NB: Lambda Functions

Programming for Data Science

What’s a Lambda Function?

Python lambda functions are small functions that can be written on one line.

Like the one line if/else statement and comprehensions, they represent Python’s lean code writing tradition.

Lambda functions don’t need to be assigned to a variable — you can put them directly in places that expect functions the return, or expressions that evaluate to, values.

They sometimes called “anonymous” functions because they don’t have to have a name.

How Do You Define a Lambda Function?

The general form of a lambda function is as follows:

lambda x: x + 1

the first x is the argument, and the x after the colon is the body of the function.

Here we just add one to the argument.

There is no return statement; the function just returns what body of the function evaluates to.

You can call the function like this:

(lambda x: x + 1)(2)

3

Note how we treat it as an expression by wrapping it in parentheses.

Since it evaluates to a function, we can use function parentheses at the end.

Assigning to a Variable

You can assign a lambda function to a variable.

For example, here are three lambda functions assigned to variables.

sum_two_vars = lambda x, y: x + y
is_non_negative = lambda x: x >= 0
is_dirty = lambda txt: 'dirty' in txt

sum_two_vars(2, 4)

6

is_non_negative(-9)

False

is_dirty('dirty dishes')

True

Lambda functions let us do things like this:

foo = {
    'addone': lambda x: x + 1,
    'square': lambda x: x * x,
    'invert': lambda x: 1 / x
}

bar = 5 

for f in foo:
    print(bar, f, '->', foo[f](bar))

5 addone -> 6
5 square -> 25
5 invert -> 0.2

Note, you can also assign function names to variables, but of course you need to define them first.

def addone(x):
    return x + 1
def square(x):
    return x * x
def invert(x):
    return 1/x

foo2 = [add_one, square, invert]

for f in foo2:
    print(f(1))

2
1
1.0

Use Cases for Lambda

Lambda functions are often used when you need use a function only once.

Some prefer them for their ability to be part of one-liners.

Lambda functions are often used in Pandas.

We will discuss their use in more detail when we get to that topic.