Global variable remains the same

Question

I am trying to change a global variable, but it still goes back to the original value. For example, look at the response I get from this code:

0
prevValue before 0
prevValue changed 644.1324516367487
1
prevValue before 0
prevValue changed 67.37908666424572

I would like the prevValue in the second iteration to be 644.

Here is my code:

prevValue = 0

def csvData():
    with open('test.csv', 'w', newline='') as f:
        thewriter = csv.writer(f)
        thewriter.writerow(['prevValue'])
        for x in range(2):
            print(x)
            parse_results(thewriter,prevValue)

def parse_results(thewriter,prevValue):
    value = getValue()
    thewriter.writerow([prevValue])
    print('prevValue before', prevValue)
    prevValue = value
    print('prevValue changed', prevValue)

Asked By: Jonathan

||

Source

Answer 1

In Python, if there is a variable in the global scope with the same name as a variable in a function, the variable in the function is a reference to another memory block if you bind that variable name in the function.

Example:

foo = 42

def spam():
    foo = "spam"
    print(foo)  # spam

spam()
print(foo)  # 42

But if you do not bind the variable to a value, if you read the value, you read the global value.

Example:

foo = 42

def print_global():
    print(foo)  # 42

spam()

So how to change the value of a global? You must use the global keyword:
Example:

foo = 42

def spam():
    global foo
    foo = "spam"
    print(foo)  # spam

spam()
print(foo)  # spam

Be careful, it’s not because you haven’t bind the variable to a value yet that python will let you read a global variable:
Example:

foo = 42

def spam():
    print(foo)  # Error : foo is undefined
    foo = "spam"
    print(foo)

spam()

In that case, since you bind foo to a value in the function, python will disable any access to the global variable in the whole function.

Answered By: Dorian Turba

Global variable remains the same

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