fix(python): Overloading date_range

[thomasaarholt]

EDIT: Now passes, see comments below.

This is my best shot at this.
Mypy still fails, with polars/internals/functions.py:208: error: Overloaded function signatures 3 and 4 overlap with incompatible return types [misc]

These are signatures 3 and 4:

# sig 3
@overload
def date_range(
    low: date | datetime | str,
    high: date | datetime | str,
    interval: str | timedelta,
    closed: ClosedWindow = "both",
    name: str | None = None,
    time_unit: TimeUnit | None = None,
    time_zone: str | None = None,
    lazy: Literal[False] = False,
) -> pli.Series:
    ...

# sig 4
@overload
def date_range(
    low: date | datetime | pli.Expr | str,
    high: date | datetime | pli.Expr | str,
    interval: str | timedelta,
    closed: ClosedWindow = "both",
    name: str | None = None,
    time_unit: TimeUnit | None = None,
    time_zone: str | None = None,
    lazy: Literal[True] = True,
) -> pli.Expr:
    ...

I think they quite explicitly don't overlap though:

# sig 3
@overload
def date_range(
    (...)
    lazy: Literal[False] = False,
) -> pli.Series:
    ...

# sig 4
@overload
def date_range(
    (...)
    lazy: Literal[True] = True,
) -> pli.Expr:
    ...

If I omit the default argument (= True or = False), then mypy passes, but we don't want to omit it.

This to me seems like a bug in mypy.

[thomasaarholt]

Fixed! I'm assuming that the test failures are unrelated to this PR.

I got one step further by realising that overloaded arguments shouldn't have default args, and then I read this comment on a relevant mypy issue, and based myself on their example.

Unfortunately, this does mean that we have to move the argument to before the default args if you want function overloading.

[ritchie46]

Thanks a lot. 🙏