'<nothing>' object is not iterable

[DevilXD]

Bug Report

The internal <nothing> object not being an iterable appears to be a problem in a case like the example below.

To Reproduce

from typing import List, Dict, Tuple, NamedTuple, Callable, TypeVar


MenuReturn = TypeVar("MenuReturn")
MenuFunction = Callable[[], MenuReturn]


class MenuOption(NamedTuple):
    description: str
    function: MenuFunction


def menu(options: List[MenuOption]) -> MenuReturn:
    options_dict: Dict[str, MenuFunction] = {}
    for num, entry in enumerate(options, start=1):
        print(f"{num}) {entry.description}")
        options_dict[str(num)] = entry.function
    while True:
        key = input("Pick an option: ")
        if key in options_dict:
            break
    return options_dict[key]()


def func1() -> Tuple[bool, bool]:
    return (False, False)


def func2() -> Tuple[bool, bool]:
    return (True, True)


results: Tuple[bool, bool] = menu([  # need type annotation for 'results' - fine, I can just add it here
    MenuOption("Return two False values", func1),
    MenuOption("Return two True values", func2),
])
print(results)

# Define the types explicitly
result1: bool
result2: bool
result1, result2 = menu([  # '<nothing>' object is not iterable - types being defined doesn't matter
    MenuOption("Return two False values", func1),
    MenuOption("Return two True values", func2),
])
print(result1, result2)  # the types are defined, but the error above remains

Expected Behavior

It'd let me at least define the types of result1 and result2 before the menu() usage, silencing the '<nothing>' object is not iterable error.

Actual Behavior

This is not the case, and while result1 and result2 do end up with the correct types, I still need to add # type: ignore on the menu() usage line.

Your Environment

• Mypy version used: 0.812
• Mypy flags: none used
• Python version used: 3.8.8
• Operating system and version: Win7x64

Notes

It seems that MyPy ends up with the MenuReturn type var being assigned to a list of whatever each function returns, those being Tuple[bool, bool] from func1 and Tuple[bool, bool] from func2. Normally, I'd expect it to go with a union of those two types, that'd collapse to just Tuple[bool, bool] - if the types would be different, for example str and int, it'd end up as Union[str, int].

[LyonsDo]

Not sure if this is the same issue but I'm getting "TypeError: 'str' object is not callable" for the string s in
@docstrings.dedent(s, stacklevel=3)

Your Environment

PyCharm 2020.3.4
Python version used: 3.9.2
Operating system and version: Win10x64

[JelleZijlstra]

@LyonsDo pycharm errors shouldn't be reported to mypy; it's a different type checker.

@DevilXD I think your problem is that NamedTuples can't be generic (#685). If you turn on the --disallow-any-generics flag (as you probably should), it will tell you about a few missing instances of [T]; some of those can be fixed but the NamedTuple cannot.

[LyonsDo]

Sorry. I have the mypy plugin enabled.

[DevilXD]

@JelleZijlstra I believe there's a different issue at play here, because getting rid of the NamedTuple and using just a normal tuple instead, generates the same error.

from typing import List, Dict, Tuple, Callable, TypeVar


MenuReturn = TypeVar("MenuReturn")
MenuFunction = Callable[[], MenuReturn]


def menu(options: List[Tuple[str, MenuFunction]]) -> MenuReturn:
    options_dict: Dict[str, MenuFunction] = {}
    for num, entry in enumerate(options, start=1):
        print(f"{num}) {entry[0]}")
        options_dict[str(num)] = entry[1]
    while True:
        key = input("Pick an option: ")
        if key in options_dict:
            break
    return options_dict[key]()


def func1() -> Tuple[bool, bool]:
    return (False, False)


def func2() -> Tuple[bool, bool]:
    return (True, True)


results: Tuple[bool, bool] = menu([  # need type annotation for 'results' - fine, I can just add it here
    ("Return two False values", func1),
    ("Return two True values", func2),
])
print(results)

# Define the types explicitly
result1: bool
result2: bool
result1, result2 = menu([  # '<nothing>' object is not iterable - types being defined doesn't matter
    ("Return two False values", func1),
    ("Return two True values", func2),
])
print(result1, result2)  # the types are defined, but the error above remains

[JelleZijlstra]

You're not setting generic parameters fully. Try using --disallow-any-generics.

[LyonsDo]

OK thanks. I'll look at that later.

It seems to be working OK now thanks..

[DevilXD]

Huh, alright, I got it to work. This is what I've got:

from typing import List, Dict, Tuple, Callable, TypeVar


MenuReturn = TypeVar("MenuReturn")
MenuFunction = Callable[[], MenuReturn]


def menu(options: List[Tuple[str, MenuFunction[MenuReturn]]]) -> MenuReturn:
    options_dict: Dict[str, MenuFunction[MenuReturn]] = {}
    for num, entry in enumerate(options, start=1):
        print(f"{num}) {entry[0]}")
        options_dict[str(num)] = entry[1]
    while True:
        key = input("Pick an option: ")
        if key in options_dict:
            break
    return options_dict[key]()


def func1() -> Tuple[bool, bool]:
    return (False, False)


def func2() -> Tuple[bool, bool]:
    return (True, True)


results = menu([
    ("Return two False values", func1),
    ("Return two True values", func2),
])
reveal_type(results)

result1, result2 = menu([
    ("Return two False values", func1),
    ("Return two True values", func2),
])
reveal_type(result1)
reveal_type(result2)

I'm guessing this isn't a real bug then. However, the error message of <nothing> object is not iterable is quite confusing. Perhaps this could be improved?

[JelleZijlstra]

Agree that this is unintuitive! #10241 seems like the underlying issue, so let's keep that one open.