The Basics

In short, the new string object’s format method in 2.6 and 3.0 (and later) uses the subject string as a template and takes any number of arguments that represent values to be substituted according to the template. Within the subject string, curly braces designate substitution targets and arguments to be inserted either by position (e.g., {1}) or keyword (e.g., {food}). As we’ll learn when we study argument passing in depth in Chapter 18, arguments to functions and methods may be passed by position or keyword name, and Python’s ability to collect arbitrarily many positional and keyword arguments allows for such general method call patterns. In Python 2.6 and 3.0, for example:

>>> template = '{0}, {1} and {2}'                             # By position
>>> template.format('spam', 'ham', 'eggs')
'spam, ham and eggs'

>>> template = '{motto}, {pork} and {food}'                   # By keyword
>>> template.format(motto='spam', pork='ham', food='eggs')
'spam, ham and eggs'

>>> template = '{motto}, {0} and {food}'                      # By both
>>> template.format('ham', motto='spam', food='eggs')
'spam, ham and eggs'

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Naturally, the string can also be a literal that creates a temporary string, and arbitrary object types can be substituted:

>>> '{motto}, {0} and {food}'.format(42, motto=3.14, food=[1, 2])
'3.14, 42 and [1, 2]'

Just as with the % expression and other string methods, format creates and returns a new string object, which can be printed immediately or saved for further work (recall that strings are immutable, so format really must make a new object). String formatting is not just for display:

>>> X = '{motto}, {0} and {food}'.format(42, motto=3.14, food=[1, 2])
>>> X
'3.14, 42 and [1, 2]'

>>> X.split(' and ')
['3.14, 42', '[1, 2]']

>>> Y = X.replace('and', 'but under no circumstances')
>>> Y
'3.14, 42 but under no circumstances [1, 2]'