Python: variable scope and "instance has no attribute"

Question

I am confused about instance and class variables in python. I am confused about how this code:

class A:
 def _init__(self):
 print "initializer called"
 #l is an instance variable
 self.l = []
 print len(l)
 def printList(self):
 print len(self.l)
a = A()
print "here"
a.printList()

produces the following output:

here
Traceback (most recent call last):
 File "C:\Python27\a.py", line 16, in <module>
 a.printList()
 File "C:\Python27\a.py", line 10, in printList
 print len(self.l)
AttributeError: A instance has no attribute 'l'

Is the initializer not being called? Why does "here" print, but the print statement in the initializer doesn't? Should 'l' be a class variable? I thought making it an instance variable using 'self' would be sufficient, is that not correct? Am I incorrect about the scope of these variables?

I have looked at these sources, but they didn't help:

Python: instance has no attribute

Python: Difference between class and instance attributes

Python - Access class variable from instance

Python Variable Scope and Classes instance has no attribute

• John Gordon: _init__ only has one underscore at the beginning. It needs two.

Answer 1

You defined a method named _init__, not __init__. The number of underscores is important. Without the exact correct name, it's just a weirdly named method, not the instance initializer at all. You end up using the default initializer (which sets no attributes). You also have an error in your _init__ (you refer to l unqualified). Fixing the two, your initializer would be:

def __init__(self): # <-- Fix name
 print "initializer called"
 #l is an instance variable
 self.l = []
 print len(self.l) # <-- Use self.l; no local attribute named l