• Choose a category
• All
• Classic-Fiction

We’ll study dictionaries in more depth in Chapter 8. See also Chapter 5 for examples that convert to hexadecimal and octal number strings with the %x and %o formatting target codes.

String Formatting Method Calls

As mentioned earlier, Python 2.6 and 3.0 introduced a new way to format strings that is seen by some as a bit more Python-specific. Unlike formatting expressions, formatting method calls are not closely based upon the C language’s “printf” model, and they are more verbose and explicit in intent. On the other hand, the new technique still relies on some “printf” concepts, such as type codes and formatting specifications. Moreover, it largely overlaps with (and sometimes requires a bit more code than) formatting expressions, and it can be just as complex in advanced roles. Because of this, there is no best-use recommendation between expressions and method calls today, so most programmers would be well served by a cursory understanding of both schemes.

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'

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]'

Adding Keys, Attributes, and Offsets

Like % formatting expressions, format calls can become more complex to support more advanced usage. For instance, format strings can name object attributes and dictionary keys—as in normal Python syntax, square brackets name dictionary keys and dots denote object attributes of an item referenced by position or keyword. The first of the following examples indexes a dictionary on the key “spam” and then fetches the attribute “platform” from the already imported sys module object. The second does the same, but names the objects by keyword instead of position:

>>> import sys

>>> 'My {1[spam]} runs {0.platform}'.format(sys, {'spam': 'laptop'})

'My laptop runs win32'

>>> 'My {config[spam]} runs {sys.platform}'.format(sys=sys,

config={'spam': 'laptop'})

'My laptop runs win32'

Square brackets in format strings can name list (and other sequence) offsets to perform indexing, too, but only single positive offsets work syntactically within format strings, so this feature is not as general as you might think. As with % expressions, to name negative offsets or slices, or to use arbitrary expression results in general, you must run expressions outside the format string itself (note