Warn on unnecessary use of `global`

[Zac-HD]

CONSTANT = 1

def fn(x):
    global CONSTANT  # <- unnecessary, unidiomatic, and a (small) performance hit
    return x + CONSTANT

The global statement is only required when assigning to a global variable; you can reference outer namespaces without it. I think it'd be nice to have a lint rule that pointed this out, to help newer Pythonistas understand how scoping works.

[JelleZijlstra]

I have seen this too and agree it would be good for linters to warn against this. Is it in scope for flake8-bugbear, though? The README says this linter is there to find likely bugs or design problems in your code, and unnecessary global statements aren't bugs, they just do nothing.

a (small) performance hit

There appears to be no runtime hit; your function compiles to the same bytecode with and without the global statement. I guess it makes the initial compilation slightly slower.

[Zac-HD]

Hmm, fair enough. Maybe a new W rule in pycodestyle?