Python - Lambda Function - Lambda Function Tutorial

Python Lambda Functions are anonymous function means that the function is without a name.

Syntax-

lambda arguments: expression

• This function can have any number of arguments but only one expression, which is evaluated and returned.
• Lambda function is a short syntax or one-line code

Example-

# creating lambda function to sum a and b, and assigning it to c
x = lambda a,b: a + b

# calling lambda function using variable x
x(10,20)

Output-

30

In the above example, we defined a lambda function(x) that sum up a two value a and b

Use Of lambda function in python-

Condition checking using python lambda function

Example-

# creating lambda function to check whether the number is even or odd
a = lambda a:"even" if a%2==0 else 'odd'

# calling lambda function using variable a
a(10)

Output-

even

Condition checking using python def function

Example-

def evenorodd(x):
    if(x%2==0):
        return "even"
    else:
        return "odd"

print(evenorodd(10))

Output-

even

Def Function vs Lambda Function

List Comprehension vs Lambda Function

We can achieve short syntax or one line of code from List Comprehension also. List Comprehension is a built-in function and can create and modify the list in fewer lines of code.

But the problem with the lambda function is, that it will return something, otherwise, it will give an error, if the value is not returned.

Example-

email=['[email protected]','[email protected]','[email protected]','[email protected]']

# Using list comprehension it will take 1 line to extract only alphabetic username
username2=[i.split('@')[0] for i in email if i.split('@')[0].isalpha()]
print(username2)

# Also using lambda function it will take 1 line to extract only alphabetic username
username3=list(map(lambda x: x.split('@')[0] if(x.split('@')[0].isalpha()) else "", email))
print(username3)

#It will give error, if value is not returned
username4=list(map(lambda x: x.split('@')[0] if(x.split('@')[0].isalpha()), email))
print(username4)

Output-

['vaibsm', 'sam', 'helpdesk']
['', 'vaibsm', 'sam', 'helpdesk']

 File "C:\Users\ssaur\AppData\Local\Temp/ipykernel_9892/2835995980.py", line 8
    username4=list(map(lambda x: x.split('@')[0] if(x.split('@')[0].isalpha()), email))
                                                                              ^
SyntaxError: invalid syntax

In the above example,

in a part of username3, it returns some value if the value is true, otherwise, it will return an empty string on false

in a part of username4, it returns some value if the value is true, but there is no else condition. hence nothing is returned in else part. which will throw an error.

Hence, list comprehension is a more readable and elegant way to define logic than the lambda function.

Using lambda() Function with filter()

The filter() function accepts two arguments in Python: a function and a list. It filters out all the elements from the list for which the function returns true.

Example-

email=['[email protected]','[email protected]','[email protected]','[email protected]']

# Using filter with lambda it will extract email whose username is alphabetic
username3=list(filter(lambda x: x.split('@')[0].isalpha(), email))
print(username3)

Output-

['[email protected]', '[email protected]']

Using filter() without lambda

Function needs to return bool value. this function will be applied to every element of the iterable.

Program to find even numbers from the list.

Example-

number=[102, 10, 99, 32, 113]

def even_check(n):
    if n%2==0:
        return True

# Using map with lambda function it will return new list in celcius
even=list(filter(even_check, number))
print(even)

Output-

[102, 10, 32]

Using lambda() Function with map()

The map() function accepts two arguments in Python: a function and a list. It return new list which contains all the lambda modified items returned by that function for each item.

Program  to find username from email using lambda and map function

Example-

email=['[email protected]','[email protected]','[email protected]','[email protected]']

# Using map with lambda function it will return new list of username extracted from email list
username3=list(map(lambda x: x.split('@')[0], email))
print(username3)

Output-

['12345', 'vaibsm', 'sam', 'helpdesk1']

Using map() without lambda

Program to find Celcius from Fahrenheit number

Example-

faherenheit=[102, 101, 99, 120, 113]

def cel(T):
    return (5/9)*(T-32)

# Using map with lambda function it will return new list in celcius
celcius=list(map(cel, faherenheit))
print(celcius)

Output-

[38.88888888888889, 38.333333333333336, 37.22222222222222, 48.88888888888889, 45.0]

Using lambda() Function with reduce()

The reduce() function accepts two arguments in Python: a function and a list. It performs a repetitive operation over the pairs of the iterable and return a new reduced result. The reduce() function belongs to the functools module.

Here, 1st element is added to 2nd element.result of both will get added to 3rd element, and this goes on till the end of the list like (((('Hello ', + 'welcome ') + 'to ') + 'fresherbell ') + 'website ')

Example-

from functools import reduce
li = ["Hello ", "Welcome ", "to ", "fresherbell ", "website"]
str1 = reduce((lambda x, y: x + y), li)
print(str1)

Output-

Hello Welcome to fresherbell website