String objects have one unique built-in operation: the %
operator (modulo) with a string left argument interprets this string
as a C sprintf format string to be applied to the right argument, and
returns the string resulting from this formatting operation.
The right argument should be a tuple with one item for each argument
required by the format string; if the string requires a single
argument, the right argument may also be a single non-tuple object.
The following format characters are understood:
%, c, s, i, d, u, o, x, X, e, E, f, g, G.
Width and precision may be a * to specify that an integer argument
specifies the actual width or precision. The flag characters -, +,
blank, # and 0 are understood. The size specifiers h, l or L may be
present but are ignored. The %s conversion takes any Python
object and converts it to a string using str() before
formatting it. The ANSI features %p and %n
are not supported. Since Python strings have an explicit length,
%s conversions don't 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.
All other errors raise exceptions.
If the right argument is a dictionary (or any kind of mapping), then
the formats in the string must have a parenthesized key into that
dictionary inserted immediately after the % character, and
each format formats the corresponding entry from the mapping. E.g.
>>> count = 2
>>> language = 'Python'
>>> print '%(language)s has %(count)03d quote types.' % vars()
Python has 002 quote types.
>>>
In this case no * specifiers may occur in a format (since they
require a sequential parameter list).
Additional string operations are defined in standard module
string and in built-in module regex.