Report incorrect assigning of dictionary entries with a colon (`:`)

[dannysepler]

Consider the following code

dct = {
    'a': 1,
}
dct['b']: 2
print(dct)  # {'a': 1}

This is actually valid python syntax, because it is typing the 'b' entry as a 2 type. But we can assume the user meant to assign an element rather than type it (especially if there's no equal on this line).

(Recommended I post this to bugbear by pyflakes maintainers here: PyCQA/pyflakes#756)

[cooperlees]

Nice catch. Agree this is perfect for bugbear, as long as we can get it accurate like @asottile mention on the pyflakes issue. Lets have tests prove that.

[asottile]

this seems to get close enough:

    def visit_AnnAssign(self, node):
        if node.value is None and not (
            isinstance(node.target, ast.Name) or (
                isinstance(node.target, ast.Attribute) and
                isinstance(node.target.value, ast.Name)
            )
        ):
            print(node)

probably need to validate a bunch of code to make sure we don't have a bunch of false positives but I think this is close