A function is an object that is able to accept some sort of input, possibly modify it, and return some sort of output. In Python, a lambda function is a one-line shorthand for function. A simple lambda function might look like this:
add_two = lambda my_input: my_input + 2
So this code:
print(add_two(3)) print(add_two(100)) print(add_two(-2))
>>> 5 >>> 102 >>> 0
Let’s break this syntax down:
- The function is stored in a variable called
lambdadeclares that this is a lambda function (if you are familiar with normal Python functions, this is similar to how we use
defto declare a function)
my_inputis what we call the input we are passing into
- We are returning
my_inputplus 2 (with normal Python functions, we use the keyword
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 "This is the master string"
So, the code:
print(is_substring('I')) print(is_substring('am')) print(is_substring('the')) print(is_substring('master'))
>>> False >>> False >>> True >>> True
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))
>>> '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:
- Declare lambda function with an input called
'Got an A!'if this statement is true:grade >= 90
- Otherwise, return
'Did not get an A...'if this statement is not true:grade >= 90
Lambda functions only work if we’re just doing a one line command. If we wanted to 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 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!