#! /usr/bin/env python

"""
This file intended to be used within an interactive instruction session

It also happens to be an example of bad programming style yet useful
for a training session.
"""


VERBOSE = True
#VERBOSE = None


class Breakfast1:
    serving = ["spam & eggs", "toast"]
    skipping = []
    ordered = None

if VERBOSE: print """
Beware subtle Python behavior: sharing pre-defined values

	If you have more than one instance of a class with pre-defined
        fields, all instances access the same lists.
"""

alice = Breakfast1()
bob = Breakfast1()


if VERBOSE: print """
	Not only do they have identical content...
"""

if bob.serving == alice.serving:
    print "Yes.  Both Bob and Alice have identical VALUES."
else:
    print "No.  Bob and Alice have different values."


if VERBOSE: print """
	Both reference the exact SAME object!
"""

if bob.serving is alice.serving:
    print "Yes.  Both Bob and Alice REFERENCE the SAME data."
else:
    print "No.  Bob and Alice REFERENCE DIFFERENT data."



if VERBOSE: print """
	So then, appending the list of what Bob is skipping with his
breakfast order effects Alice's too, even though it might be only
Bob who wants spam & eggs without the spam.
"""

bob.skipping.append("spam")
print "Bob skipping:", bob.skipping
print "Alice skipping:", alice.skipping

if VERBOSE: print """
	The above subtlety applies to any pre-assigned value other than
None.
"""


bob.ordered = 1
print "Bob ordered:", bob.ordered
print "Alice ordered:", alice.ordered

if VERBOSE: print """
	As Python permits creating class attributes "on demand" unlike
compiled object oriented languages, you can identify unshared fields
by assigning a value of None (think: NULL).
"""

#End.