change value of local variable from function inside function

Question

I have something like this:

def a():
  #do something
  foo = 0
  def b():
    foo += 2
    # do something
  b()
  #do something

a()

but it says

Traceback (most recent call last):
  File "<pyshell#6>", line 1, in <module>
    a()
  File "<pyshell#5>", line 7, in a
    b()
  File "<pyshell#5>", line 5, in b
    foo += 2
UnboundLocalError: local variable 'foo' referenced before assignment

How can I access foo without making it global?

chepner · Accepted Answer · 2020-05-08 12:30:21Z

3

The assignment to foo in b makes it a local variable, unrelated to the variable of the same name in a. Use the nonlocal statement to change that.

def a():
    #do something
    foo = 0
    def b():
        nonlocal foo
        foo += 2
        # do something
    b()
  #do something

Now foo in b is the same variable as foo in a. nonlocal is like global, but uses the closest enclosing scope that contains the name foo rather than jumping straight to the global scope.

answered May 8, 2020 at 12:30

chepner

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