8000 Document how (not) to use basestring · Issue #1537 · python/mypy · GitHub
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JukkaL opened this issue May 16, 2016 · 3 comments
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Document how (not) to use basestring #1537

JukkaL opened this issue May 16, 2016 · 3 comments
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@JukkaL
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JukkaL commented May 16, 2016

Users have started using basestring in stubs and elsewhere. Using basestring as a function argument type in a stub seems relatively benign (though inconsistent), but using as a return type is problematic for a few reasons:

  • basestring currently has no operations and can't be passed to functions expecting unicode or even Union[bytes, unicode].
  • Using isinstance() doesn't work well, since it's not a union. It's often necessary to use something like if isinstance(x, bytes): ... elif isinstance(x, unicode) instead of if isinstance(x, bytes): ... else: ....

Not sure what we should tell users. Perhaps say that using basestring as an argument type is not recommended (at least for now), and Union[bytes, unicode] is the recommended, hopefully future-proof alternative in Python 2 -- even though it's currently equivalent to just unicode.

Also say the basestring should not be used as a return value or attribute type, but Union[bytes, unicode] should also be avoided there. In these contexts str, bytes, unicode and AnyStr are better, whenever possible.

See also python/typeshed#202.
@gvanrossum
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See also python/typeshed#138

@JukkaL JukkaL mentioned this issue May 16, 2016
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@ddfisher ddfisher added this to the 0.5 milestone May 19, 2016
@lucaswiman
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lucaswiman commented Jan 28, 2017 •
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This comes up in attempting to make python-3 compatible code with six, since six.string_types is aliased to basestring (both in six and in the stub). Here's a simplified example from a codebase I'm attempting use mypy on:

import six
from typing import Any

def c(a, b):  # type: (Any, Any) -> int
    if isinstance(a, six.string_types):
        return getattr(b, a)
    else:
        return b[a]

This fails with:

$ mypy --py2 getattr_six.py 
getattr_six.py: note: In function "c":
getattr_six.py:6: error: Argument 2 to "getattr" has incompatible type "basestring"; expected "unicode"

@JukkaL commented:

Having basestring aliased to Union[bytes, unicode] might work better (I think somebody suggested it), though the resulting error messages could be confusing, as they wouldn't refer to basestring.

I think updating the type signature in the stub type of six.text_type to (Union[bytes, unicode], ) wouldn't be more confusing, since six.text_type already doesn't mention basestring.

@lucaswiman lucaswiman mentioned this issue Jan 28, 2017
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@gvanrossum gvanrossum removed this from the 0.5 milestone Mar 29, 2017
@emmatyping emmatyping mentioned this issue Dec 2, 2017
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@hauntsaninja
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What is Python 2?

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