MRO inconsistency: why?

[Ravi]

Why the following code gives inconsistent method resolution order
error:

class X(object):
x = 4
def f(self):
print 'f in X'
print dir(X)
X.g(self)
def g(self):
print 'g in X'

class Y(object, X):
def g(self):
print 'g in Y'

o = Y()
o.f()

While this code doesn't:

class X(object):
x = 4
def f(self):
print 'f in X'
print dir(X)
X.g(self)
def g(self):
print 'g in X'

class Y(X, object):
def g(self):
print 'g in Y'

o = Y()
o.f()

 

[Terry Reedy]

Why the following code gives inconsistent method resolution order
error:

class X(object): x = 4 def f(self): print 'f in X' print dir(X)
X.g(self) def g(self): print 'g in X'

class Y(object, X): def g(self): print 'g in Y'

o = Y() o.f()

Calculating a linear MRO from a non-tree inheritance graph is a fairly
arcane subject. Here, I believe, are the two rule relevant here:
1. Direct superclass must remain in the same order as in the class header.
2. A class cannot appear before any of its subclasses.
Violation of these rules can lead to unexpected anomalies.

(object, X) violates rule 2.
(X, object) violates rule 1.
There are no other possible orders.

The only reason to add object to the header first would be if you want Y
to only inherit new methods defined in X but not object methods
redefined in X. To do that, define a class Z that at least copies from
object those redefined methods and "class Y(Z,X)" should work.

While this code doesn't:

class X(object): x = 4 def f(self): print 'f in X' print dir(X)
X.g(self) def g(self): print 'g in X'

class Y(X, object): def g(self): print 'g in Y'

o = Y() o.f()

(X, object) satifies both rules. That said, putting object in the class
Y header is redundant since object would follow X in the MRO anyway!

Terry Jan Reedy

 

[Michele Simionato]

Why the following code gives inconsistent method resolution order
error:
<snip>

If you want to know all the nitty-gritty details about
the MRO (including the reason for the error you get)
you should read this:
http://www.python.org/download/releases/2.3/mro/

HTH,

M. Simionato