[Python-projects] pylint: E0601 false negative

[Maarten ter Huurne]

Hi,

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.

Bye,
                Maarten