re's Match does not support indexing: "Value of type "Optional[Match[str]]" is not indexable"

[blueyed]

The following is valid, but mypy complains about it:

import re

m = re.search(".", "foo")
print(m[0])

t-mypy.py:4: error: Value of type "Optional[Match[str]]" is not indexable

Seems to be similar to #2751 (#2753).

mypy 0.710+dev.5f08ccf029aa3046b15e1afc60743ba692e4758d.dirty (dirty due to using latest typshed master (53500c8)

[JelleZijlstra]

That's because of the Optional part. It's telling you that if there is no match, this code will break. Mypy will show no errors if you first check that m is not None.

[blueyed]

@JelleZijlstra
Thanks - figured this out with something else already (forgot to report back here).
assert m also works.

[dongweiming]

@JelleZijlstra if use Assignment expressions will throw an error:

❯ cat a.py 
import re

if (m := re.search(".", "foo")):
    print(m[0])

❯ mypy a.py 
a.py:4: error: Value of type "Optional[Match[str]]" is not indexable
        print(m[0])
              ^
Found 1 error in 1 file (checked 1 source file)

[srittau]

This is a mypy problem:

from typing import Optional

def foo() -> Optional[str]:
    pass

x = foo()
if x:
    x.lower()  # no error
    
if (y := foo()):
    y.lower()  # error: Item "None" of "Optional[str]" has no attribute "lower"

(mypy 0.761 with Python 3.8.0)

[JelleZijlstra]

Yes, python/mypy#7316.