[Python-Dev] Package ambiguities

7 Jun 2000

      We hit some very weird behaviour recently while setting up a package
hierarchy.  Robin Becker managed to distil this into a simple example.
Can anyone shed any light on what is happening below?  Is Python
behaving as it should?

Create a package A, empty __init__.py, with modules as follows:
--------parent.py----------
class Parent:
    pass

--------child.py------------
from parent import Parent
class Child(Parent):
    pass

---------test.py--------
from parent import *
class Examiner:
    def examine(self, arg):
        print 'examining argument:'
        print 'class of arg =', arg.__class__
        print 'bases of arg =', arg.__class__.__bases__
        print 'arg =', arg
        if isinstance(arg, Parent):
            print "arg is an instance of Parent"
        else:
            print "arg IS NOT an instance of Parent"
        print

if __name__=='__main__':
        from traceback import print_exc
        import sys
        def run0():
                from A.child import Child
                e = Examiner()
                e.examine(Child())

        def run1():
                from A.child import Child
                from A.test import Examiner
                e = Examiner()
                e.examine(Child())
        run0()
        run1()

Running this script produces the following output:

  C:\users\andy\A>test.py
  examining argument:
  class of arg = A.child.Child
  bases of arg = (<class A.parent.Parent at 7f9150>,)
  arg = <A.child.Child instance at 7f9410>
  arg IS NOT an instance of Parent

  examining argument:
  class of arg = A.child.Child
  bases of arg = (<class A.parent.Parent at 7f9150>,)
  arg = <A.child.Child instance at 7f83c0>
  arg is an instance of Parent

Many thanks,

Andy Robinson

[Python-Dev] Package ambiguities

andy＠robanal.demon.co.uk