228 – Filter with a predicate

Suppose you have a predicate function and a loop that only acts on elements that satisfy the predicate:

def predicate(value):
    ...  # returns True or False

for value in iterable:
    if predicate(value):
        ...

The body of the loop only acts on elements that satisfy the predicate. In this case, you can use the built-in filter instead of spelling out the conditional statement:

def predicate(value):
    ...
    
for value in filter(predicate, iterable):
    ...

By using the built-in filter, you’re communicating early the intent to filter the data and to not act on the entirety of it.

This tip works well if the function predicate is already defined, or worth defining. If you’re working with a Boolean expression to determine which values to process, it’s preferable to spell out the conditional statement.