Can a nested function assign to variables from the parent function

examples and workarounds in Python

Posted by Alexander Todorov on Sun 11 June 2017

While working on a new feature for Pelican I've put myself in a situation where I have two functions, one nested inside the other and I want the nested function to assign to variable from the parent function. Turns out this isn't so easy in Python!

hello.py
def hello(who):
    greeting = 'Hello'
    i = 0

    def do_print():
        if i >= 5:
            return

        print i, greeting, who
        i += 1
        do_print()

    do_print()

if __name__ == "__main__":
    hello('World')

The example above is a recursive Hello World. Notice the i += 1 line! This line causes i to be considered local to do_print() and the result is that we get the following failure on Python 2.7:

Traceback (most recent call last):
  File "./test.py", line 16, in <module>
    hello('World')
  File "./test.py", line 13, in hello
    do_print()
  File "./test.py", line 6, in do_print
    if i >= 5:
UnboundLocalError: local variable 'i' referenced before assignment

We can workaround by using a global variable like so:

hello.py using global variable
i = 0
def hello(who):
    greeting = 'Hello'

    def do_print():
        global i

        if i >= 5:
            return

        print i, greeting, who
        i += 1
        do_print()

    do_print()

However I prefer not to expose internal state outside the hello() function. Only if there was a keyword similar to global. In Python 3 there is nonlocal!

hello.py using nonlocal, Python 3
def hello(who):
    greeting = 'Hello'
    i = 0

    def do_print():
        nonlocal i

        if i >= 5:
            return

        print(i, greeting, who)
        i += 1
        do_print()

    do_print()

nonlocal is nice but it doesn't exist in Python 2! The workaround is to not assign state to the variable itself, but instead use a mutable container. That is instead of a scalar use a list or a dictionary like so:

hello.py where i is a list, Python 2
def hello(who):
    greeting = 'Hello'
    i = [0]

    def do_print():
        if i[0] >= 5:
            return

        print i[0], greeting, who
        i[0] += 1
        do_print()

    do_print()

Thanks for reading and happy coding!

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