diff --git a/docs/source/common_issues.rst b/docs/source/common_issues.rst index 704c9a23f76d..8b326408abc6 100644 --- a/docs/source/common_issues.rst +++ b/docs/source/common_issues.rst @@ -809,40 +809,46 @@ not necessary: def test(self, t: List[int]) -> Sequence[str]: # type: ignore[override] ... -Unreachable code during typechecking ------------------------------------- +Unreachable code +---------------- -Sometimes a part of the code can become unreachable, even if not immediately obvious. -It is important to note that in such cases, that part of the code will *not* be type-checked -by mypy anymore. Consider the following code snippet: +Mypy may consider some code as *unreachable*, even if it might not be +immediately obvious why. It's important to note that mypy will *not* +type check such code. Consider this example: .. code-block:: python class Foo: - bar:str = '' + bar: str = '' def bar() -> None: foo: Foo = Foo() return - x:int = 'abc' + x: int = 'abc' # Unreachable -- no error -It is trivial to notice that any statement after ``return`` is unreachable and hence mypy will -not complain about the mis-typed code below it. For a more subtle example, consider: +It's easy to see that any statement after ``return`` is unreachable, +and hence mypy will not complain about the mis-typed code below +it. For a more subtle example, consider this code: .. code-block:: python class Foo: - bar:str = '' + bar: str = '' def bar() -> None: foo: Foo = Foo() assert foo.bar is None - x:int = 'abc' + x: int = 'abc' # Unreachable -- no error -Again, mypy will not throw any errors because the type of ``foo.bar`` says it's ``str`` and never ``None``. -Hence the ``assert`` statement will always fail and the statement below will never be executed. -Note that in Python, ``None`` is not a null-reference but an object of type ``NoneType``. This can -also be demonstrated by the following: +Again, mypy will not report any errors. The type of ``foo.bar`` is +``str``, and mypy reasons that it can never be ``None``. Hence the +``assert`` statement will always fail and the statement below will +never be executed. (Note that in Python, ``None`` is not an empty +reference but an object of type ``None``.) + +In this example mypy will go on to check the last line and report an +error, since mypy thinks that the condition could be either True or +False: .. code-block:: python @@ -853,10 +859,7 @@ also be demonstrated by the following: foo: Foo = Foo() if not foo.bar: return - x:int = 'abc' - -Here mypy will go on to check the last line as well and report ``Incompatible types in assignment``. + x: int = 'abc' # Reachable -- error -If we want to let mypy warn us of such unreachable code blocks, we can use the ``--warn-unreachable`` -option. With this mypy will throw ``Statement is unreachable`` error along with the line number from -where the unreachable block starts. +If you use the :option:`--warn-unreachable ` flag, mypy will generate +an error about each unreachable code block. pFad - Phonifier reborn

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