Lambda

In python, a lambda function relates to the concept of anonymous functions.

Problem

We want to sort a list in alphabetical order.

animal_names = ['Fievel', 'Buck', 'lasSie', 'Zedicus']
animal_names.sort()

Easy enough right?

Result

['Buck', 'Fievel', 'Zedicus', 'lasSie']

The issue here is that due to case differences, we are unable to sort them alphabetically.

Normal function

To understand how lambda functions work.

We’ll contrast with a normal function to solve the issue above

In the function below, we have a function that converts a string to uppercase.

#!/usr/bin/python3

def makeuppercase(yourstring):
    """ To convert string to all uppercase """
    return yourstring.upper()

animal_names = ['Fievel', 'Buck', 'lasSie', 'Zedicus']
animal_names.sort(key=makeuppercase)

print (animal_names)

Result

['Buck', 'Fievel', 'lasSie', 'Zedicus']

Because, it first transforms all elements to uppercase, it is able to sort them properly.

Lambda function

Because our previous function uses only one line of code.

We can convert it to a lambda function instead, which does just that.

The syntax for a lambda function is:

lambda parameters : expression

We can replace the lambda parameters with what we used in the previous function.

lambda yourstring : yourstring.upper()

Since, the function has been stripped from its name, it’s now an anonymous (lambda) function.

Putting it together

animal_names = ['Fievel', 'Buck', 'lasSie', 'Zedicus']
animal_names.sort (key = lambda yourstring : yourstring.lower())
print(animal_names)

Result

['Buck', 'Fievel', 'lasSie', 'Zedicus']

Making lambda less anonymous

They are supposed to be, but don’t need to be :).

make_uppercase = lambda yourstring : yourstring.lower()

animal_names.sort(key=make_uppercase)
print (animal_names)

Result

['Buck', 'Fievel', 'lasSie', 'Zedicus']

\ Codarren /

Credits

modified example from python all in one for dummies