Python: I am a bit lost in local and global scope...

Question

def ZeroRecursive(a):
if a == []:
return
a[0] = 0
ZeroRecursive(a[1:])
a = [1,2,3,4]
ZeroRecursive(a)
# [0, 2, 3, 4]

Why only the first digit will be zero in the output? I am a bit lost in local and global variables here...

Answer 1

I think maybe it's because `a[1:]` is a slice, which is a different `list` object, not the actual `list` <a>.

Answer 2

Prof. Dr. Zoltán Vass
I would avoid using the same identifier for function argument and actual object to pass as argument. Especially if the actual object is global 😁

By using a different name for parameter and actual object (passed as argument), plus a little print outs, it becomes clearer which one is being modified in the function.

def ZeroRecursive(b):
print(f'PRE-CHECK\nList <a> is {a} and argument <b> is {b}')
if b == []:
print(f'Argument <b> is {b}. Leave recursion.')
return
b[0] = 0
print(f'POST-CHECK\nList <a> is {a} and argument <b> is {b}\n')
ZeroRecursive(b[1:])

a = [1,2,3,4]
ZeroRecursive(a)
print(f'\nFinally List <a> is {a}') # [0, 2, 3, 4]

Answer 3

Ipang I think so but why only the first digit of the global list is replaced by 0?

Answer 4

Prof. Dr. Zoltán Vass
The actual `list` <a> is only passed on and modified at the first recursion phase, the successive recursion phases only modifies a slice of the previous recursion phase. A slice obviously be a different `list` object, so I guess that is why only the first element of actual `list` <a> is actually getting modified.