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1. JukkaL

   added

   feature

   on Oct 12, 2014
2. JukkaL commented on Oct 12, 2014

   

   JukkaL

   on Oct 12, 2014

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   This already works if the condition only has an isinstance check:

   def f(x: A) -> None:
       if isinstance(x, B):
           print(x.value)  # Ok
   
   class A:
       pass
   class B(A):
       value = 0

   Mypy should support more cases, including and expressions (your example should be valid).

   Here is another case that could be useful (this could be a separate issue):

   def f(x: object) -> None:
       if isinstance(x, int) or isinstance(x, str):   # or isinstance(x, (int, str))
           # type of x should be Union[int, str]
           pass

   These improvements are important for proper type checking of None values (currently None is treated pretty much like null in Java, i.e. it is implicitly included in every type). For example, consider this code:

   def f(x: Union[int, None]) -> None:
       if x and x > 3:   # Ok
           print(x * 2)   # Ok
       print(x * 2)   # Error, since can't multiply None

   Currently there is no error reported for the the second print statement.
3. JukkaL commented on Oct 12, 2014

   

   JukkaL

   on Oct 12, 2014

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   Added related issues #475 and #476.
4. refi64 commented on Oct 12, 2014

   

   refi64

   on Oct 12, 2014

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   Just to put it out there, the Cobra approach (see Compile-time Nil Tracking) to null values seems nicer, in my opinion, than using Union[T, None]. Basically, it would mean something like this:

   def x(a: Nullable[int]) -> int:
       print(a*a) # ok; a is still an int
       return a # error; cannot convert Nullable type to none-Nullable type
       if a: return a # ok; a cannot be None
5. gvanrossum commented on Oct 12, 2014

   

   gvanrossum

   on Oct 12, 2014

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   My preferred spelling would be Optional[int]. (Nullable reeks of SQL. :-)

   On Sun, Oct 12, 2014 at 1:57 PM, Ryan Gonzalez notifications@github.com
   wrote:

   Just to put it out there, the Cobra
   http://cobra-language.com/docs/quality/ approach (see Compile-time Nil
   Tracking) to null values seems nicer, in my opinion, than using Union[T,
   None]. Basically, it would mean something like this:

   def x(a: Nullable[int]) -> int:
   print(a*a) # ok; a is still an int
   return a # error; cannot convert Nullable type to none-Nullable type
   if a: return a # ok; a cannot be None

   —
   Reply to this email directly or view it on GitHub
   #473 (comment).

   --Guido van Rossum (python.org/~guido)
6. refi64 commented on Oct 12, 2014

   

   refi64

   on Oct 12, 2014

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   @gvanrossum You're very right; I like Optional better.
7. JukkaL commented on Oct 12, 2014

   

   JukkaL

   on Oct 12, 2014

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   @kirbyfan64 I agree, special syntax would be preferable. But I'm not sure why a*a should be okay if a has an optional type. None * None raises an exception.
8. JukkaL commented on Oct 12, 2014

   

   JukkaL

   on Oct 12, 2014

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   Added issue #477 for Optional[T].
9. JukkaL

   changed the title [-]Type of variable after isinstance check[/-] [+]Type of variable after isinstance check with 'and'[/+] on Oct 13, 2014
10. rwbarton commented on Jun 15, 2016

    

    rwbarton

    on Jun 15, 2016

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    The original program here has worked since commit 0b151c6. Also, Jukka's "another case" will be addressed by #1712, and the example with Union[int, None] already works under strict optional checking. So closing this as fixed.
11. rwbarton

    closed this as completedon Jun 15, 2016