CODEWITHSUNDEEP
python
Bob5421
Bob5421

Reputation: 9073

Strange behaviour with python global variables

Look at this very basic code:

s2 = 'prefixe'
cpt = 1

def test():
    cpt += 1
    str = "%s%d" % (s2,cpt)
    print(str)

test()

I have an error. It says that cpt is read before assignment. It is normal to my opinion because cpt should be declared as a global variable:

s2 = 'prefixe'
cpt = 1

def test():
    global cpt
    cpt += 1
    str = "%s%d" % (s2,cpt)
    print(str)

test()

In this case, i have no error and the program works fine.

But, why there is no error for s2 variable ? This variable should be declared as a global variable too ? Why do not i have error ?

Thanks

Upvotes: 1

Views: 536

Answers (2)

Aaron Brock
Aaron Brock

Reputation: 4536

From the Python Docs

In Python, variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.

Since s2 is only read (not assigned) inside the function it's implicitly global, and there is no error. However, if you tried to modify s2 it would throw an error unless you define it as global, since by default it would assume the variable is local, and there isn't a local variable named s2.

Personally, I agree that this is a bit unintuitive.

Upvotes: 2

Vishal Taj PM
Vishal Taj PM

Reputation: 1359

You can refer this:

Using global variables in a function other than the one that created them

You can use a global variable in other functions by declaring it as global in each function that assigns to it:

s2 = 'prefixe'    
cpt = 1

def test():
    global cpt # Needed to modify global copy of cpt
    cpt += 1
    str = "%s%d" % (s2,cpt) # No need for global declaration to read value
    print(str)

test()

I imagine the reason for it is that, since global variables are so dangerous, Python wants to make sure that you really know that's what you're playing with by explicitly requiring the global keyword.

Upvotes: 1

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