Python lambda (Anonymous) Function

The Python lambda function is anonymous means, a function without a definition keyword and name. To create this function, we have to use the lambda keyword, and the syntax of the expression is

lambda arguments: expression

This function accepts any number of arguments. However, it takes only one expression. For instance, lambda a, b: a + b. Here, a and b are the arguments the function accepts; a + b is the expression.

Python lambda Function Examples

The following list of examples helps to learn the lambda functions.

Use the following statement to display the True or False as an output. It can make you understand that you do not need any argument to create an expression.

num = lambda: True
 
print(num())

With no arguments, output

True

Python lambda Sum

We use the lambda expression to add 5 to the given argument value. It accepts one value because we specified one argument after the keyword. After the colon, it is an expression or the functionality it has to perform when we call this anonymous function.

num = lambda a: a + 5
 
print(num(10))

We passed ten as the argument. 10 + 5 = 15.

In this lambda Sum example, we are using two arguments to perform addition or find the sum of two numbers. It means we have to assign two values while calling this expression.

add = lambda a, b : a + b
 
print(add(10, 20))

Generally, we can achieve the same by declaring or creating a method. This time, we are using the expression and normal function. Both give the same result.

add = lambda x, y : x + y
print(add(10, 20))
 
print("\nResult from a Function")
def add_func(x, y):
    return x + y

print(add_func(10, 20))

Both this and regular functions return the same result. However, the normal Python function needs a def keyword for function definition, method name, and a return value. Whereas, this method does not need any of them. By default, it returns the expression result.

30

Result from a Function
30

These expressions are not about adding two values. Instead, we can perform multiplication, subtractions, or any other calculations in Python lambda expression. Here, we are multiplying two argument values.

For better understanding of this, we are placing the regular function as well. Please refer to the Functions in Python.

multi = lambda a, b : a * b
print(multi(5, 20))
 
print("\nResult from a multi")
def multi_func(a, b):
    return a * b
 
print(multi_func(5, 20))

100
 
Result from a multi
100

Multiple Values

In this example, we are using three arguments. Next, we are multiplying those three argument values.

multi =lambda a, b, c : a * b * c
print(multi(5, 2, 6))
 
print("\nResult from a multi")
def multi_func(a, b, c):
    return a * b * c
 
print(multi_func(5, 2, 6))

With three arguments output

60

Result from a multi
60

Until now, we used this Python lambda Function expression to calculate something and return results. However, use the print statement to print the output as well. The below code prints Hello World! as an output.

We can also achieve the same result by calling that statement with parenthesis from the shell itself.

Default Argument Values

In this lambda expression example, we assigned default values to all three arguments. Next, we are adding, and multiply three of them. If we have the default values, we do not have to pass values while calling them.

add = lambda x = 10, y = 20, z = 30 : x + y + z
print(add()) # 10 + 20 + 30
 
multi = lambda x = 10, y = 20, z = 30 : x * y * z
print(multi()) # 10 * 20 * 30

60
6000

However, you can override the default values by passing new values as arguments. Here, the first print statement overrides 10 with 12, 20 with 14 and 30 with 16. It means, x = 12, y = 14 and z = 16. In the second statement, x = 75, y = 126 and z = 30.

add = lambda x = 10, y = 20, z = 30 : x + y + z
print(add(12, 14, 16)) # 12 + 14 + 16
print(add(75, 126)) # 75 + 126 + 30
print(add(222)) # 222 + 20 + 30
print(add()) # 10 + 20 + 30

print("Multiplication Values")
multi = lambda x = 10, y = 20, z = 30 : x * y * z
print(multi(2, 4, 5)) # x = 2, y = 4, z = 5
print(multi(100, 22)) # x = 100, y = 22, z = 30
print(multi(9)) # x = 9, y = 20, z = 30
print(multi()) # 10 * 20 * 30

Python lambda function without Arguments

If you don’t want to pass any arguments, but wish to return something from it, then you can use this kind of statement.

We do not have to pass any argument values to call these expressions. Whenever we call them, they return the same result.

add = lambda : 10 + 20
print(add())

If we replace + with * (10 * 20) in the above example, we can get the multiplication result. And the output of 10 * 20 = 200.

