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'

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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 the use of *parts here to unpack a tuple's items into individual arguments, as we did on Page 132 when studying fractions; more on this in Chapter 18):

>>> somelist = list('SPAM')
>>> somelist
['S', 'P', 'A', 'M']

>>> 'first={0[0]}, third={0[2]}'.format(somelist)
'first=S, third=A'

>>> 'first={0}, last={1}'.format(somelist[0], somelist[-1])   # [-1] fails in fmt
'first=S, last=M'

>>> parts = somelist[0], somelist[-1], somelist[1:3]          # [1:3] fails in fmt
>>> 'first={0}, last={1}, middle={2}'.format(*parts)
"first=S, last=M, middle=['P', 'A']"