如何检查全局变量是否存在，如果不存在则将其定义为全局变量？

Question

I wish to do :

if var not in globals():
     global var
     var = -1

or

try : var
except NameError :
    global var
    var = -1

The problem is :

global var
^
SyntaxError: name 'var' is used prior to global declaration

So what can be done to achieve similar effect ?

---

EDIT :

This is not at module level, but rather in a function. The variable is supposed to store a position value that the function uses and updates in it but needs to persist between calls.

I have been made aware that the above code is bad practise, and I agree, and request those agreeing to suggest alternative "good practise" methods, if possible.

---

EDIT 2 :

This question is required to help me fix my previous question . There, the obj_cnt is getting reset at every call, bugging up the function that accesses objects from a pickled file by an "index" (an abstraction), as I try to avoid unnecessary seeking and reading by querying indexes in sorted order.

The above should clear up the "Why?" part.

---

EDIT 3:

I did not end up needing to do this convoluted stuff. I solved the above question by using classes.

Nevertheless, thanks to everyone who helped me here.

Answer 1

It is considered bad practice to alter the global variables (see Why are global variables evil? ) , but for the sake of knowledge, you can do:

if 'var' not in globals():
     globals()['var'] = var
     var = -1

for what you're trying to do.

Answer 2

The global keyword doesn't unilaterally declare a variable name as a module level variable. It is used within a function to declare that assignments to a variable should be in the module namespace instead of the function instance namespace. The global keyword makes no sense at module level because the module namespace is already the place variable assignments go.

So, just leave the global out.

try:
    var
except NameError:
    var = 1
print(var)

In a function you need the global, but it needs to be before first use.

def foo():
    global var
    try:
        var += 1
    except NameError:
        print("making var")
        var = 1
    return var

print(foo())
print(foo())

Outputs

making var
1
2