In Python, why is list[] automatically global?

Question

This is a weird behavior.

Try this :

rep_i=0
print \"rep_i is\" , rep_i
def test():
  global rep_i #without Global this gives error but list , dict ,         


        

Answer 1

There is an error in python called UnboundLocalError which often confuses newcomers. The confusing thing is: future assignment does change the way a variable is looked up.

When the interpreter sees a variable name for the first time, it looks ahead to the end of current code block, and if you don't have an assignment to it anywhere within the same block of code, the interpreter considers it global. If you do, however, then it is considered local, and any reference to it before assignment generates an UnboundLocalError. That's the error you got. That's why you need to declare global rep_i. If you did not assign rep_i, you wouldn't need this line.

Also, this has nothing to do with variable type. Also, assigning or appending an item to the list (which you probably meant to do, but did not) is not assignment of the list itself, it is essentially calling a method on a list object, which is different from assignment: assignment creates a new object (possibly under a name that already exists), while manipulating a list just changes an existing list. You can try:

In [1]: # It won't work with small integers, as they are cached singletons in CPython

In [2]: a = 123123

In [3]: id (a)
Out[3]: 9116848

In [4]: a = 123123

In [5]: id(a)
Out[5]: 9116740

In [6]: # See, it changed

In [7]: # Now with lists

In [8]: l = [1,2,3]

In [9]: id(l)
Out[9]: 19885792

In [10]: l[1] = 2

In [11]: id(l)
Out[11]: 19885792

In [12]: # See, it is the same

In [13]: # But if i reassign the list, even to the same value

In [14]: l = [2,2,3]

In [15]: id(l)
Out[15]: 19884272