Typing: Is there a "cast operator"?
Paulo da Silva
p_d_a_s_i_l_v_a_ns at nonetnoaddress.pt
Sun Oct 23 21:45:45 EDT 2022
Às 23:56 de 23/10/22, Cameron Simpson escreveu:
> On 23Oct2022 21:36, Paulo da Silva
> <p_d_a_s_i_l_v_a_ns at nonetnoaddress.pt> wrote:
>> I am in the process of "typing" of some of my scripts.
>> Using it should help a lot to avoid some errors.
>> But this is new for me and I'm facing some problems.
>>
>> Let's I have the following code (please don't look at the program
>> content):
>>
>> f=None # mypy naturally assumes Optional(int) because later, at open,
>> it is assigned an int.
>> ..
>> if f is None:
>> f=os.open(...
>> ..
>> if f is not None:
>> os.write(f, ...)
>> ..
>> if f is not None:
>> os.close(f)
>>
>> When I use mypy, it claims
>> Argument 1 to "write" has incompatible type "Optional[int]"; expected
>> "int"
>> Argument 1 to "close" has incompatible type "Optional[int]"; expected
>> "int"
>>
>> How to solve this?
>> Is there a way to specify that when calling os.open f is an int only?
>>
>> I use None a lot for specify uninitialized vars.
>
> Maybe you shouldn't. The other way is to just not initialise the var at
> all. You could then just specify a type. Example:
>
> Python 3.8.13 (default, Aug 11 2022, 15:46:53)
> [Clang 12.0.0 (clang-1200.0.32.29)] on darwin
> Type "help", "copyright", "credits" or "license" for more
> information.
> >>> f:int
> >>> f
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> NameError: name 'f' is not defined
> >>>
>
> So now `f` has `int` type definitely (for `mypy`'s purposes), and if
> used before assignment raises a distinctive error (versus having the
> value `None`, which you might then pass around, and perhaps successfully
> use in some contexts).
>
> It is probably better on the whole to specify types up front rather than
> relying on `mypy` or similar to infer them. That way (a) you're stating
> your intent and (b) not relying on an inferred type, which if you've got
> bugs may be inferred _wrong_. If `mypy` infers a type incorrectly all
> the subsequent checks will also be flawed, perhaps subtly.
>
Yes.
I also use to make f unavailable (f=None) when something goes wrong and
I don't want to stop the script but of course I could use "del f". I
also need to care about using "try", which might be better than "if" tests.
A thing to think of ...
Thanks.
Paulo
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