Question

Please see the following code:

def good():
  foo[0] = 9         # why this foo isn't local variable who hides the global one

def bad():
  foo    = [9, 2, 3] # foo is local, who hides the global one

for func in [good, bad]:
  foo = [1,2,3]

  print('Before "{}": {}'.format(func.__name__, foo))
  func()
  print('After  "{}": {}'.format(func.__name__, foo))

The result is as below:

# python3 foo.py
Before "good": [1, 2, 3]
After  "good": [9, 2, 3]
Before "bad" : [1, 2, 3]
After  "bad" : [1, 2, 3]

Answer 1

Because you’re not setting foo, you’re getting something in foo (foo[0] to be exact).

In bad you create a new variable foo. In good you do something like foo.set(0, 9) (set item 0 to value 9). Which is using a variable, and not defining a new name.