从格式化字符串内部切割字符串会产生'TypeError：字符串索引必须是整数'

Question

Shouldn't both these commands do the same thing?

>>> "{0[0:5]}".format("lorem ipsum")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: string indices must be integers

>>> "{0}".format("lorem ipsum"[0:5])
'lorem'

The commands

>>> "{0[0]}".format("lorem ipsum")
'l'

and

>>> "{0}".format("lorem ipsum"[0])
'l'

evaluate the same. (I know that I can use other methods to do this, I am mainly just curious as to why it dosen't work)

Answer 1

The str.format syntax is handled by the library and supports only a few “expression” syntaxes that are not the same as regular Python syntax. For example,

"{0[foo]}".format(dict(foo=2))  # "2"

works without quotes around the dictionary key. Of course, there are limitations from this simplicity, like not being able to refer to a key with a ] in it, or interpreting a slice, as in your example.

Note that the f-strings mentioned by kendall are handled by the compiler and (fittingly) use (almost) unrestricted expression syntax . They need that power since they lack the obvious alternative of placing those expressions in the argument list to format .