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Puzzle 16: Explanation

Let’s look at the scope of variables in Python and the cause of the `UnboundLocalError` exception.

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Try it yourself

Try executing the code below to verify the result:

Python 3.8
from functools import wraps
def metrics(fn): 
    ncalls = 0
    name = fn.__name__
    @wraps(fn)
    def wrapper(*args, **kw):
        ncalls += 1
        print(f'{name} called {ncalls} times')
        
    return wrapper
@metrics 
def inc(n):
    return n + 1 
inc(3)

Explanation

When we have a variable, or name, in Python like cart = [‘lamp’]), we can do two operations:

  • Mutate the variable by changing the object that the variable is pointing to ( cart.append(‘mug’)).
  • Rebind the variable by having the variable point to another
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