Using 'try' vs. 'if' in Python
python
Solution
You often hear that Python encourages EAFP style ("it's easier to ask for forgiveness than permission") over LBYL style ("look before you leap"). To me, it's a matter of efficiency and readability.
In your example (say that instead of returning a list or an empty string, the function were to return a list or `None`), if you expect that 99 % of the time `result` will actually contain something iterable, I'd use the `try/except` approach. It will be faster if exceptions really are exceptional. If `result` is `None` more than 50 % of the time, then using `if` is probably better.
To support this with a few measurements:
>>> import timeit
>>> timeit.timeit(setup="a=1;b=1", stmt="a/b") # no error checking
0.06379691968322732
>>> timeit.timeit(setup="a=1;b=1", stmt="try:\n a/b\nexcept ZeroDivisionError:\n pass")
0.0829463709378615
>>> timeit.timeit(setup="a=1;b=0", stmt="try:\n a/b\nexcept ZeroDivisionError:\n pass")
0.5070195056614466
>>> timeit.timeit(setup="a=1;b=1", stmt="if b!=0:\n a/b")
0.11940114974277094
>>> timeit.timeit(setup="a=1;b=0", stmt="if b!=0:\n a/b")
0.051202772912802175
So, whereas an `if` statement always costs you, it's nearly free to set up a `try/except` block. But when an `Exception` actually occurs, the cost is much higher.
Moral:
- It's perfectly OK (and "pythonic") to use `try/except` for flow control,
- but it makes sense most when `Exception`s are actually exceptional.
From the Python docs:
EAFP
Easier to ask for forgiveness than permission. This common Python coding style assumes the existence of valid keys or attributes and catches exceptions if the assumption proves false. This clean and fast style is characterized by the presence of many `try` and `except` statements. The technique contrasts with the LBYL style common to many other languages such as C.
Problem
Is there a rationale to decide which one of `try` or `if` constructs to use, when testing variable to have a value? For example, there is a function that returns either a list or doesn't return a value. I want to check result before processing it. Which of the following would be more preferable and why? ``` result = function(); if (result): for r in result: #process items ``` or ``` result = function(); try: for r in result: # Process items except TypeError: pass; ``` Related discussion: Checking for member existence in Python