Question

I have a question about local and global variables and global objects in Python. Look at this code:

var = 0

class Test():
    var = 0

test = Test()

def sum():
    Test.var += 1
    test.var += 1
    var += 1

sum()

If I run that code, the exception is triggered in the line “var += 1” only. The two previous lines work. I read this question from the Python FAQ. I think that there is no exception in the first two lines of the function because the “Test” and “test” are referenced. The member “var” is assigned, but “Test” and “test” are global because are referenced to get the member. The FAQ said: “In Python, variables that are only referenced inside a function are implicitly global. If a variable is assigned a new value anywhere within the function’s body, it’s assumed to be a local.”

So, the question is… is my assumption true?

Answer 1

I think that there is no exception in the first two lines of the
function because the “Test” and “test” are referenced.

Correct. And they refer to the class attribute var, not the global one you defined.

The member “var” is assigned, but “Test” and “test” are global because are referenced to get the member.

Or to put it another way, Test and test are available in the global namespace so Test.var and test.var work.

If the value of var was not changed in sum(), you would get 0 since the lines above it have changed the Test class attribute not the global. Adding some prints in sum and removing the var += 1

def sum():
    Test.var += 1
    print Test.var
    test.var += 1
    print test.var
    print var

sum()

…gives:

1
2
0

But the moment I try to assign a value to var within the sum function, I get an error even before that line:

>>> def sum():
...     Test.var += 1
...     print Test.var
...     test.var += 1
...     print test.var
...     print var
...     var += 1
...     print var
... 
>>> sum()
1
2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in sum
UnboundLocalError: local variable 'var' referenced before assignment

Because var is now being assigned a value in sum(), it’s considered local but has not been defined prior to that line. (Which implies that python is doing some ‘looking ahead’ or checking variable scope in sum() since it raised the error for the first print var before var was re-assinged. Putting var = 50 instead of var += 1 raises the same error.)

To work with the global var:

def sum():
    Test.var += 1
    print Test.var
    test.var += 1
    print test.var
    global var #added the global keyword
    print var
    var += 1
    print var

output:

1
2
0    # notice that global var is still 0 because the above var+=1 are for Test.var
1

---

Edit: Regarding the ‘look ahead’ behaviour I mentioned. Was going to post a question about it but it’s been explained well in this answer: https://stackoverflow.com/a/370380/1431750 (to Python variable scope error)