8000 isinstance does not impact the type detected · Issue #1702 · python/mypy · GitHub
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ttsiodras opened this issue Jun 12, 2016 · 2 comments
Closed

isinstance does not impact the type detected #1702

ttsiodras opened this issue Jun 12, 2016 · 2 comments

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@ttsiodras
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I believe the example below depicts a bug in mypy's handling of isinstance. According to the documentation, isinstance informs the type of the variable inside the if block - but the following example shows that this is not the case:

$ pip freeze | grep mypy
mypy-lang==0.4.2

$ cat -n bug.py
 1  class A:
 2      pass
 3
 4  class B(A):
 5      def __init__(self):
 6          self._members = [1]
 7
 8  class C(A):
 9      def __init__(self):
10          self._members = [2]
11
12  class D(A):
13      pass
14
15  def foo(a: A) -> bool:
16      if isinstance(a, B) or isinstance(a, C):
17          for m in a._members:
18              print(m)
19
20  foo(B())
21  foo(C())

$ mypy bug.py
bug.py: note: In function "foo":
bug.py:17: error: "A" has no attribute "_members"

In line 17, the a variable is of type B or C - so it does have access to _members.
@ttsiodras
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Further testing (thanks, Maxime!) shows that replacing...

if isinstance(a, B) or isinstance(a, C)

with

if isinstance(a, (B, C))

works properly. But the other form should work too, I think.

@JukkaL
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JukkaL commented Jun 13, 2016

Yes, the first example should be okay. There is an existing issue for or + isinstance: #942, so I'm closing this.

@JukkaL JukkaL closed this as completed Jun 13, 2016
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@ttsiodras @JukkaL
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