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Python Basics
Function: Lambda Function

Lambda Function

The lambda keyword in Python provides a shortcut for declaring small anonymous functions. Lambda functions behave just like regular functions declared with the def keyword.

Example:

>>> add = lambda x, y: x + y 
>>> add(5, 3)
8

It is equivalent to declaring the same add function with the def keyword (which would be slightly moer verbose):

>>> def add(x, y): 
... 	return x + y 
>>> add(5, 3)
8

We can also define a lambda function and then immediately evaluated it inline, without binding the function object to a name:

>>> (lambda x, y: x + y)(5, 3) 
8

The syntactic difference between lambdas and regular function definitions is: Lambda functions are restricted to a single expression. This means a lambda function can’t use statements or annotations—not even a return statement.

Executing a lambda function evaluates its expression and then automatically returns the expression’s result, so there’s always an implicit return statement.

When to use lambda?

Lambdas provide a handy and “unbureaucratic” shortcut to defining a function in Python. A typical use case for lambdas is writing short and concise key funcs for sorting iterables by an alternate key:

>>> tuples = [(1, 'd'), (2, 'b'), (4, 'a'), (3, 'c')] 
>>> sorted(tuples, key=lambda x: x[1])
[(4, 'a'), (2, 'b'), (3, 'c'), (1, 'd')]

Just like regular nested functions, lambdas also work as lexical closures.

Example:

>>> def make_adder(n):
... 	return lambda x: x + n

>>> plus_3 = make_adder(3)

>>> plus_3(4) 
7

The x + n lambda can still access the value of n even though it was defined in the make_adder function (the enclosing scope).

When NOT to use lambda?

Lambda functions should be used sparingly and with extraordinary care!!!

If you’re tempted to use a lambda, spend a few seconds (or minutes) to think if it is really the cleanest and most maintainable way to achieve the desired result.

For example:

# Bad readablility with lambda:
>>> list(filter(lambda x: x % 2 == 0, range(16))) [0, 2, 4, 6, 8, 10, 12, 14]

# Better readablility with list comprehension
>>> [x for x in range(16) if x % 2 == 0] [0, 2, 4, 6, 8, 10, 12, 14]

Do not use lambda just for show-off purpose.

  • If you find yourself doing anything remotely complex with lambda expressions, consider defining a standalone function with a proper name instead.
  • Saving a few keystrokes won’t matter in the long run, but your col- leagues (and your future self) will appreciate clean and readable code more than terse wizardry.