__Classes and type tests

[Brian van den Broek]

Hi all,

The code below exhibits an attempt to refer to the type of a __Class
from within a method of that class. I've been unable to figure out how
to make it work as I want, and would appreciate any insight.

The problem emerged out of a bad design that the good folks on the
tutor list helped me repair. With that repair, I no longer need nor
want to do this sort of thing in actual code. But the academic issue
"How/Can it be done?" still itches.
class _OneUnderBase(object):
def __init__(self):
if type(self) == _OneUnderBase:
print "From _OneUnderBase"
else:
print "From subclass",

class __TwoUnderBase(object):
def __init__(self):
# What to write in the if type() test to secure same
# behaviour as _OneUnderBase
if type(self) == __TwoUnderBase:
print "From __TwoUnderBase"
else:
print "From subclass",

class SubOne(_OneUnderBase):
def __init__(self):
super(SubOne, self).__init__()
print "SubOne"

class SubTwo(__TwoUnderBase):
def __init__(self):
super(SubTwo, self).__init__()
print "SubTwo"

s1 = SubOne()
s2 = SubTwo()
When run, this gives:

From subclass SubOne
Traceback (most recent call last):
File "C:/Documents and Settings/Owner/My
Documents/PythonFiles/foo.py", line 28, in ?
s2 = SubTwo()
File "C:/Documents and Settings/Owner/My
Documents/PythonFiles/foo.py", line 24, in __init__
super(SubTwo, self).__init__()
File "C:/Documents and Settings/Owner/My
Documents/PythonFiles/foo.py", line 12, in __init__
if type(self) == __TwoUnderBase:
NameError: global name '_TwoUnderBase__TwoUnderBase' is not defined

So, the _OneUnderBase version works fine. I've been unable to work out
what to write in the __TwoUnderBase version to make it work the same.

(In case it is not clear, I'm aiming at the output:
From subclass SubOne
From subclass SubTwo
)

I *think* I understand how __ name mangling works when making
references from one module to another. But I cannot work it out
reference to the class itself from within a __ class.

Should it matter: Python 2.4.2 on Win32.

Thanks,

Brian vdB

[Peter Otten]

Brian van den Broek wrote:

The code below exhibits an attempt to refer to the type of a __Class
from within a method of that class. I've been unable to figure out how
to make it work as I want, and would appreciate any insight.

The problem emerged out of a bad design that the good folks on the
tutor list helped me repair. With that repair, I no longer need nor
want to do this sort of thing in actual code. But the academic issue
"How/Can it be done?" still itches.

Define the method outside the class to defeat mangling:

def __init__(self): .... if type(self) == __Two:
.... print "two leading underscores"
.... class __Two(object): .... __init__ = __init__
.... __Two()

two leading underscores
<__main__.__Two object at 0x4029360c>

Peter