Python Anonymous Functions using lambda

It is powerful when we use this anonymous function inside a method. Here, we declared a function that accepts one argument. Then, inside the method, we used this expression to multiply that value by the unknown number of times.

def new_func(n):
    return lambda a : a * n
 
number = new_func(2)
 
print(number(50))

It calls new_func method where n = 2. It means, the method returns a: a * 2. Then, we assigned that value to the number (object)

number = new_func(2)

Next, we called that number with 50 (this is the argument value. It means, 50 : 50 * 2 => 100

We are calling this method with different values. First with 2, and then with 3.

def new_func(n):
    return lambda a : a * n
 
number1 = new_func(2)
number2 = new_func(3)
 
print(number1(50))
print(number2(50))

100
150

lambda Built-in Functions

We can use the available Built-in methods along with this.

With filter Example

We use the built-in filter function to filter the sequence of list items. First, we declared a list of numbers from 1 to 15. Next, we used the filter method with this expression. This Python lambda filter expression checks whether a number is divisible by two or not. Next, the filter method returns all the True values.

This filters method and returns the Even Number, and Odd Numbers. Refer to the List article

number = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
print(number)
 
even_num = list(filter(lambda x : x % 2 == 0, number))
print(even_num)
 
odd_num = list(filter(lambda x : x % 2 != 0, number))
print(odd_num)

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
[2, 4, 6, 8, 10, 12, 14]
[1, 3, 5, 7, 9, 11, 13, 15]

map expressions Example

Unlike filter, the map function takes each list item and returns both the True and False values. In this lambda map example, we used map method to return the boolean value. It checks each individual value is divisible by 2 equals to 0. If true, it returns True. Otherwise, it returns false.

number = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
print(number)
 
new_num = list(map(lambda x : x % 2 == 0, number))
print(new_num)

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[False, True, False, True, False, True, False, True, False, True]

This time, we are performing multiplication using the map function. It takes one individual list item at a time and performs the multiplication. At last, the expression returns the modified list.

number = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
print(number)
 
double_num = list(map(lambda x : x * 2, number))
print(double_num)

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

If you replace the above expression x : x * 2 with x ** 2, the output will be [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

You can also perform calculations on multiple lists using the map method. For instance, this example performs addition, subtraction, and multiplication of two lists. It takes one value from both the list in the same position and performs the calculation.

number1 = [10, 20, 30]
number2 = [15, 25, 35]
print(number1)
print(number2)
 
print()
 
mul_num = list(map(lambda x, y : x * y, number1, number2))
print(mul_num)

First, x = 10 from number1, y = 15 from number2 list. By adding both of them, get 25.

[10, 20, 30]
[15, 25, 35]

[150, 500, 1050]

reduce Example

The Python lambda reduce function accepts two values and a list as arguments values and we are using the reduce along with this.

from functools import reduce
number = [10, 20, 30, 15, 25, 35, 45]
print(number)
 
print("==========")
add_num = reduce((lambda x, y : x + y), number)
print(add_num)

[10, 20, 30, 15, 25, 35, 45]]
==========
180

reduce Analysis

First, x = 10, y = 20. Or write it as ((((((10 + 20) + 30) + 15) + 25) + 35) + 45)

Built-in function Example

Until now, we are using the expression to calculate something or perform numerical operations. However, you can use this on string data as well.

In this example, we declared a string list. Next, we used the sorted method to sort the list items. Finally, however, we used the sorted key as a expression.

Within this expression, we are using the len method to find the length of each word. It means sorting is done based on the item length.

x = ['apple', 'Mango Fruit', 'Banana', 'oranges', 'cherry','kiwi']
print(x)
 
y = sorted(x, key =lambda a: len(a))
print(y)
 
z = sorted(x, key =lambda a: a.casefold())
print(z)

lambda len and casefold functions output.

['apple', 'Mango Fruit', 'Banana', 'oranges', 'cherry', 'kiwi']
['kiwi', 'apple', 'Banana', 'cherry', 'oranges', 'Mango Fruit']
['apple', 'Banana', 'cherry', 'kiwi', 'Mango Fruit', 'oranges']