Data gives clarity to Decision Making. : June 2015

What are closures ?

In a simplest sense, a Closure is a function instance, just like class instance, with each function instance is instantiated with the specific value. In Python for example where everything is an object - modules, classes, functions, attributes etc., a closure is an object returned by a function which is instantiated with parameters it is invoked with. The object returned in this case is a function ( an inner function ) instance returned by another function (an outer function).

Now a more canonical explanation definition of closure.
"A closure is data attached to code " . A Closure is about maintaining state of some kind in functions.

Many functional languages ( Lisp, Haskel, Scala and other ) rely heavily on closures. Python, support it, but it's not at the very core of the language. Python, however, is a fine language for getting to know closures. So, even if your expertise is in some other language, you might learn a thing or two here.

An Example

Now consider this :

def make_log(level):
    ''' a closure that manufactures different logs'''
    def _anon(message):
        print ("{}: {}".format(level, message))
    return _anon

if __name__ == '__main__' :
    main()
    print(main.__doc__)
    # log_info closure
    log_info = make_log("info")
    # log warn closure
    log_warn = make_log("warn")
    # log error closure
    log_error = make_log("error")
    
    log_info("loading configuration file")  #invoking log_info closure
    log_warn("config value missing. using default value") # invoking log_warn closure
    log_error("network socket error. can not connect to server") # invoking log_error closure

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What are closures ?

Spark - RDD how do they work ?

Original RDD research publication by Matei Zharia - Creater of Apache Spark

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