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I have a questions regarding the inner workings of the global statement in Python. With the syntax

def f():
    global x
    x = 1

Variable x is turned into a global variable appears in globals() and is accessible outside of f. So far understood.

Until now I assumed, that with global x the name x is defined and should reference to a memory address, however if x is not initialized, such as

def f():
    global x
id(x)

a name "x" not defined error pops up.

Therefore I am wondering what immediate effect the global x line has in the background,as neither locals, globals, vars seem to be affected by the line shown by the comparison below:

def test():
    global x
    print("Vars")
    print([i for i in vars().keys() if i[0] != "_"])
    print("Globals")
    print([i for i in globals().keys() if i[0] != "_"])
    print("Locals")
    print([i for i in locals().keys() if i[0] != "_"])
    x = 1
    print("Vars")
    print([i for i in vars().keys() if i[0] != "_"])
    print("Globals")
    print([i for i in globals().keys() if i[0] != "_"])
    print("Locals")
    print([i for i in locals().keys() if i[0] != "_"])
    
test()

After global x the scopes are all empty, and after assignment, x is rightfully set as global.

Vars
[]
Globals
['In', 'Out', 'get_ipython', 'exit', 'quit', 'test']
Locals
[]
Vars
[]
Globals
['In', 'Out', 'get_ipython', 'exit', 'quit', 'test', 'x']
Locals
[]

def test():
    print("Vars")
    print([i for i in vars().keys() if i[0] != "_"])
    print("Globals")
    print([i for i in globals().keys() if i[0] != "_"])
    print("Locals")
    print([i for i in locals().keys() if i[0] != "_"])
    x = 1
    print("Vars")
    print([i for i in vars().keys() if i[0] != "_"])
    print("Globals")
    print([i for i in globals().keys() if i[0] != "_"])
    print("Locals")
    print([i for i in locals().keys() if i[0] != "_"])
    
test()

Without global x it is local

Vars
[]
Globals
['In', 'Out', 'get_ipython', 'exit', 'quit', 'test']
Locals
[]
Vars
['x']
Globals
['In', 'Out', 'get_ipython', 'exit', 'quit', 'test']
Locals
['x']

So basically the question is, where the heck does python save the information of x going to be global between definition and assignment.

0x5453
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FloLie
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    The line `global x` has no immediate effect. It doesn't define `x`. It simply tells the interpreter that when you use `x` you're intending to refer to a global variable named `x` instead of a local one. It should be separately defined at the module scope. – Kemp Jul 22 '21 at 14:27
  • Does this answer your question? https://stackoverflow.com/questions/52310792/python3-globals-and-locals-contents – PIG208 Jul 22 '21 at 14:28
  • Why do you expect this information to be exposed at runtime? `global` tells the interpreter how to interpret future references to `x`, but by itself is not required to have any observable side effects. Somewhat similar to how the line `x: int` does nothing by itself, but still can produce linting errors if followed by `x = ''`. – 0x5453 Jul 22 '21 at 14:32

0 Answers0