Source: đź“– Python Cookbook ch7.9 p231
A closure is a function that lives (is defined) inside of another function. The reason closures are used is because they remember the environment in which they are defined—that is, a closure remembers any values that exist within the function within which it is defined.
def func(my_arg):
def closure(other_arg):
return my_arg + other_arg
return closure # returns callable `closure` function
In the above example, the closure function is defined inside the func function. This means that each time func is called, closure is defined and it will remember any arguments which the enclosing func was passed for my_arg. func then returns the callable closure, which expects other_arg. This returned callable can be assigned to any variable name and then used as its own function, with the key feature that this variable contains within it (contained within the closure function) whatever value was originally passed to func.
testing = func(5)
# testing now holds the `closure` function, inside which is `my_arg=5`
result = testing(2) # result is `my_arg=5` + `other_arg=2`, so 7
other_result = testing(10) # 15
using_str = func('Hello')
restult_str = using_str('World!') # 'HelloWorld!'
Because of this 'remembering' behaviour, closures can be used instead of simple single-method classes for cleaner code. You may want to use a single-method class when you need a simple function that remembers one or more values—for such cases, using a closure may be a more elegant option.