lambda.md

Lambda Expressions

Lambda expressions are sort of anonymous function which you might use in lieu of a named function.

The syntax for a lambda expression is lambda arguments : expression. Strictly speaking, lambda functions differ from regular ones only in syntax. For example,

def square(num):
  return num ** 2

is the same as

lambda num: num ** 2

Note that return is not used. Whatever follows the : is what is returned.

As as iffy expression

A lambda can be used as an Immediately Invoked Function Expression by surrounding it in parentheses, like as follows:

(lambda num: num ** 2)(2)  # returns 4

(lambda a, b: a + b)(5, 7)  # returns 12

Binding to variables

Lambdas can be bound to variables, which results in something much like normal function definition:

square = lambda num: num ** 2
square(2)  # returns 4

Using a lambda in this manner is a bit less robust than using a normal function. For example, if an exception occurs the traceback will only show that the exception occured in <lambda>, whereas the traceback for a function would give the function name.

With other functions

The typical use case for a lambda expression is in conjunction with other functions. For example, below is a lambda used in conjunction with the built-in map() and filter() functions:

list(map(lambda num: num ** 2, [1, 2, 3, 4, 5]))
# returns [1, 4, 9, 16, 25]

list(filter(lambda num: num % 2 == 0, [1, 2, 3, 4, 5]))
# returns [2, 4]

Sources

• https://realpython.com/python-lambda/