Typing: Is there a "cast operator"?

[Paulo da Silva]

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