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Lambda Functions

Updated Oct 30, 2019 ·

Overview​

Lambda functions are shorter and don't need a name. They are used for simple tasks where you don’t need to create a full function.

  • Syntax: lambda arguments: expression
  • You can use x for the argument, or any other name.
  • The expression is like the function body.
  • The result is automatically returned.

When to use Lambda:

  • Lambda:
    • Use for simple tasks
    • When the function will only be used once
  • Custom Functions:
    • Use for more complex logic
    • When the function will be reused many times

Using Lambda​

If you only need to use the function once, you can directly define and call it with Lambda like this:

(lambda x: function(x))(x)

Example: To calculate the average, use sum(x)/len(x) as the function and pass the argument x.

## Computes the average of multiple values
(lambda x: sum(x)/len(x))([10, 20, 30])   # Output: 20.0

Storing Lambda​

You can also store a lambda function in a variable and then call it like a regular function.

Example:

average = lambda x: sum(x) / len(x)
print(average([10, 20, 30]))  # Output: 20.0

Multiple Arguments​

Lambda functions can take more than one argument.

Example: Raising one number to the power of another.

(lambda x, y: x ** y)(2,3) # Output: 8

Which could also be written as:

power = lambda x, y: x ** y
print(power(2, 3))  # Output: 8

Using Lambda with Iterables​

Lambda functions work well with iterables such as lists. They are commonly used with functions like map() to apply an operation to every element, then converted to a list using list().

Example 1: Adding text to each item​

Adds "!!!" to every string in the list using a lambda function with map().

spells = ["protego", "accio", "expecto patronum", "legilimens"]

shout_spells = map(lambda x: x + "!!!", spells)
shout_spells_list = list(shout_spells)
print(shout_spells_list)

Output:

['protego!!!', 'accio!!!', 'expecto patronum!!!', 'legilimens!!!']

Example 2: Capitalizing each name​

Capitalizes the first letter of each name in the list using a lambda function with map().

names = ['alice', 'bob', 'charlie']
capitalized_names = list(map(lambda x: x.capitalize(), names))
print(capitalized_names)

Output:

['Alice', 'Bob', 'Charlie']

Example 3: Filtering long names​

Keeps only the names longer than 6 letters from the list using filter() and a lambda function.

fellowship = ['frodo', 'samwise', 'merry', 'pippin', 'aragorn', 'boromir', 'legolas', 'gimli', 'gandalf']

result = filter(lambda name: len(name) > 6, fellowship)
result_list = list(result)

print(result_list)

Output:

['samwise', 'aragorn', 'boromir', 'legolas', 'gandalf']

Example 4: Concatenating all names​

Combines all names in the list into a single string using reduce() and a lambda function.

from functools import reduce

stark = ['robb', 'sansa', 'arya', 'brandon', 'rickon']
result = reduce(lambda a, b: a + b, stark)

print(result)

Output:

robbsansaaryabrandonrickon

Calling Lambda In-Line​

You can also define the function and print the output in one line.

Example: alculate the tax (20% of the sale price)

sale_price = 29.99

print((lambda x: x * 1.2)(sale_price)) ## Output: 35.988

If sale_price is a list of prices, you can use map to apply the tax calculation to each value, and list to convert the result into a list.

sales_prices = [29.99, 9.95, 14.50, 39.75, 60.00]

add_taxes = map(lambda x: x*1.2, sales_prices)
print(list(add_taxes))

Output:

[35.988, 11.94, 17.4, 47.699999999999996, 72.0]