• tabs
  • indentation

Python 2 and 3 allow tabs to be used to indent lines. Python will always expand these to a multiple of 8 (with Python 3 throwing an exception if there were spaces preceding the tab), but many editors and Stack Overflow expand tabs to multiples of 4 instead. This can lead to subtle and weird flow errors.

The canonical example is a return statement that uses a tab, placed after a for loop using spaces for indentation; it looks like the return is indented to 4 spaces, but in reality it is part of the loop:

def test(word):
    for l in word:
        print l
    return total

The function is exited during the first iteration, rather than print all elements of word first.

Running with python -tt won’t help in this class of errors.