Typing: Do we need an explicit cast() function?

[gvanrossum]

In mypy there's a cast(TYPE, EXPR) function. Do we still need this, given that we can do the same thing with # type: comments? (However, see issue #9.)

[JukkaL]

Mypy treats # type: comments differently from casts. When using a comment, types still have to be compatible. For example, this is not valid:

def f(x: Sequence[int]) -> None:
    y = x # type: List[int]   # Error, since Sequence is not compatible with List

However, this is okay:

def f(x: Sequence[int]) -> None:
    y = cast(List[int], x)   # Fine

Effectively, type annotations (comments) are type safe (or as type safe as anything in mypy), but casts can do arbitrary things and work around the restrictions of the type system.

Also, cast can be used within expressions, whereas a type comment is only valid for assignment statements. This is often useful. Here is a toy example:

foo(cast(List[int], x))

If we used a type annotation as a cast, this would require an extra local variable:

tmp = x # type: List[int]
foo(tmp)

In more complex cases the refactoring could be non-trivial (it could break the order of evaluation in a boolean expression, for example).

[ambv]

@JukkaL, do you have non-toy examples where working around the type system with cast() is necessary? That would be good to put in the PEP.

[JukkaL]

It's actually a bit tricky to give a short, realistic example where a cast is clearly useful. Here is one inspired by a similar example in mypy:

def find_first_str(a: Sequence[object]) -> str:
    index = next((i for i, s in enumerate(a) if isinstance(s, str)), -1)
    if index >= 0:
        return cast(str, a[index])
    else:
        raise ValueError

Every example with a cast can be rewritten to not need a cast, but when adapting existing dynamically typed code, introducing a cast is often the simplest way of getting a piece of code to type check.

[gvanrossum]

It's good enough for me. :-)

I also like using cast() to initialize result variable with an empty container of the right type:

def word_count(input: Iterable[str]) -> Dict[str, int]:
    res = cast(Dict[str, int], {})
    for w in input:
        res[w] = res.get(w, 0) + 1
    return res

[JukkaL]

Yeah, cast() can be used with empty containers, but it has the drawback that you don't get an error from mypy if you give a bogus type:

a = cast(Dict[str, int], [])   # ouch, no error

If you use a # type: comment, you'll get an error:

a = []  # type: Dict[str, int]   # error, incompatible types in assignment

cast() just blindly accepts whatever you give it. This is different from Java, for example, where the compiler catches some bogus casts. Mypy could also do that, but only for built-in types (since they don't support arbitrary multiple inheritance).

[gvanrossum]

Maybe we need different kinds of casts, like in C++? This makes me a little
sad, so much complexity to satisfy the type checker in rare cases.

On Wednesday, January 7, 2015, Jukka Lehtosalo [email protected]
wrote:

Yeah, cast() can be used with empty containers, but it has the drawback
that you don't get an error from mypy if you give a bogus type:

a = cast(Dict[str, int], []) # ouch, no error

If you use a # type: comment, you'll get an error:

a = [] # type: Dict[str, int] # error, incompatible types in assignment

cast() just blindly accepts whatever you give it. This is different from
Java, for example, where the compiler catches some bogus casts. Mypy could
also do that, but only for built-in types (since they don't support
arbitrary multiple inheritance).

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[ambv]

How about:

cast(Dict[str, int], [])   # error
with typing.skip_type_checks:
    cast(Dict[str, int], [])   # okay

[gvanrossum]

Łukasz, I presume you're talking about errors from the static checker only. After Jukka's comments in #33 I think it's better to use comments to disable the type checker than with-statements.

Doing runtime checks in cast() is madness anyway (it took me a little while to realize this, but there are too many ways it can fail incorrectly, modify the value by even looking at it, or waste a lot of time).

[ambv]

Yes, those would be static checker errors only.

Fixed in e2e6fc4 (advertises usage of @no_type_checks for routines and "# type: ignore" for lines).

[ambv]

Oh, the core issue, whether we need cast() is still open. @JukkaL, @gvanrossum, now that we endorse "# type: " comments, maybe a specific comment would be better suited for this?

Jukka, you were worried about runtime performance of cast(), the comment would resolve that.

[JukkaL]

As mentioned earlier, a # type: comment is not a replacement for a cast, since # type: comments only act as hints and the type checker will still verify that the initializer type is compatible, whereas the type checker will assume that casts are valid and may allow code that does bad things at runtime. This is similar to Java variable type definitions vs. casts -- only the latter can be used to write 'unsafe' code.

So this would be rejected by the type checker:

x = <expression> # type: object
y = x # type: int  # error, since object is not compatible with int

But this would be okay:

x = <expression> # type: object
y = cast(int, x)  # okay (casts are usually used to go from a general to a narrower type)

Cast should be an expression, since casts are often used within subexpressions. # type: comments, on the other hand, are only used for declaring the types of variables, so they are always applied to an assignment statement, and thus there is no need to nest them within expressions.

[gvanrossum]

Decision: we'll add cast() to the PEP. Closing this issue; the TODO is added to the README.rst.

[refi64]

Yes: https://www.python.org/dev/peps/pep-0484/#casts

On Sat, Nov 30, 2019, 12:37 PM Omry Yadan [e-mail] wrote:

Did the cast operator made it to the PEP?
I have a use case where I think it will be handy but as far as I can tell
it's not supported in 3.7 or 3.8, and it's not clear from this task which
PEP this is about.

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