Closure Functions

Closure are higher-order functions which return lower-order function (usually first-order function) with its environment. Defining closure functions is possible in languages which has first-class functions like Python, JavaScript, Lua, Scheme, and others.

Example

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def divisiblity_check(divisor):
    """Factory for producing functions that check divisiblity with divisor."""
    def inner_func(divident):
        return (divident % divisor == 0)

    return inner_func

This closure function (divisiblity_check) is return another function as evident by syntax. It should be noted that environment for inner function (i.e., the variable divisor) is also saved with the returned function. Hence, following line of codes work:

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div_by_7 = divisiblity_check(7)
div_by_19 = divisiblity_check(19)

div_by_7(38)  # return false
div_by_19(38) # return true
div_by_7(35)  # return true
div_by_19(35) # return false

Here, although the functions div_by_7 and div_by_19 are returned by same closure function; however they have different environment attached to them and hence produce different results. The example closure function (i.e., divisiblity_check) can also be defined using anonymous function.

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def divisiblity_check(divisor):
    """Factory for producing functions that check divisiblity with divisor."""

    return lambda divident: (divident % divisor == 0)

This usage is sometimes called anonymous closure function.