There was a paper at the 1997 Python conference on this topic titled "Standard Class Exceptions in Python". A copy is at https://web.archive.org/web/20030610173145/http://barry.wars... . It evaluated the performance of try/except vs. has_key and concluded:
> This indicates that the has_key() idiom is usually the best one to choose, both because it is usually faster than the exception idiom, and because its costs are less variable.
The take-home lesson is that actually raising an exception in Python 1.5 was about 10x more expensive than a function call, but the try/except block when there is no exception was not expensive.
Interesting. My information was not only out of date; it was also wrong. The dangers of cargo-culting, although in my case, more theoretical than real, as I never had performance-sensitive python code in production.
There was a paper at the 1997 Python conference on this topic titled "Standard Class Exceptions in Python". A copy is at https://web.archive.org/web/20030610173145/http://barry.wars... . It evaluated the performance of try/except vs. has_key and concluded:
> This indicates that the has_key() idiom is usually the best one to choose, both because it is usually faster than the exception idiom, and because its costs are less variable.
The take-home lesson is that actually raising an exception in Python 1.5 was about 10x more expensive than a function call, but the try/except block when there is no exception was not expensive.