python TypeGuard (PEP 647) (intermediate) anthony explains #439

today we go over narrowings and `TypeGuard` -- which enables you to write your own user-defined narrowings!

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[luck]

Wow, now I don't have to use C++ to ponder at template typing mismatch errors, now I can do it with Python!

[ChrisBNisbet]

Anthony: "What!?"
That's what I find myself saying way more often than I'd like.

[tannerbobanner]

I know this is an old video, but for those watching: instead of binding T to type, type[T] should be used as the parameter type.

[sadhlife]

i suppose you can still do `bound=type` for the TypeVar there, it's just like `bound=object` but accepts any class instead of any object.

[sadhlife]

consider this case for the negative case:

```
def is_zero(x: int | None) -> TypeGuard[int]:
return x == 0

def foo(x: int | None) -> None:
if is_zero(x):
print("got zero")
else:
reveal_type(x) # int | None
```

[RoyAAD]

@4:40 you say this is a variance issue. Do you have a video that explains more about it?

[marsma]

Oh. I just want to write comment that this is not your style to let unclear stuff exist and then i see this addition. You met my expectation :)

[aldrickdev]

when i try to import TypeGuard mypy tells me that its not a valid type. I tried importing it from the typing module and the typing_extensions module and I still get the same error.

[OrCarmi]

I see that "fumble" not as a failure of you Anthony, but as a testament to how clumsy and unintuitive python typing becomes at higher levels

[xaro]

Why did you used `object` instead of `typing.Any` ? is there any practical difference?

[alexeiguirik]

Anthony: "This is a disaster..." Sometimes I think that about type hints in Python in general.