"cannot assign to name" looks wrong. Usually we can assign to name, unless it is a keyword. And the problem is not that we cannot assign to name (we can), but that is is an invalid syntax, assignment is a statement, not an expression.

Seems it handles only simplest cases with "=" in "if". It does not handle "while x = 3" and "if y and x = 3". Could it be possible to make it more general.