Python: What is a Lambda Function?

In Python, the lambda keyword is used to define an anonymous (i.e., nameless) function, using the following syntax:

lambda parameters: expression

Assume we have the following list of fruits:

fruits = ['apple', 'orange', 'grape', 'lemon', 'mango', 'banana']

Now imagine we need to filter our list to print only fruit names which are 5 characters long. We could do so by defining a named function to test word lengths, and then passing it (and our list of fruits) to filter():

def five_letter_words(word):
  if len(word) == 5:
    return True

five_letter_fruits = filter(five_letter_words, fruits)

for fruit in five_letter_fruits:
  print(fruit)

apple
grape
lemon
mango
>>>

Or, the same task can be accomplished directly in filter() using a lambda expression, without needing to define a separate named function:

five_letter_fruits = filter(lambda word: len(word) == 5, fruits)

for fruit in five_letter_fruits:
  print(fruit)

apple
grape
lemon
mango
>>>

Because lambda functions are Python expressions, they can be assigned to variables.

So, this:

add_two = lambda x, y: x+y
add_two(3, 5)

Is equivalent to this:

def add_two(x, y):
  return x + y

add_two(3, 5)

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