Science&Technology info.: Python Lambda

A lambda function is a small anonymous function.

A lambda function can take any number of arguments, but can only have one expression.

Syntax

lambda arguments : expression

The expression is executed and the result is returned:

A lambda function that adds 10 to the number passed in as an argument, and print the result:

x = lambda a : a + 10
print(x(5))

Run example »
C:\Users\My Name>python demo_lambda.py
15

Lambda functions can take any number of arguments:

A lambda function that multiplies argument a with argument b and print the result:

x = lambda a, b : a * b
print(x(5, 6))

Run example »
C:\Users\My Name>python demo_lambda2.py
30

A lambda function that sums argument a, b, and c and print the result:

x = lambda a, b, c : a + b + c
print(x(5, 6, 2))

Run example »
C:\Users\My Name>python demo_lambda3.py
13

The power of lambda is better shown when you use them as an anonymous function inside another function.

Say you have a function definition that takes one argument, and that argument will be multiplied with an unknown number:

def myfunc(n):
  return lambda a : a * n

Use that function definition to make a function that always doubles the number you send in:

def myfunc(n):
  return lambda a : a * n

mydoubler = myfunc(2)

print(mydoubler(11))

Run example »
C:\Users\My Name>python demo_lambda_double.py
22

Or, use the same function definition to make a function that always triples the number you send in:

def myfunc(n):
  return lambda a : a * n

mytripler = myfunc(3)

print(mytripler(11))

Run example »
C:\Users\My Name>python demo_lambda_triple.py
33

Or, use the same function definition to make both functions, in the same program:

def myfunc(n):
  return lambda a : a * n

mydoubler = myfunc(2)
mytripler = myfunc(3)

print(mydoubler(11)) 
print(mytripler(11))

Run example »
C:\Users\My Name>python demo_lambda_both.py
22
33

Use lambda functions when an anonymous function is required for a short period of time.