derived / base class name conflicts

[christopherlmarshall]

Suppose you want to write a subclass of some existing class you are
importing from a module you didn't write and that you don't want to
study the internals of, and you want to define a data member i in your
constructor.

As in the following:

from module1 import A

class B(A):
def __init__(self):
A.__init__(self )
self.i= 0
b= B()

Now, 'i' might have already been defined by A or by the call to
A.__init__() so if you define it without knowing that, you could be
changing the behavior of A's methods in unknown ways, which is
obviously a bad thing.

One way to avoid this is to run the following program to clear the name
'i' first:

from module1 import A
a= A()
print a.i

If you get an AttributeError, you know the name 'i' is safe to use. If
you actually get some sort of report from the print statement, then you
will know that 'i' is not safe to use.

This strikes me as a rather odd procedure to go through, but I don't
see any way around it.

It there some other way to handle this issue?

Do I actually need to study the sources for / implementation of
Tkinter.Canvas before I can safely subclass it (and assign attributes
to an instance of the subclass) or clear attribute names as I outlined
above?

Chris Marshall

[Steve Juranich]

This should prove most enlightening:

import Tkinter
dir(Tkinter.Can vas)

On 10 Nov 2005 14:53:04 -0800, ch************* *****@yahoo.com
<ch************ ******@yahoo.co m> wrote:

Suppose you want to write a subclass of some existing class you are
importing from a module you didn't write and that you don't want to
study the internals of, and you want to define a data member i in your
constructor.

As in the following:

from module1 import A

class B(A):
def __init__(self):
A.__init__(self )
self.i= 0
b= B()

Now, 'i' might have already been defined by A or by the call to
A.__init__() so if you define it without knowing that, you could be
changing the behavior of A's methods in unknown ways, which is
obviously a bad thing.

One way to avoid this is to run the following program to clear the name
'i' first:

from module1 import A
a= A()
print a.i

If you get an AttributeError, you know the name 'i' is safe to use. If
you actually get some sort of report from the print statement, then you
will know that 'i' is not safe to use.

This strikes me as a rather odd procedure to go through, but I don't
see any way around it.

It there some other way to handle this issue?

Do I actually need to study the sources for / implementation of
Tkinter.Canvas before I can safely subclass it (and assign attributes
to an instance of the subclass) or clear attribute names as I outlined
above?

Chris Marshall

--
http://mail.python.org/mailman/listinfo/python-list

--
Steve Juranich
Tucson, AZ
USA

[Fredrik Lundh]

ch************* *****@yahoo.com wrote:

Now, 'i' might have already been defined by A or by the call to
A.__init__() so if you define it without knowing that, you could be
changing the behavior of A's methods in unknown ways, which is
obviously a bad thing.

http://docs.python.org/tut/node11.ht...00000000000000

</F>

[christopherlmarshall]

Steve Juranich wrote:

This should prove most enlightening:

import Tkinter
dir(Tkinter.Can vas)

Huh?

Chris Marshall

[christopherlmarshall]

Fredrik Lundh wrote:

ch************* *****@yahoo.com wrote:

Now, 'i' might have already been defined by A or by the call to
A.__init__() so if you define it without knowing that, you could be
changing the behavior of A's methods in unknown ways, which is
obviously a bad thing.

http://docs.python.org/tut/node11.ht...00000000000000

</F>

I see. Thanks for the link.

So putting two underscores in front of an instance variable (or any
identifier used inside the scope of a class statement) invokes a name
mangling mechanism ( __x becomes __classname_x internally)

so the following would not result in any conflicts

class A:
def __init__(self):
self.__i= 0

class B(A):
def __init__(self):
A.__init__(self )
self.__i= 1

Is it commonplace to use underscores when defining derived class
instance variables, or is this considered against the grain of python?

Chris Marshall

[Ben Finney]

ch************* *****@yahoo.com wrote:

Suppose you want to write a subclass of some existing class you are
importing from a module you didn't write and that you don't want to
study the internals of

No need to study its internals. Fire up a Python interpreter and
inspect its outside:

import foomodule
dir(foomodule.F ooClass)

Any attributes named there should be avoided in your subclasses if you
don't want to clobber existing behaviour.

--
\ "Those who will not reason, are bigots, those who cannot, are |
`\ fools, and those who dare not, are slaves." -- "Lord" George |
_o__) Gordon Noel Byron |
Ben Finney

[bruno at modulix]

ch************* *****@yahoo.com wrote:
(snip)

So putting two underscores in front of an instance variable (or any
identifier used inside the scope of a class statement) invokes a name
mangling mechanism
(snip)
Is it commonplace to use underscores
I assume you mean double underscore...
when defining derived class
instance variables,
or is this considered against the grain of python?

I don't know if it's 'commonplace', but that's something I only do when
I absolutely want to protect a given attribute of a base class from
being accidentally overriden by a derived class. Else I use the usual
convention ('_name' => implementation, 'name' => API), and assume users
of my code will read the documentation (or the source FWIW) before
subclassing. Note that since 'users of my code' are mostly my coworkers
and I, this is a quite sensible assumption !-)

--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'o****@xiludom. gro'.split('@')])"

[christopherlmarshall]

I see what you mean now.

It would indeed be enlightening if I wanted to study the internals of
Tkinter, and perhaps one day I will.

[christopherlmarshall]

Your suggestion ('_name' -> implementation, 'name' -> API) makes sense
as a convention between programmers that know a fair amount about each
other's classes before using them.

I don't think it is reasonable in general to only subclass from base
classes you have studied the full API of, however. The double
underscore is a decent solution to my problem.

I imagine it must be used a lot in domains where people are making
heavy use of third party python code.

Chris Marshall

[Alex Martelli]

<ch************ ******@yahoo.co m> wrote:
...

I don't think it is reasonable in general to only subclass from base
classes you have studied the full API of, however. The double

I think you underestimate the level of coupling that inevitably occurs
between base and derived classes. In general, such coupling being
strong, knowing the full API of the base class is inevitable (although
in some special corner-case, when you do only need to add private data,
you may, exceptionally, get away with less knowledge).
Alex