Useful lambda functions in python. refer to : https://www.geeksforgeeks.org/python-lambda-anonymous-functions-filter-map-reduce/

In Python, an anonymous function means that a function is without a name. As we already know that the def keyword is used to define a normal function in Python. Similarly, the lambda keyword is used to define an anonymous function in Python. 

• def() and lambda() comparison

# Python code to illustrate cube of a number
# showing difference between def() and lambda().
def cube(y):
    return y*y*y

lambda_cube = lambda y: y*y*y

# using the normally
# defined function
print(cube(5)).  # 125

# using the lambda function
print(lambda_cube(5)).  # 125

• Using lambda() Function with filter()

  The filter() function in Python takes in a function and a list as arguments. This offers an elegant way to filter out all the elements of a sequence “sequence”, for which the function returns True. Here is a small program that returns the odd numbers from an input list: 

# Python code to illustrate
# filter() with lambda()
li = [5, 7, 22, 97, 54, 62, 77, 23, 73, 61]

final_list = list(filter(lambda x: (x%2 != 0) , li))
print(final_list)
# [5, 7, 97, 77, 23, 73, 61]

# Python 3 code to people above 18 yrs
ages = [13, 90, 17, 59, 21, 60, 5]

adults = list(filter(lambda age: age>18, ages))

print(adults)
# [90, 59, 21, 60]

• Using lambda() Function with map()

  The map() function in Python takes in a function and a list as an argument. The function is called with a lambda function and a list and a new list is returned which contains all the lambda modified items returned by that function for each item. Example: 

# Python code to illustrate
# map() with lambda()
# to get double of a list.
li = [5, 7, 22, 97, 54, 62, 77, 23, 73, 61]

final_list = list(map(lambda x: x*2, li))
print(final_list)
# [10, 14, 44, 194, 108, 124, 154, 46, 146, 122]

# Python program to demonstrate
# use of lambda() function
# with map() function
animals = ['dog', 'cat', 'parrot', 'rabbit']

# here we intend to change all animal names
# to upper case and return the same
uppered_animals = list(map(lambda animal: str.upper(animal), animals))

print(uppered_animals)
# ['DOG', 'CAT', 'PARROT', 'RABBIT']

• Using lambda() Function with reduce()

The reduce() function in Python takes in a function and a list as an argument. The function is called with a lambda function and an iterable and a new reduced result is returned. This performs a repetitive operation over the pairs of the iterable. The reduce() function belongs to the  functools module. 

# Python code to illustrate
# reduce() with lambda()
# to get sum of a list

from functools import reduce
li = [5, 8, 10, 20, 50, 100]
sum = reduce((lambda x, y: x + y), li)
print (sum)
# 193
Here the results of previous two elements are added to the next element and this goes on till the end of the list like (((((5+8)+10)+20)+50)+100).

# python code to demonstrate working of reduce()
# with a lambda function

# importing functools for reduce()
import functools

# initializing list
lis = [ 1 , 3, 5, 6, 2, ]

# using reduce to compute maximum element from list
print ("The maximum element of the list is : ",end="")
print (functools.reduce(lambda a,b : a if a > b else b,lis))

#output:

The maximum element of the list is : 6