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

it's not a global variable anymore when you put it inside class. — Ashwini Chaudhary
– Ashwini Chaudhary, Commented Oct 17, 2012 at 19:21

Zaur Nasibov · Accepted Answer · 2014-12-18 09:25:07Z

12

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 :)

edited Dec 18, 2014 at 9:25

answered Oct 17, 2012 at 19:42

Zaur Nasibov

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1 Comment

Phil Over a year ago

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!

mgilson · Accepted Answer · 2012-10-17 19:42:04Z

11

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

edited Oct 17, 2012 at 19:42

answered Oct 17, 2012 at 19:20

mgilson

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7 Comments

ronak Over a year ago

if you put it in __init__, this means that this is only initialized when an object is created.

mgilson Over a year ago

@ronak -- I'm not sure I understand your comment. Care to elaborate?

jsbueno Over a year ago

@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 Over a year ago

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 Over a year ago

@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 ...

|

kindall · Accepted Answer · 2012-10-17 19:21:09Z

2

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.

answered Oct 17, 2012 at 19:21

kindall

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Comments

retracile · Accepted Answer · 2012-10-17 19:20:58Z

1

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...

answered Oct 17, 2012 at 19:20

retracile

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