Python Lambda

A lambda is a small function without a name (an anonymous function), written in one line with the lambda keyword. Python took this idea from functional programming.

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Syntax

lambda arguments: expression

A lambda can take any number of arguments, but it can have only one expression. The value of that expression is returned automatically, so you do not write return.

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Examples

add_one = lambda x: x + 1
print(add_one(1))

print((lambda x: x + 2)(1))

Here x is the argument and x + 1 is the expression. In the second line the lambda is called right away by putting brackets around it and passing the argument.

The same function written with def looks like this:

def add_one(x):
    return x + 1

print(add_one(1))

More than one argument

multiply = lambda a, b: a * b
print(multiply(3, 4))

Lambda with a condition

bigger = lambda a, b: a if a > b else b
print(bigger(5, 9))

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Where lambda is used

Lambdas are mostly used as a short function inside another function. Three common places are sorted(), map() and filter().

students = [("Robin", 85), ("Maria", 92), ("John", 78)]
print(sorted(students, key=lambda s: s[1]))

nums = [1, 2, 3, 4, 5]
print(list(map(lambda x: x * 2, nums)))
print(list(filter(lambda x: x % 2 == 0, nums)))

A function that returns a lambda

def multiplier(n):
    return lambda x: x * n

double = multiplier(2)
print(double(5))

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Lambda vs def

Point	lambda	def
Name	No name needed	Has a name
Body	One expression	Many statements
Return	Automatic	Use return
Best for	Short, one-time use	Anything bigger or reused

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Summary

• A lambda is a one-line anonymous function: lambda args: expression.
• It returns the value of the expression automatically.
• It is handy with sorted(), map() and filter().
• Use def when the function is long or used in many places.
• Read more in the interview question What is a lambda function?

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