How do I check if a static variable exists in Python?

Question

This is a related question: How do I check if a variable exists?

However, it did not work well for static variables.

What I am trying to do is the following,

class A:
    def __init__(self):
        if A.var is null: # this does not work, okay
            A.var = 'foo'
            print 'assigned'

Okay, since A.var is not even assigned. It raises en error. So, I tried this:

class A:
    def __init__(self):
        if 'A.var' not in globals(): # this seems to okay, but ..
            A.var = 'foo'
            print 'assigned'

a = A()
b = A()

It results:

assigned
assigned

Which shows that if 'A.var' not in globals(): line not working properly.

So, how do I check if a static variable exists in Python?

Answer 1

Either you use hasattr:

if not hasattr(A, 'var'):
    A.var = 'foo'

or, as some would prefer according to the "Easier to ask for forgiveness than permission" principle:

try:
    A.var
except NameError:
    A.var = 'foo'

Finally, you can simply define the default value in the class body:

class A(object):
    var = None
    ...

if A.var is None:
    a.var = 'foo'

(Note that neither approach is thread-safe)