Description
During #2221 (which I have little hope to be actually accepted) I ran into this function in binder.py:
def get(self, expr: Union[Expression, Var]) -> Type:
return self._get(expr.literal_hash)The Var case comes from analyze_var_ref() in checkexpr.py.
Well, I couldn't find where Var.literal_hash is initialized to anything other than None, and returning None in the Var case passes all the tests. So I tried to replace it with something like the following:
def get(self, expr: Union[Expression, Var]) -> Type:
if isinstance(expr, Var):
new_expr = NameExpr(expr.name())
else:
new_expr = expr
return self._get(new_expr.literal_hash)I then ran mypy on itself, but it raised many errors. So I ran the test suite, and I got many failures; however, it seems that these failures are due to a more precise typechecking. For example, take testModifyLoopFor4 (simplified):
x = None # type: Union[int, str]
x = 1
if 1:
x = 'a'The test does not say anything bad in this part, but mypy now complains that x is an int and therefore 'a' cannot be assigned into it. It might be too restrictive, but it is my understanding that this is the desired behavior, since "x=1" rebinds the variable into int (it's valid since int is part of the Union) and then it tries to rebind it to str, but now it is not possible.
I am not 100% sure, since I am not 100% sure how the code works and what is the expected behavior, but it looks like a bug - and if so, there are ~40 "type errors" to fix in the source code (although I suspect none of them is an actual error).
So what is the expected behavior? When and how should types rebind?