pylint #5719 E0601 false negative (variable referenced before assignment) [resolved]
Maarten wrote :
In the following Python fragment, the variable "x" is referenced before it is assigned in two occasions:
def f(n): if n == 0: if x == 1: pass else: print x x = 3
There is a rule in the "variables" checker that should catch errors like this: message E0601. But for some reason this message is not issued.
The reason seems to be the "if x == 1" statement. If this is replaced by "pass", the message will be issued.
I looked at the code of the variables checker and the implementation of E0601 is in the visit_name() method. This is the final part of the decision whether to issue the message or not:
if (maybee0601 and stmt.source_line() <= defstmt.source_line() and not is_defined_before(node) and not are_exclusive(stmt, defstmt)): self.add_message('E0601', args=name, node=node)
I added some debug prints and the reason the message is not issued is that "are_exclusive(stmt, defstmt)" returns True. Here "stmt" is the "if x == 1" statement and "defstmt" is the "x = 3" assignment. Indeed those two branches are exclusive. However, that is not a valid reason why the use of "x" should be considered correct. In fact, when the branch in which a variable is assigned is exclusive to a branch in which a variable is used, that would be a reason to consider its use wrong.
I hope someone who is more familiar with the pylint code can figure out what the proper role of are_exclusive() should be for triggering E0601.
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