Python Patterns: Map and Filter

August 31, 2020 #python #software-development

Computers are great at performing a simple action over and over again. A common way to make them do such a task is to store data in a list and iterate over it with a for loop, calling a function for each item.

But Python has some great functions to replace for loops, which I will cover below after a quick example.

Playing Cards

Given a list of playing cards as tuples, like so:

cards = [
  ("Spades", 14),
  ("Diamonds", 13),
  ("Hearts", 2),
  ("Spades", 8),
  ("Clubs", 11),
  ...  # etc.
]

We want to convert them to PlayingCard objects as defined in my previous post on enums. To do this, we need a function to convert a tuple into the class:

def tuple_to_card(card_tuple):
  suit, rank = card_tuple

  card = PlayingCard(
    CardSuit(suit),
    CardRank(rank),
  )

  return card

This makes it easy to parse the list with a quick loop:

new_cards = []
for card_tuple in cards:
  new_card = tuple_to_card(card_tuple)
  new_cards.append(new_card)

And we can even filter the cards so that we only keep hearts:

just_hearts = []
for card_tuple in cards:
  new_card = tuple_to_card(card_tuple)
  if new_card.suit is CardSuit.HEARTS:
    just_hearts.append(new_card)

These code snippets are fine: short and clean, with not a lot that can go wrong. But I love to replace custom code with Python built-ins whenever possible, because they are fast, well tested, and concise. Python provides two functions that can simplify even these already simple code fragments: map() and filter().

Map

The map() function replaces a for loop that calls a function on each item of a list, just as we did in the above when making PlayingCard objects. Here is how we could rewrite the above code using map:

new_cards = map(tuple_to_card, cards)

Simple!

If the function is not too complicated, it can be useful to define it inline with a lambda function:

get_card_rank = lambda card_tuple: card_tuple[1]
ranks = map(get_card_rank, cards)

But what if instead of tuples we had two lists: one of suits and one of ranks? We could use the zip function to combine the two lists like a zipper:

new_cards = []
for card_tuple in zip(card_suits, card_ranks):
  new_card = tuple_to_card(card_tuple)
  new_cards.append(new_card)

But map already allows us to do pairwise operations:

new_cards = map(tuple_to_card, card_suits, card_ranks)

Of course, we could write our own map using list comprehension:

new_cards = [tuple_to_card(card_tuple) for card_tuple in cards]

Which, perhaps, is a little more Pythonic.

Filter

But how would we filter the list so that we only keep hearts, as in our second example? We could wrap the map call in a for loop:

just_hearts = []
for card in map(tuple_to_card, cards):
  if card.suit is CardSuit.HEARTS:
    just_hearts.append(card)

But the filter() function does that for us! It takes a function and an iterable and returns only the elements of the iterable that evaluate to True when the function is called on them. This allows us to rewrite the above as:

is_a_heart = lambda card: card.suit is CardSuit.HEARTS

just_hearts = filter(
  is_a_heart,
  map(tuple_to_card, cards)
)

Of course, again, we could write this as a comprehension:

just_hearts = [
  tuple_to_card(card_tuple) for card_tuple in cards
  if tuple_to_card(card_tuple).suit is CardSuit.HEARTS
]

However, this is not as readable as the map and filter example, which is very short, very readable, and even a bit… functional.¹

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1. But what about reduce(), the third function of the classic “filter-map-reduce” triplet? Python does have a reduce function, but it was moved to functools.reduce() because “a loop is more readable most of the time”. ↩