Capturing Variables in Anonymous Functions

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

Visit the Python Cookbook: Functions Lab to test out this Shortcut in a live practice environment.

Problem

You’ve defined an anonymous function using lambda but you also need to capture the values of certain variables at the time of definition.

Solution

Consider the behavior of the following code:

>>> x = 10
>>> a = lambda y: x + y
>>> x = 20
>>> b = lambda y: x + y
>>>

Now ask yourself a question. What are the values of a(10) and b(10)? If you think the results might be 20 and 30, you would be wrong:

>>> a(10)
30
>>> b(10)
30
>>>

The problem here is that the value of x used in the lambda expression is a free variable that gets bound at runtime, not definition time. Thus, the value of x in the lambda expressions is whatever the value of the x variable happens to be at the time of execution. For example:

>>> x = 15
>>> a(10)
25
>>> x = 3
>>> a(10)
13
>>>

If you want an anonymous function to capture a value at the point of definition and keep it, include the value as a default value, like this:

>>> x = 10
>>> a = lambda y, x=x: x + y
>>> x = 20
>>> b = lambda y, x=x: x + y
>>> a(10)
20
>>> b(10)
30
>>>

Discussion

The problem addressed in this Shortcut is something that tends to come up in code that tries to be just a little bit too clever with the use of lambda functions—for example, creating a list of lambda expressions using a list comprehension or in a loop ...