Python Pitfall: Alleged Decrement Operator

The best way to make oneself more familiar with the possibilities and pitfalls of a newly learned programming language is to start pet projects using that language. That’s just what I did to dive deeper into Python. While working on my Python pet project I made a tiny mistake which took me quite a while to figure out. The code was something like (highly simplified):

for i in range(someRange):
  # lots of code here
  doSomething(--someNumber)
  # even more code here

For me, with a strong background in Java and C, this looked perfectly right. Yet, it was not. Since it compiled properly, I immediately excluded syntax errors from my mental list of possible reasons and began to search for a semantic or logical error.

After a while, I remembered that there is no such thing as post-increment or post-decrement operator, so why should there be a pre-decrement? Well, there isn’t. But, if there is no pre-decrement operator, why does –someNumber compile? Basically, the answer is pretty simple: To Python –someNumber is the same as -(-(someNumber)).

A working version of the above example could be:

for i in range(someRange):
  # lots of code here
  someNumber -= 1
  doSomething(someNumber)
  # even more code here