Python Lambda Functions with Practical Examples

A lambda function in Python is a small anonymous function that is defined using the “lambda” keyword. It can have any number of arguments, but can only have one expression.

A simple lambda function might look like this:

times_two = lambda number: number * 2

So this code:

print(times_two(7))
print(times_two(150))
print(times_two(-5))

would print:

>>> 14
>>> 300
>>> -10

Let’s break this syntax down:

1. The function is stored in a variable called times_two
2. lambda declares that this is a lambda function (if you are familiar with normal Python functions, this is similar to how we use def to declare a function)
3. number is what we call the input we are passing into times_two
4. We are returning number * 2 (with normal Python functions, we use the keyword return)

Let’s write a lambda function that checks if a string is a substring of the string “This is the master string”.

is_substring = lambda my_string: my_string in "I am learning Python"

So, the code:

print(is_substring('I'))
print(is_substring('am'))
print(is_substring('playing'))
print(is_substring('basketball'))

would print:

>>> True
>>> True
>>> False
>>> False

Lambda can also take more than one argument. For example, the following lambda function takes two arguments (x and y) and returns their sum:

sum = lambda x, y: x + y
print(sum(3, 4))
# Output: 7

We might want a function that will perform differently based on different inputs. Let’s say that we have a function check_if_A_grade that outputs 'Got an A!' if a grade is at least 90, and otherwise says you 'Did not get an A…'. So, the code:

print(check_if_A_grade(91))
print(check_if_A_grade(70))
print(check_if_A_grade(20))

would print:

>>> 'Got an A!'
>>> 'Did not get an A...'
>>> 'Did not get an A...'

We can do this using an if statement in our lambda function, with syntax that looks like:

<WHAT TO RETURN IF STATEMENT IS TRUE> if <IF STATEMENT> else <WHAT TO RETURN IF STATEMENT IS FALSE>

So this is what our check_if_A_grade function might look like:

check_if_A_grade = lambda grade: 'Got an A!' if grade >= 90 else 'Did not get an A...'

This is what this line of code does:

1. Declare lambda function with an input called grade (lambda grade:)
2. Return 'Got an A!' if this statement is true:grade >= 90
3. Otherwise, return 'Did not get an A...' if this statement is not true:grade >= 90

add_or_subtract = lambda input_number: input_number - 1 if input_number >= 0 else input_number + 1

>>> add_or_subtract(0)-1
>>> add_or_subtract(8)7
>>> add_or_subtract(-4)-3

Lambda functions only work if we’re just doing a one line command. If we wanted to write something longer, we’d need a more complex function. Lambda functions are great when you need to use a function once. Because you aren’t defining a function, the reusability aspect of functions is not present with lambda functions. By saving the work of defining a function, a lambda function allows us to efficiently run an expression and produce an output for a specific task, such as defining a column in a table, or populating information in a dictionary.

Now you can make simple Python functions in one line!