How to deal with non-existent variables “reference before assignment”

Question

I have some code that creates a variable if a certain condition is met:

if self.left: 
    left_count = self.left.isUnivalTree()

A few lines down I have the code:

if left_count and right_count: 
    total_count = right_count + left_count

This throws the following error:

local variable 'left_count' referenced before assignment

How do I get around this? Surely by saying if left_count... I account for the fact it may not

(This error is fixed by setting a default value to all the parameters in the function but I am just wondering if there is a simpler way around this as I need to set a default value for 5 parameters?)

Answer 1

You aren't assigning if self.left isn't True , try the below which will set a default of None :

left_count = self.left.isUnivalTree() if self.left else None

As per your question update, setting default parameters on the function is better, that's exactly what they are there for.

Answer 2

Instead of:

left_count = False
if self.left: left_count = self.left.isUnivalTree()

Just use the short circuiting behaviour of and :

left_count = self.left and self.left.isUnivalTree()

Answer 3

If isUnivalTree returns a bool (which it should because of the name):

left_count = self.left and self.left.isUnivalTree()

Otherwise:

left_count = self.left.isUnivalTree() if self.left else None

Answer 4

or catch the NameError

try:
    if left_count and right_count: total_count = right_count + left_count
except NameError:
    print('NameError catched')
    total_count = 0

or check if it is defined:

if 'left_count' in locals():
    print('left_count is defined')

it all depends on what fits best in your work flow