Typing: Is there a "cast operator"?

[Thomas Passin]

On 10/23/2022 11:14 PM, Dan Stromberg wrote:
> On Sun, Oct 23, 2022 at 2:11 PM Paulo da Silva <
> p_d_a_s_i_l_v_a_ns at nonetnoaddress.pt> wrote:
> 
>> Hello!
>>
>> 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.
>>
> 
> I've found that mypy understands simple assert statements.
> 
> So if you:
> if f is not None:
>          assert f is not None
>          os.write(f, ...)
> 
> You might be in good shape.

I'm not very familiar with anything but the simplest typing cases as 
yet, but mypy is happy with these two fragments.

if f:
     os.write(int(f)) # f must be an int if it is not None, so we can 
cast it to int.

Or something like this (substitute write() for print() as needed) -

from typing import Optional, Any

def f1(x:int)->Optional[int]:
     if x == 42:
         return x
     return None

def zprint(arg:Any):
     if type(arg) == int:
         print(arg)

y0 = f1(0)  # None
y42 = f1(42) # 42

zprint(y0)  # Prints nothing
zprint(y42) # Prints 42

Another possibility that mypy is happy with (and probably the simplest) 
- just declare g:int = None instead of g = None:

g: int = None
def yprint(arg: int):
     if arg:
         yprint(arg)
     else:
         print('arg is None')

yprint(g)  # Prints "arg is None"


And **please** let's not go doing this kind of redundant and inelegant 
construction:

if f is not None:
     assert f is not None
     os.write(f, ...)