Detect invalid string literal formatting

[asmeurer]

It would be nice if pyflakes could detect invalid formatting errors on string literals. Something like

'abc' % stuff
'%s' % (a, b)
'{}'.format(1, 2)
'{x} {y}'.format(x=1, y=2, z=3)

It would only operate on string literals, so there's no type inferencing or anything.

[Ian Cordasco (icordasc)]

So while the first two are certainly wrong, the last one can be difficult realistically.

>>> '{}'.format(1, 2)
'1'
>>> '{a} {b}'.format(a=1, b=2, c=3)
'1 2'

It doesn't balk at the fact that you provided more parameters than necessary. (Not say like '%s' % (1, 2).)

This can also be tricky because you could build up a dictionary and then do '{a} {b}'.format(**my_dict).

I definitely agree that the simpler cases (using %) should be somewhat easy to handle. I'm not sure about the latter two since str.format seems to allow more arguments than necessary.

[asmeurer (asmeurer)]

Ah, that was a mistake on my part. I didn't realize that new-style formatting allows too many arguments. It doesn't allow too few arguments, so that could still be checked.

Regarding format(**mydict), it shouldn't bother with it. It should only check the easy obvious case where it 100% definitely *will* raise an exception, i.e., string _literal_ with a % operator or .format method, and explicit arguments in either case. For % if the rhs is not a tuple literal we can only detect no format strings as we can't know if the rhs will be a tuple or not. Similarly, if .format() is used with * or ** we cannot do anything.

[Phil Frost (bitglue)]

Agreed, this would be a nice addition.