Scope: global and nonlocal

#python #code

• They are multiple variable scopes: local, global, nonlocal

>>> b = 6
>>> def f3(a):
...     global b
...     print(a)
...     print(b)
...     b = 9
...
>>> f3(3)
3
6
>>> b
9

• Code above would have been failed if global was not used in the function: UnboundLocalError: local variable 'b' referenced before assignment
• nonlocal is used for free variables

def make_averager():
    count = 0
    total = 0

    def averager(new_value):
        nonlocal count, total
        count += 1
        total += new_value
        return total / count

    return averager

Closures

def make_averager():
    series = []

    def averager(new_value):
        series.append(new_value)
        total = sum(series)
        return total / len(series)

    return averager

>>> avg = make_averager()
>>> avg(10)
10.0
>>> avg(11)
10.5
>>> avg(15)
12.0

• In the example above, series is a free variable
• series is where we call closures
• Closures are functions holding free variables when the function is defined in a way that these variables can be used late, when the function is invoked but the scope of the definition is not available anymore