Question

I read in an earlier answer that exception handling is cheap in Python so we shouldn’t do pre-conditional checking.

I have not heard of this before, but I’m relatively new to Python. Exception handling means a dynamic call and a static return, whereas an if statement is static call, static return.

How can doing the checking be bad and the try-except be good, seems to be the other way around. Can someone explain this to me?

Answer 1

You might find this post helpful: Try / Except Performance in Python: A Simple Test where Patrick Altman did some simple testing to see what the performance is in various scenarios pre-conditional checking (specific to dictionary keys in this case) and using only exceptions. Code is provided as well if you want to adapt it to test other conditionals.

The conclusions he came to:

From these results, I think it is fair to quickly determine a number of conclusions:

1. If there is a high likelihood that the element doesn’t exist, then you are better off checking for it with has_key.
2. If you are not going to do anything with the Exception if it is raised, then you are better off not putting one have the except
3. If it is likely that the element does exist, then there is a very slight advantage to using a try/except block instead of using has_key, however, the advantage is very slight.