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Hello.
I have found a rather annoying bug in Python, present in both Python 1.5
and Python 2.0.
If a class has an argument with a default of an empty dictionary, then
all instances of the same class will point to the same dictionary,
unless the dictionary is explictly defined by the constructor.
I attach a piece of code that demostrates the problem
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"""
Bug description:
A class is defined. in the __init__ method, we define an options "attribs"
argument, which defaults to {}.
We create two instances of class foo, each of them without argument.
we then modify the attribs attribute in one of them. in a suprising manner,
the change if reflected in BOTH instances, where it should only appear in the
first one.
Workaround:
explictly define an empty dictionary as the argument, or define the empty dictionary
inside the method body.
"""
class foo:
def __init__(self,attribs={}):
self.attribs=attribs;
return None;
print "";
print "Defining Two instances of class foo:";
print "a=foo()"
print "b=foo()"
a=foo();
b=foo();
print "";
print "The 'attribs' attribute of both looks like this:";
print "a.attribs = %s" % a.attribs
print "b.attribs = %s" % b.attribs
print ""
print "Now we modify 'attribs' in a:"
print 'a.attribs["bug"]= "exists"';
a.attribs["bug"]= "exists";
print ""
print "Now, things should now look like this:"
print "a.attribs = %s" % a.attribs
print "b.attribs = %s" % "{}";
print ""
print "However, things look like this:"
print "a.attribs = %s" % a.attribs
print "b.attribs = %s" % b.attribs
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