8000 No warning for uninitialised instance variable when using instance variable annotation · Issue #4206 · python/mypy · GitHub
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ned opened this issue Nov 4, 2017 · 1 comment
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No warning for uninitialised instance variable when using instance variable annotation #4206

ned opened this issue Nov 4, 2017 · 1 comment

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@ned
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ned commented Nov 4, 2017 •
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If you intend to initialise an instance variable in __init__ but forget to do so, mypy won't warn if PEP 526 style instance variable annotation is used. For example, the following code emits a runtime warning but no warning from mypy. Tested on mypy version 0.540.

class MyClass:
    my_attr: str
    def __init__(self, my_attr: str) -> None:
        pass
        # missing line: self.my_attr = my_attr

x = MyClass("grapefruit")
s: str = x.my_attr
#          ^ AttributeError at runtime

However, if the instance variable annotation is removed, mypy does warn.

class MyClass:
    def __init__(self, my_attr: str) -> None:
        pass
        # missing line: self.my_attr = my_attr

x = MyClass("grapefruit")
s: str = x.my_attr

From PEP 526 (emphasis mine):

In particular, the value-less notation a: int allows one to annotate instance variables that should be initialized in __init__ or __new__.
@emmatyping
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Duplicate of #4019

@emmatyping emmatyping marked this as a duplicate of #4019 Nov 4, 2017
@ned ned mentioned this issue Nov 4, 2017
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