Charm the Python: Lambdas

If coding tutorials with math examples are the bane of your existence, keep reading. This series uses relatable examples like dogs and cats.

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Lambda Function

Lambda functions are unnamed functions. This means it is not defined like a normal function.

Defining lambdas is similar to defining a regular function, but it has a different syntax.

# syntax of a function
def name(args):

# syntax of lambda function
lambda args: expression  # can only have one expression
lambda args, args, args: expression  # may have multiple parameters

Note: Lambdas can only handle expressions. Expressions must have return values like numbers, True, and False

Lambda, by Itself

# A regular function
def add_two_numbers(a, b):
    return a + b

# Same function written as a lambda function
add_two_numbers = lambda a, b: a + b

Notice, both take args a and b, then add them together. The only difference is that the first one is defined as a function.

For my math loathing pals

# A regular function
def generate_full_name(first_name, last_name):
    return first_name + ' ' + last_name

# Same function written as a lambda function
generate_full_name = lambda first_name, last_name: first_name + ' ' + last_name

Lambda function inside other function

Lambda functions can be used inside of other functions.

# future home of an example
# if you could help me with a simple example that would be badass
# a non-math example would be appreciated forever
# simple math would be cool too (something like 2+2)

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Further Investigation

If all of this makes sense to you and you'd like a little more fanciness and excitement, lookup 'Self invoking lambda function'

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Note: Lambdas are not something I'm really comfortable with. For more details check out this thorough article about Lambdas in Python.

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Series based on

30 Days Of Python Challenge

Asabeneh ・ Nov 20 '19 ・ 1 min read

#python #javascript #react #mongodb

Comments

[Ben Sinclair]

For your example of a "function inside another function", do you mean something like this?

my_list = (
    {
        "name": "Ben",
        "age": "99"
    },
    {
        "name": "Antigone",
        "age": "49"
    },
    {
        "name": "Jonas",
        "age": "10"
    },
    {
        "name": "Samantha",
        "age": "52"
    },
    {
        "name": "Vicki",
        "age": "22"
    }
)

print (sorted(my_list, key = lambda x: x["age"]))

[Vicki Langer]

I think this is what I was referring to, but I’m not super sure. All the examples I saw were using functions like map(), filter(), and sort()

I’ve never seen key used before. Help me out, what is x doing in this?

If I’m reading this correctly, it’s just sorting these key-value pairs by age, right?

[Ben Sinclair]

yes, array.sort and sorted take a named parameter in Python 3 (I had to learn this a couple of days ago!) which is run as a callback, once per key, immediately before sorting the elements. So that example calls x: x["age"] on every element first.
It's the equivalent of this:

sort([x["age"] for x in my_list])

and this:

def by_age(x):
    return x["age"]

sort(my_list, key = by_age)