These functions are called anonymous because they are not declared in the standard manner by using the def keyword
Instead, lambda keyword is used to create anonymous functions
They are defined without a name, but are assigned to variables, which can be called like 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
The syntax of lambda functions contains only a single statement, which is as follows -
lambda [arg1 [,arg2,.....argn]]:expression Following is the example to show how lambda form of function works -
#!/usr/bin/python
# Function definition is here
sum = lambda arg1, arg2: arg1 + arg2
# Now, sum can be called as a function
print("sum of 10, 20:", sum(10,20))
print("sum of 20, 20:", sum(20,20)) Output :
sum of 10, 20: 30
sum of 20, 20: 40Following example shows a lambda function that doubles the input value
>>>double = lambda x: x * 2
>>>double(91)
182 In the above example, 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 but returns a function object which is assigned to the identifier double
This identifier can then be called as a function
The statement
double = lambda x: x * 2 is analogous to
def double(x):
return x * 2 Lambda functions are used when a nameless function is required for a short period of time
In Python, it is typically used 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
The filter() function in Python takes in a function and a list as arguments
That 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
Following example uses filter() function to filter out only even numbers from a list
>>> y = filter(lambda x: x%2==0, list(range(1, 20)))
>>> list(y)
[2, 4, 6, 8, 10, 12, 14, 16, 18] map() function in Python takes in a function and a list
That 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
Following example uses map()function with lambda to get the square of all the items in a list
>>> y = map(lambda x: x*x, list(range(1, 20)))
>>> list(y)
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361]