list and global variable： why there is no "global a" in test().

Question

a = [10,20]

def test(m):
m.append(30)

test(a)

print(a)

a is a global variable and it's changed by append in test(), but why there is no "global a" statement.

Answer 1

When you give an object like a list as argument in a function, it doesn't copy the object but modify it.

Answer 2

That's hard to understand if you never learn c or c++ language.