[Python] How to comprehend the following recursive function in binary tree?

The following is a Binary search function (a root has a left and a right child) which I don't quite comprehend. In the code it returns a list that is the longest path in the binary tree. However for the part:

return_path_left = list_longest_path(node.left)
return_path_right = list_longest_path(node.right)
if len(return_path_left) > len(return_path_right):


how can you compare two recursive call? For example, if the tree is

1
/ \
2


list_longest_path(node.right) will surely return []. But how do you compare list_longest_path(2) with []?

Someone help will be great.

def list_longest_path(node):
"""
List the data in a longest path of node.

@param BinaryTree|None node: tree to list longest path of
@rtype: list[object]

>>> list_longest_path(None)
[]
>>> list_longest_path(BinaryTree(5))
[5]
>>> b1 = BinaryTree(7)
>>> b2 = BinaryTree(3, BinaryTree(2), None)
>>> b3 = BinaryTree(5, b2, b1)
>>> list_longest_path(b3)
[5, 3, 2]
"""
if node is None:
return []
else:
return_path_left = list_longest_path(node.left)
return_path_right = list_longest_path(node.right)
if len(return_path_left) > len(return_path_right):
return [node.data] + return_path_left
else:
return [node.data] + return_path_right

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