What is the correct type hint for an empty list?

Question

What is the correct type hint for x = []?

The type checker in my PyCharm editor flagged this as an error:

labelframes: List[ttk.LabelFrame] = []

‘Optional’ is not an option as in:

labelframes: List[Optional[ttk.LabelFrame]] = []

because the documentation for typing.Optional states that this is equivalent to:

labelframes: List[Union[ttk.LabelFrame, None]] = []

and [None] is not [].

I ought to mention that PyCharm doesn’t like this either:

labelframes: List[Union[ttk.LabelFrame, None]] = [None]

Whatever type hint I try. PyCharm flags it as an error with,
“Expected to return my type hint here, got no return,” so I tried:

labelframes: Optional[List[ttk.LabelFrame, None]] = []

That didn’t work.

I am aware that PEP 526 has numerous examples which follow the pattern:

x: List[str] = []

Asked By: lemi57ssss

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Source

Answer 1

Looking at PEP 484, specifically the section on type comments (the precursor to variable annotations) this does indeed seem like a bug with PyCharm’s checker.

Quoting from the PEP:

In non-stub code, there is a similar special case:
from typing import IO

stream = None  # type: IO[str]
Type checkers should not complain about this (despite the value None not matching the given type), nor should they change the inferred type to Optional[...] (despite the rule that does this for annotated arguments with a default value of None ). The assumption here is that other code will ensure that the variable is given a value of the proper type, and all uses can assume that the variable has the given type.

So, in a similar fashion, type-checkers (as mypy currently does), should not complain about the fact that the initializing value you provide doesn’t strictly match the annotation.

Answered By: Dimitris Fasarakis Hilliard

Answer 2

Let’s first write sample code for an empty list and check it using mypy.

from typing_extensions import reveal_type

l = []
reveal_type(l)

Now let’s run it in the shell, as below.

$ mypy test.py

test.py:4: error: Need type annotation for "l" (hint: "l: List[<type>] = ...")
test.py:6: note: Revealed type is "builtins.list[Any]"
Found 1 error in 1 file (checked 1 source file)

Now pay attention to the second line as it says that the type is List[Any].

Lets make changes and re-run

from typing import Any, List
from typing_extensions import reveal_type

l : List[Any] = []

reveal_type(l)

And now run mypy.

$ mypy test.py

test.py:6: note: Revealed type is "builtins.list[Any]"
Success: no issues found in 1 source file

Answered By: Lawhatre

What is the correct type hint for an empty list?

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