checking character classes and some useful string functions. See the module `re` for string functions based on regular expressions.

The constants defined in this module are are:

`digits`
  The string `'0123456789'`.

`hexdigits`
  The string `'0123456789abcdefABCDEF'`.

`letters`
  The concatenation of the strings `lowercase()` and `uppercase()` described below.

`lowercase`
  A string containing all the characters that are considered lowercase letters. On most systems this is the string `'abcdefghijklmnopqrstuvwxyz'`. Do not change its definition -- the effect on the routines `upper()` and `swapcase()` is undefined.

`octdigits`
  The string `'01234567'`.

`uppercase`
  A string containing all the characters that are considered uppercase letters. On most systems this is the string `'ABCDEFGHIJKLMNOPQRSTUVWXYZ'`. Do not change its definition -- the effect on the routines `lower()` and `swapcase()` is undefined.

`whitespace`
  A string containing all characters that are considered whitespace. On most systems this includes the characters space, tab, linefeed, return, formfeed, and vertical tab. Do not change its definition -- the effect on the routines `strip()` and `split()` is undefined.

The functions defined in this module are:

`atof` (s)
  Convert a string to a floating point number. The string must have the standard syntax for a floating point literal in Python, optionally preceded by a sign ("`+`" or "`-`"). Note that this behaves identical to the built-in function `float()` when passed a string.

  Note: When passing in a string, values for NaN and Infinity may be returned, depending on the underlying C library. The specific set of strings accepted which cause these values to be returned depends entirely on the C library and is known to vary.

`atoi` (s[, base])
  Convert string s to an integer in the given base. The string must consist of one or more digits, optionally preceded by a sign ("`+`" or "`-`"). The base defaults to 10. If it is 0, a default base is chosen depending on the leading characters of the string (after stripping the sign): "`0x`" or "`0X`" means 16, "`0`" means 8, anything else means 10. If base is 16, a leading "`0x`" or "`0X`" is always accepted. Note that when invoked without base or with base set to 10, this behaves identical to the built-in function `int()` when passed a string. (Also note: for a more flexible interpretation of numeric literals, use the built-in function `eval()`.)

`atol` (s[, base])
  Convert string s to a long integer in the given base. The string must consist of one or more digits, optionally preceded by a sign ("`+`" or "`-`"). The base argument has the same meaning as for `atoi()`. A trailing "`l`" or "`L`" is not allowed, except if the base is 0. Note that when invoked without base or with base set to 10, this behaves identical to the built-in function `long()` when passed a string.

`capitalize` (word)
  Capitalize the first character of the argument.

`capwords` (s)
  Split the argument into words using `split()`, capitalize each word using `capitalize()`, and join the capitalized words using `join()`. Note that this replaces runs of whitespace characters by a single space, and removes leading and trailing whitespace.

`expandtabs` (s, [tabsize])
  Expand tabs in a string, i.e. replace them by one or more spaces, depending on the current column and the given tab size. The column number is reset to zero after each newline occurring in the string. This doesn't understand other non-printing characters or escape sequences. The tab size defaults to 8.

`find` (s, sub[, start[,end]])
  Return the lowest index in s where the substring sub is found such that sub is wholly contained in `s[start:end]`. Return `-1` on failure. Defaults for start and end and interpretation of negative values is the same as for slices.

`rfind` (s, sub[, start[, end]])
  Like `find()` but find the highest index.

`index` (s, sub[, start[, end]])
  Like `find()` but raise `ValueError` when the substring is not found.

`rindex` (s, sub[, start[, end]])
  Like `rfind()` but raise `ValueError` when the substring is not found.

`count` (s, sub[, start[, end]])
  Return the number of (non-overlapping) occurrences of substring sub in string `s[start:end]`. Defaults for start and end and interpretation of negative values is the same as for slices.

`lower` (s)
  Return a copy of s, but with upper case letters converted to lower case.

`maketrans` (from, to)
  Return a translation table suitable for passing to `translate()` or `regex.compile()`, that will map each character in from into the character at the same position in to; from and to must have the same length.

  Warning: don't use strings derived from `lowercase` and `uppercase` as arguments; in some locales, these don't have the same length. For case conversions, always use `lower()` and `upper()`.

`split` (s[, sep[, maxsplit]])
  Return a list of the words of the string s. If the optional second argument sep is absent or `None`, the words are separated by arbitrary strings of whitespace characters (space, tab, newline, return, formfeed). If the second argument sep is present and not `None`, it specifies a string to be used as the word separator. The returned list will then have one more items than the number of non-overlapping occurrences of the separator in the string. The optional third argument maxsplit defaults to 0. If it is nonzero, at most maxsplit number of splits occur, and the remainder of the string is returned as the final element of the list (thus, the list will have at most `maxsplit+1` elements).

`splitfields` (s[, sep[, maxsplit]])
  This function behaves identically to `split()`. (In the past, `split()` was only used with one argument, while `splitfields()` was only used with two arguments.)

`join` (words[, sep])
  Concatenate a list or tuple of words with intervening occurrences of sep. The default value for sep is a single space character. It is always true that "`string.join(string.split(s, sep), sep)`" equals s.

`joinfields` (words[, sep])
  This function behaves identical to `join()`. (In the past, `join()` was only used with one argument, while `joinfields()` was only used with two arguments.)

`lstrip` (s)
  Return a copy of s but without leading whitespace characters.

`rstrip` (s)
  Return a copy of s but without trailing whitespace characters.

`strip` (s)
  Return a copy of s without leading or trailing whitespace.

`swapcase` (s)
  Return a copy of s, but with lower case letters converted to upper case and vice versa.

`translate` (s, table[, deletechars])
  Delete all characters from s that are in deletechars (if present), and then translate the characters using table, which must be a 256-character string giving the translation for each character value, indexed by its ordinal.

`upper` (s)
  Return a copy of s, but with lower case letters converted to upper case.

`ljust` (s, width)
`rjust` (in string) (s, width)
`center` (in string) (s, width)
  These functions respectively left-justify, right-justify and center a string in a field of given width. They return a string that is at least width characters wide, created by padding the string s with spaces until the given width on the right, left or both sides. The string is never truncated.

`zfill` (s, width)
  Pad a numeric string on the left with zero digits until the given width is reached. Strings starting with a sign are handled correctly.

`replace` (str, old, new[, maxsplit])
  Return a copy of string str with all occurrences of substring old replaced by new. If the optional argument maxsplit is given, the first maxsplit occurrences are replaced.

This module is implemented in Python. Much of its functionality has been reimplemented in the built-in module `strop`. However, you should never import the latter module directly. When `string` discovers that `strop` exists, it transparently replaces parts of itself with the implementation from `strop`. After initialization, there is no overhead in using `string` instead of `strop`.