why this python program have the following output?

Question

def makeInc (x, step):
    def next():
        nonlocal x, step
        x = x + step
        return x
    return next

x = makeInc (0, 1)
y = makeInc (0, 10)

x1=x()
x2=x()
y1=y()
y2=y()

print( x1, x2, y1, y2)

The output is 1 2 10 20. I am not sure why it gives these outputs, can anyone explain it in detail? Thanks!

@Hariprasad it worked correctly for me. I'm using Python 3.3.1, which version are you using? — steveha, Dec 20 '13 at 07:25
Does this answer your question? [Python nonlocal statement / keyword](https://stackoverflow.com/questions/1261875/python-nonlocal-statement-keyword) and [What do lambda function closures capture?](https://stackoverflow.com/questions/2295290/) — Karl Knechtel, Sep 11 '22 at 07:40

score 0 · Accepted Answer · edited May 23 '17 at 11:56

The function makeInc() is a "factory" that makes function objects and returns them. The nonlocal declaration makes the function "close over" a variable. Usually you would make an explicit variable and close over that; in this case, the nonlocal declaration is closing over an argument variable from makeInc().

If you want to learn about closures and nonlocal here are a few references:

http://www.mondoinfo.com/blog/C182263547/E20060918200706/index.html

Python nonlocal statement

So, makeInc() makes a function object that has a start value and a "step" by which the start value will be incremented. Once you have made the custom function object, the final code calls the functions and gets the incrementing values.

why this python program have the following output?

1 Answers1