About assigning value to variables in Python

Question

Here I wrote two simple programs and expected the results to be same: program 1(result=123):

price=[]
def checkCondition(a,b):
    if a<b:
        price.append(123)
if  __name__ == '__main__':
    checkCondition(1,2)
print price[0]

program 2(result=''):

price=''
def checkCondition(a,b):
    if a<b:
        price=123
if  __name__ == '__main__':
    checkCondition(1,2)
print price

Why it couldn't assign 123 to price in program 2?

These are not the same operation at all. In the first you are mutating an object, in the second you are reassigning something. — Daniel Roseman, Jun 12 '15 at 18:22
@TigerhawkT3: I disagree,it's not a dupe since there's no parameter at all. — zw324, Jun 12 '15 at 18:26

tynn · Accepted Answer · 2015-06-12T18:33:18.570

You're assigning a local variable. Tell Python to use the global variable instead:

price=''
def checkCondition(a,b):
    global price # now reassigning price
    if a<b:
        price=123
if  __name__ == '__main__':
    checkCondition(1,2)
print price

From the Python doc Naming and binding:

If a name binding operation occurs anywhere within a code block, all uses of the name within the block are treated as references to the current block. [...]

If the global statement occurs within a block, all uses of the name specified in the statement refer to the binding of that name in the top-level namespace.

score 0 · Answer 2 · answered Jun 12 '15 at 18:26

There are two types of python variables: globals and locals. Globals are reachable from all positions in your code. But locals are only reachable from the def() - definition they were created from. By default variables you create in a function are local and so there is a global price (='') amd a local price in your function(=123). To make it global and replace the existing global variable with the new one type on the beginning of your function definition:

global price

This makes the price variable in your functuon global.

The first one worked because your were not making a local variable. You did something on an existing variable (price). If the existing variable is global or not is not important then.

score 0 · Answer 3 · answered Jun 12 '15 at 18:26

You need to tell the interpreter that you are referring to global name, as names inside functions are local to their scope by default:

price=''
def checkCondition(a,b):
    global price
    if a<b:
        price=123
if  __name__ == '__main__':
    checkCondition(1,2)
print price

zw324 · Answer 4 · 2015-06-12T18:28:45.880

Note that inside a function, you can use a global variable, but can't change it without declaring it with global (see here).

The quote in the doc that matters:

It would be impossible to assign to a global variable without global, although free variables may refer to globals without being declared global.

The first program used the variable by calling a method, which is okay and it price resolves to the global variable. For the second you intended to change a global variable's value, which isn't doable unless the variable is declared with global. Since you can't change the global variable's value, it will create a new local variable and assign the value to it.

To fix it, you need to use global to declare it first:

price=[]
def checkCondition(a,b):
    global price
    if a<b:
        price.append(123) # also works without the global
        price = 123
if  __name__ == '__main__':
    checkCondition(1,2)
print price

About assigning value to variables in Python

4 Answers4