return two list python?

Question

I am not sure how the return works in the following compare function? Why can it return the format like this?

def func(self, num):
      num = sorted([str(x) for x in num], cmp=self.compare) 

def compare(self, a, b):
      return [1, -1][a + b > b + a]

Answer 1

It's not returning two lists. It's returning one of the two values from the first list. Consider this rewriting:

def compare(self, a, b):
      possible_results = [1, -1]
      return possible_results[a + b > b + a]

It's taking advantage of the fact that True in Python is treated as the value 1, and False is treated as the value 0, and using those as list indices.

Answer 2

The boolean value of False is zero and the boolean value of True is one. They can both be used as indexes into a list:

# Normal indexing with integers
>>> ['guido', 'barry'][0]
'guido'
>>> ['guido', 'barry'][1]
'barry'

# Indexing with booleans
>>> ['guido', 'barry'][False]
'guido'
>>> ['guido', 'barry'][True]
'barry'

# Indexing with the boolean result of a test
>>> ['guido', 'barry'][5 > 10]
'guido'
>>> ['guido', 'barry'][5 < 10]
'barry'

Hope that makes it all clear :-)