mypy bug with try/except conditional imports

try:
    import simplejson
except ImportError:
    import json as simplejson

resulst in the error error: Name 'simplejson' already defined

Pretty sure this is a dup, but I can't find the issue.

Related to #649.

Note that #649 was closed; a buch of "leftover" bugs were opened in its
place, but I don't see one that matches this pattern, so I think it's a new
case.

On Mon, Jan 25, 2016 at 3:19 PM, Ryan Gonzalez notifications@github.com
wrote:

Related to #649 #649.

—
Reply to this email directly or view it on GitHub
#1153 (comment).

--Guido van Rossum (python.org/~guido)

Closed by mistake.

@RitwikGupta

Another example from #2251 (reported by @RitwikGupta):

try:
    # Python 3
    from urllib.request import urlopen
except ImportError:
    # Python 2
    from urllib2 import urlopen

And another from #2253:

This python code generates an error. It's a common idiom and it would be good if mypy could check for it in some way.
try:
    import cPickle as pickle
except ImportError:
    import pickle
$ mypy --py2 --silent-imports bad.py
bad.py:4: error: Name 'pickle' already defined

The current workaround is to add a # type: ignore comment. For example:

try:
    import cPickle as pickle
except ImportError:
    import pickle  # type: ignore     # <<-- add this

FYI, this doesn't appear to work with sub-modules:

# This will error
try:
    import foo.bar
except ImportError:
    foo = None  # type:ignore

This seems to work though:

# This will pass
try:
    import doesnt.exist  # type:ignore
except ImportError:
    doesnt = None

# So will this
try:
    import collections.abc  # type:ignore
except ImportError:
    collections = None

assert doesnt is None
assert collections is not None

This is particularly difficult when the import that's happening is a typing import.

For example, this works:

$ mypy --version
mypy 0.530

$ cat no_try.py
from typing import Dict, Any

JSON = Dict[str, Any]

def accept(obj: JSON) -> Any:
    return obj.pop('hello')

$ mypy --ignore-missing-imports no_try.py

$ cat with_try.py
try:
    from typing import Dict, Any
except:
    from backports.typing import Dict, Any  # type: ignore

JSON = Dict[str, Any]

def accept(obj: JSON) -> Any:
    return obj.pop('hello')

$ mypy --ignore-missing-imports with_try.py
with_try.py:10: error: Invalid type "with_try.JSON"
with_try.py:11: error: JSON? has no attribute "pop"

This program should typecheck.

Add basic support for variable mappings (#1124)

A pattern that might be nicer for some?

if TYPE_CHECKING or sys.version_info < (3, 8, 0):
    from typing_extensions import Literal
else:
    from typing import Literal

Fix mypy failures

ignore type checking on importlib metadata try/except; see python/myp…

Workaround python/mypy#1153

Building on an answer from @cjerdonek, would this be an acceptable workaround instead of # type: ignore?:

try:
    from py3_pkg import module as _module
except:
    from py2_pkg import module
else:
    module = _module

Mypy doesn't handle importing from submodules well

Is this going to be fixed anytime soon?

No, it's not, and it's not clear that it should.

A majority of cases I've seen here are to work around Python stdlib changes, especially Python 2 to Python 3 stuff. sys.version_info checks are the preferred way to do this. Also Python 2 is dead.

if sys.version_info >= (3, 8):
    import importlib.metadata as importlib_metadata
else:
    import importlib_metadata

For the remaining cases, it's often not clear that it's sound, especially given nominal typing. Even if the API is structurally the same, nominal isinstance checks could result in surprising behaviour from mypy.

The only thing we should do in this space is #5018 where you can explicitly define exactly the structure you want with a Protocol, and mypy will confirm.

For all other cases, if you want to lie to the typechecker, please, go ahead and lie to the type checker (or type ignore):

if TYPE_CHECKING:
    import json
else:
    try:
        import simplejson as json
    except ImportError:
        import json

chore: Remove comments about python/mypy#1153 (orjson != json, but it…

fight mypy[1]

FYI "type ignore" doesn't work well because in some cases it will cause Unused "type: ignore" comment errors elsewhere (e.g. GitHub Actions).

@lilydjwg

@lilydjwg Issue #8823 was fixed last week and covers what you just described - unused-ignore errors in unreachable code.

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