Python Variable Scope and Classes

Question

In Python, if I define a variable:

my_var = (1,2,3)

and try to access it in __init__ function of a class:

class MyClass:
 def __init__(self):
 print my_var

I can access it and print my_var without stating (global my_var).

If I put my_var right after class MyClass however, I get scope error (no global variable found).

What is the reason for this? How should I do this? Where can I read about this to learn? I did read Python Class page but I did not encounter its explanation.

Thank you

• Ashwini Chaudhary: it's not a global variable anymore when you put it inside class.

Answer 1

Complementing @mgilson's answer: Note that Python Class variables are shared among the class instances. And the behaviour might be VERY unexpected and seem weird. In practice it works like this:

class MyClass(object):
 my_var = 10
 def __init__(self):
 print(self.my_var)
m1 = MyClass()
print(m1.my_var)
>>> 10 # this is fine
MyClass.my_var = 20
print(m1.my_var)
>>> 20 # WTF? :) --> shared value
m2 = MyClass()
print(m2.my_var)
>>> 20 # this is expected
m1.my_var = 30
print(MyClass.my_var)
>>> 20 # this is also expected
MyClass.my_var = 40 
print(m1.my_var)
>>> 30 # But WHY? Isn't it shared? --> 
 # The value WAS shared until m1.my_var = 30 has happened.
print(m2.my_var)
>>> 40 # yep m2.my_var's value is still shared :)

• Phil: BasicWolf, since @mgilson's answer was a bit more on the subject, I could not help but choose it as the correct answer. But I sincerely thank you so much for this excellent, truly great explanation. It made me understand a lot, on top of what mgilson clearified. Thanks a lot!

Answer 2

When you put it right after class MyClass, it becomes a class attribute and you can get access to it via MyClass.my_var or as self.my_var from within the class (provided you don't create an instance variable with the same name).

Here's a little demo:

my_var = 'global'
class MyClass(object):
 my_var = 'class' 
 def __init__(self):
 print my_var #global
 print MyClass.my_var #class
 print self.my_var #class -- Only since we haven't set this attribute on the instance
 self.my_var = 'instance' #set instance attribute.
 print self.my_var #instance
 print MyClass.my_var #class

• ronak: if you put it in __init__, this means that this is only initialized when an object is created.
• mgilson: @ronak -- I'm not sure I understand your comment. Care to elaborate?
• jsbueno: @mgilson: beware: in this case the first `print self.my_var" will print the class variable (containing (4,5,6), not (1,2,3).
• jsbueno: While an instance attribute with the name my_var is not created, tries to access it will yield the class attribute instead - not the global variable.
• mgilson: @jsbueno -- I did run it beforehand -- the class variable is (1,2,3), the global is (4,5,6), I'm not sure I understand what you're saying ...

Answer 3

If you write:

class MyClass(object):
 my_var = (1, 2, 3)

you are defining an attribute of MyClass, not a variable. In your __init__ method, then, it is MyClass.my_var.

Answer 4

Once it's inside the class definition, it's no longer global, it's now in the class object's namespace. You can access it with self.my_var within __init__ though...