different treatment of untyped defs, typed defs, and lambdas

[hashbrowncipher]

Edit: see first comment

Test case:

  1 from typing import Callable
  2 from typing import Union
  3
  4
  5 def works(arg: Union[int, str]) -> Callable[[], int]:
  6     if isinstance(arg, str):
  7         def ret():
  8             return len(arg)
  9         return ret
 10     elif isinstance(arg, int):
 11         def ret():
 12             return arg + 8
 13         return ret
 14
 15
 16 def fails1(arg: Union[int, str]) -> Callable[[], int]:
 17     if isinstance(arg, str):
 18         def ret() -> int:
 19             return len(arg)
 20         return ret
 21     elif isinstance(arg, int):
 22         def ret() -> int:
 23             return arg + 8
 24         return ret
 25
 26
 27 def fails2(arg: Union[int, str]) -> Callable[[], int]:
 28     if isinstance(arg, str):
 29         return lambda: len(arg)
 30     elif isinstance(arg, int):
 31         return lambda: arg + 8

Actual output:

$ mypy --version
mypy 0.640+dev.e12be3ba9e6be3e9242e3a81ffaa9b3a136876fe
$ python3 --version
Python 3.7.0
$ mypy lambdas_n_defs.py
lambdas_n_defs.py:19: error: Argument 1 to "len" has incompatible type "Union[int, str]"; expected "Sized"
lambdas_n_defs.py:23: error: Unsupported operand types for + ("str" and "int")
lambdas_n_defs.py:23: note: Left operand is of type "Union[int, str]"
lambdas_n_defs.py:29: error: Argument 1 to "len" has incompatible type "Union[int, str]"; expected "Sized"
lambdas_n_defs.py:31: error: Unsupported operand types for + ("str" and "int")
lambdas_n_defs.py:31: note: Left operand is of type "Union[int, str]"

Expected output: none, because I believe mypy should treat fails1() and fails2() identically to works()

[hashbrowncipher]

I've realized that the only reason that works() works is because I wasn't using --check-untyped-defs, so here's a second test case that gets to what I actually care about:

$ python3 --version
Python 3.7.0
$ mypy --version
mypy 0.640+dev.e12be3ba9e6be3e9242e3a81ffaa9b3a136876fe
$ nl -ba lambdas_n_defs.py
     1	from typing import Callable
     2	from typing import Union
     3
     4
     5	def works(arg: Union[int, str]) -> Callable[[], int]:
     6	    if isinstance(arg, str):
     7	        answer = len(arg)
     8
     9	        def ret() -> int:
    10	            return answer
    11	        return ret
    12	    elif isinstance(arg, int):
    13	        answer = arg + 8
    14
    15	        def ret() -> int:
    16	            return answer
    17	        return ret
    18
    19
    20	def fails(arg: Union[int, str]) -> Callable[[], int]:
    21	    if isinstance(arg, str):
    22	        def ret() -> int:
    23	            return len(arg)
    24	        return ret
    25	    elif isinstance(arg, int):
    26	        def ret() -> int:
    27	            return arg + 8
    28	        return ret
$ mypy --strict lambdas_n_defs.py
lambdas_n_defs.py:23: error: Argument 1 to "len" has incompatible type "Union[int, str]"; expected "Sized"
lambdas_n_defs.py:27: error: Unsupported operand types for + ("str" and "int")
lambdas_n_defs.py:27: note: Left operand is of type "Union[int, str]"

[ilevkivskyi]

Thanks for reporting!

This is a duplicate of #2608 (types don't propagate to nested functions, including lambdas), see also #4973.