6python-lambda-functions

Lambda Functions¶

Lambda functions are "disposable" nameless functions. They are often used as arguments in other functions. For example:

In [1]:

pairs = [(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]
pairs.sort(key = lambda p : p[1])
print(pairs)

[(4, 'four'), (1, 'one'), (3, 'three'), (2, 'two')]

The expression "lambda p : p[1]" defines a nameless function equivalent to the following (see its visualization here):

In [2]:

def getkey(p):
    return p[1]

In [3]:

pairs = [(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]
pairs.sort(key = getkey)
print(pairs)

[(4, 'four'), (1, 'one'), (3, 'three'), (2, 'two')]

The following is another example where we use a lambda function to extract from a list with the filter command:

In [4]:

squares = [x ** 2 for x in range(100)]
sq = list(filter(lambda x : x % 2 == 0 and x % 3 == 0, squares))
sq

Out[4]:

It is equivalent to the following:

In [5]:

def multiple_of_6(x):
    return x % 2 == 0 and x % 3 == 0

squares = [x ** 2 for x in range(100)]
sq = list(filter(multiple_of_6, squares))
sq

Out[5]:

7. Lambda Functions