In Python, 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. Hence, anonymous functions are also called lambda functions.
Lambda Functions
A lambda function has the following syntax.
Syntax of Lambda Function
lambda arguments: expression
Lambda functions can have any number of arguments but only one
expression. The expression is evaluated and returned. Lambda functions
can be used wherever function objects are required.
Example of Lambda Function
Here is an example of lambda function that doubles the input value.
# Program to show the
# use of lambda functions
double = lambda x: x * 2
print(double(5))
Output
10
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
Use of Lambda Function
We use lambda functions 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 functions are used along with built-in functions like
filter(),
map() etc.
Example 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 evaluats to
True.
Here is an example use of
filter() function to filter out only even numbers from a list.
# Program to filter out
# only the even items from
# a list using filter() and
# lambda functions
my_list = [1, 5, 4, 6, 8, 11, 3, 12]
new_list = list(filter(lambda x: (x%2 == 0) , my_list))
print(new_list)
Output
[4, 6, 8, 12]
Example use 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.
# Program to double each
# item in a list using map() and
# lambda functions
my_list = [1, 5, 4, 6, 8, 11, 3, 12]
new_list = list(map(lambda x: x * 2 , my_list))
print(new_list)
Output
[2, 10, 8, 12, 16, 22, 6, 24]
