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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.
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