Python Anonymous/lambda Function

Python Anonymous/lambda Function#

Learn about the anonymous function, also known as lambda functions. You’ll learn what they are, their syntax and how to use them (with examples).

What are lambda functions in Python?#

In Python, an anonymous function is a function that is defined without a name.

While normal functions are defined using the def keyword in Python, anonymous functions are defined using the lambda keyword.

APython lambda function is a single expression. But, in a lambda body, we can expand with expressions over multiple lines using parentheses () or a multiline string """ """.

For example: lambda n:n+n

The reason behind the using anonymous function is for instant use, that is, one-time usage and the code is very concise so that there is more readability in the code.

Hence, anonymous functions are also called lambda functions.

  • Lambda forms can take any number of arguments but return just one value in the form of an expression. They cannot contain commands or multiple expressions.

  • An anonymous function cannot be a direct call to print because lambda requires an expression.

  • lambda functions have their own local namespace and cannot access variables other than those in their parameter list and those in the global namespace.

  • Although it appears that lambdas are a one-line version of a function, they are not equivalent to inline statements in C or C++, whose purpose is to stack allocation by passing function, during invocation for performance reasons.

Syntax:

lambda argument_list: expression

Let’s see an example to print even numbers without a lambda function and with a lambda function. See the difference in line of code as well as readability of code.

# Example 1: Program for even numbers without lambda function

def even_numbers(nums):
    even_list = []
    for n in nums:
        if n % 2 == 0:
            even_list.append(n)
    return even_list

num_list = [10, 9, 16, 78, 2, 3, 7, 1]
ans = even_numbers(num_list)
print("Even numbers are:", ans)

Even numbers are: [10, 16, 78, 2]

# Example 1: Program for even number with a lambda function

l = [10, 9, 16, 78, 2, 3, 7, 1]
even_nos = list(filter(lambda x: x % 2 == 0, l))
print("Even numbers are: ", even_nos)

Even numbers are:  [10, 16, 78, 2]

# Example 2: Program to show the use of lambda functions

double = lambda x: x * 2

print(double(6))

12

Explanation:

In the above program, lambda x: x * 2 is the lambda function. Here x is the argument and x * 2 is the expression that gets evaluated and returned.

This function has no name. It returns a function object which is assigned to the identifier double. We can now call it as a normal function. The statement

>>> double = lambda x: x * 2

is nearly the same as:

>>> def double(x):
>>>     return x * 2

# Example 3: Normal Function definition is here

def square(x):
    return x*x

# anonymous function
sqr = lambda x: x*x

#Calling square function
print("Square of number is",square(10)) #call normal function
print("Square of number is",sqr(2)) #call anonymous function

Square of number is 100
Square of number is 4

Use of lambda Function in python#

We use lambda function when we require a nameless function for a short period of time.

In Python, we generally use it as an argument to a higher-order function (a function that takes in other functions as arguments). lambda function are used along with built-in functions like filter(), map(), reduce() etc.

lambda function use with filter()#

The filter() function in Python takes in a function and a list as arguments.

The function is called with all the items in the list and a new list is returned which contains items for which the function evaluates to True.

Here is an example use of filter() function to filter out only even numbers from a list.

# Example 1: Program to filter out only the even items from a list

my_list = [1, 5, 4, 6, 8, 11, 3, 12]  # total 8 elements

new_list = list(filter(lambda x: (x%2 == 0), my_list)) # returns the output in form of a list

print("Even numbers are: ", new_list)

Even numbers are:  [4, 6, 8, 12]

# Example 2: 

l = [-10, 5, 12, -78, 6, -1, -7, 9]
positive_nos = list(filter(lambda x: x > 0, l))
print("Positive numbers are: ", positive_nos)

Positive numbers are:  [5, 12, 6, 9]

lambda function with map()#

The map() function in Python takes in a function and a list.

The function is called with all the items in the list and a new list is returned which contains items returned by that function for each item.

Here is an example use of map() function to double all the items in a list.

# Example 1: Program to double each item in a list using map()

my_list = [1, 5, 4, 6, 8, 11, 3, 12]  # total 8 elements
new_list = list(map(lambda x: x * 2, my_list)) # returns the output in form of a list
print("Double values are: ", new_list)

Double values are:  [2, 10, 8, 12, 16, 22, 6, 24]

# Example 2:

list1 = [2, 3, 4, 8, 9]
list2 = list(map(lambda x: x*x*x, list1))
print("Cube values are:", list2)

Cube values are: [8, 27, 64, 512, 729]

lambda function with reduce()#

The reduce() function is used to minimize sequence elements into a single value by applying the specified condition.

The reduce() function is present in the functools module; hence, we need to import it using the import statement before using it.

# Example 1:

from functools import reduce
list1 = [20, 13, 4, 8, 9]
add = reduce(lambda x, y: x+y, list1)
print("Addition of all list elements is : ", add)

Addition of all list elements is :  54
Contents