2.1.5.1 More String Operations

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. ^2.4 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. ^2.5 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. For example:

>>> 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 re.