New submission from Ben Wolfson <[email protected]>:
As near as I can make out from
<http://docs.python.org/library/string.html#formatstrings>, the following
should return the string "hi":
"{0[!]}".format({"!":"hi"})
We have a "{", followed by a field name, followed by a "}", the field name
consisting of an arg_name, which is 0, a "[", an element index, and a "]". The
element index, which the docs say may be any source character except "]", is
here "!". And, according to the docs, "An expression of the form '.name'
selects the named attribute using getattr(), while an expression of the form
'[index]' does an index lookup using __getitem__()".
However, it doesn't work:
>>> "{0[!]}".format({"!":"hi"})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: Missing ']' in format string
The same thing happens with other strings that are significant in other places
in the string-formatting DSL:
>>> "{0[:]}".format({":":"hi"})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: Missing ']' in format string
If there are more characters the error message changes:
>>> class spam:
... def __getitem__(self, k): return "hi"
...
>>> "{0[this works as expected]}".format(spam())
'hi'
>>> "{0[I love spam! it is very tasty.]}".format(spam())
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: expected ':' after format specifier
>>> "{0[.]}".format(spam()) # periods are ok
'hi'
>>> "{0[although curly braces, }, are not square brackets, they also don't work
>>> here]}".format(spam())
Right square brackets work fine, though:
>>> "{0[[]}".format(spam())
'hi'
The failure of the expected result with curly braces presumably indicates at
least part of the cause of the other failures: namely, that they stem from
supporting providing flags to one replacement field using another, as in
"{1:<{0}}". Which is quite useful. But it obviously isn't universally supported
in the case of index fields anyway:
>>> "{0[recursive {1[spam]}]}".format(spam(), spam())
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: Only '.' or '[' may follow ']' in format field specifier
(Note that this is a very strange error message itself, asis the following, but
since one isn't, according to the grammar, allowed to include a "]" where I've
got one *anyway*, perhaps that's to be expected:
>>> "{0[recursive {1[spam].lower} ]}".format(spam(), spam())
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'lower} ]'
)
But, even if that would explain why one can't use a "{" in the index field, it
wouldn't explain why one can't use a "!" or ":", since if those aren't already
part of a replacement field, as indicated by some initial "{", they couldn't
have the significance that they do when they *are* part of that field.
----------
components: Interpreter Core
messages: 135258
nosy: Ben.Wolfson
priority: normal
severity: normal
status: open
title: str.format parses replacement field incorrectly
versions: Python 2.6, Python 3.1
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Python tracker <[email protected]>
<http://bugs.python.org/issue12014>
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