What is proper way to require a method to be overridden?

[jeremito]

I am writing a class that is intended to be subclassed. What is the
proper way to indicate that a sub class must override a method?

Thanks,
Jeremy

[Thomas Ploch]

jeremito schrieb:

I am writing a class that is intended to be subclassed. What is the
proper way to indicate that a sub class must override a method?

Thanks,
Jeremy

What do you mean by 'indicate'? Writing it to the docstring of the
class/method? Writing a comment?

class Foo:
"""
When inheriting from Foo, method foo must be
overridden. Otherwise SPAM.
"""
def foo(self):
print 'bar'

class Bar(Foo):
def __init__(self):
Foo.__init__(se lf)

# Has to be defined to override the base class's method
# when inheriting from class Foo. Otherwise: SPAM
def foo(self):
print 'foo'

I don't know any other way.

Thomas

[Robert Kern]

jeremito wrote:

I am writing a class that is intended to be subclassed. What is the
proper way to indicate that a sub class must override a method?

raise NotImplementedE rror

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

[Gabriel Genellina]

At Thursday 4/1/2007 23:52, jeremito wrote:

>I am writing a class that is intended to be subclassed. What is the
proper way to indicate that a sub class must override a method?

If any subclass *must* override a method, raise NotImplementedE rror
in the base class (apart from documenting how your class is supposed
to be used).
--
Gabriel Genellina
Softlab SRL


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[Thomas Ploch]

Gabriel Genellina schrieb:

At Thursday 4/1/2007 23:52, jeremito wrote:

>I am writing a class that is intended to be subclassed. What is the
proper way to indicate that a sub class must override a method?

If any subclass *must* override a method, raise NotImplementedE rror in
the base class (apart from documenting how your class is supposed to be
used).

I learn so much from this list. I didn't even know this error existed.

Thomas

[jeremito]

Gabriel Genellina wrote:

At Thursday 4/1/2007 23:52, jeremito wrote:

I am writing a class that is intended to be subclassed. What is the
proper way to indicate that a sub class must override a method?

If any subclass *must* override a method, raise NotImplementedE rror
in the base class (apart from documenting how your class is supposed
to be used).
--
Gabriel Genellina
Softlab SRL

Thanks, that's what I needed. Since I am a complete novice at
Exceptions, I'll have to learn about it.
Jeremy

[Grant Edwards]

On 2007-01-05, Thomas Ploch <Th**********@g mx.netwrote:

>>I am writing a class that is intended to be subclassed. What
is the proper way to indicate that a sub class must override a
method?

If any subclass *must* override a method, raise
NotImplemented Error in the base class (apart from documenting
how your class is supposed to be used).

I learn so much from this list. I didn't even know this error existed.

And remember: even if it didn't, you could have created your
own:

------------------------------foo.py------------------------------
class NotImplementedE rror(Exception) :
pass

def foo():
print "hi there"
msg = "there's a penguin on the telly!"
raise NotImplementedE rror(msg)
print "how are you?"

foo()
------------------------------------------------------------------

$ python foo.py
hi there
Traceback (most recent call last):
File "foo.py", line 10, in ?
foo()
File "foo.py", line 7, in foo
raise NotImplementedE rror(msg)
__main__.NotImp lementedError: there's a penguin on the telly!
A few carefully thought-out exceptions can often eliminate the
need for a lot of messy code.

--
Grant Edwards grante Yow! I'll show you MY
at telex number if you show
visi.com me YOURS...

[Thomas Ploch]

Grant Edwards schrieb:

On 2007-01-05, Thomas Ploch <Th**********@g mx.netwrote:

>>>I am writing a class that is intended to be subclassed. What
is the proper way to indicate that a sub class must override a
method?
If any subclass *must* override a method, raise
NotImplemente dError in the base class (apart from documenting
how your class is supposed to be used).

I learn so much from this list. I didn't even know this error existed.

And remember: even if it didn't, you could have created your
own:

Erm, it wasn't me who asked. I just wanted to say that I didn't know
that there is a NotImplementedE rror. Havn't seen it before.

:-)

Thomas

[Carl Banks]

jeremito wrote:

I am writing a class that is intended to be subclassed. What is the
proper way to indicate that a sub class must override a method?

You can't (easily).

If your subclass doesn't override a method, then you'll get a big fat
AttributeError when someone tries to call it. But this doesn't stop
someone from defining a subclass that fails to override the method.
Only when it's called will the error show up. You can, as others have
noted, define a method that raises NotImplementedE rror. But this still
doesn't stop someone from defining a subclass that fails to override
the method. The error still only occurs when the method is called.

There are some advantages to using NotImplementedE rror:

1. It documents the fact that a method needs to be overridden
2. It lets tools such as pylint know that this is an abstract method
3. It results in a more informative error message

But, in the end, if someone wants to define a class that defiantly
refuses to declare a method, you can't stop them.
Carl Banks

[Grant Edwards]

On 2007-01-05, Thomas Ploch <Th**********@g mx.netwrote:

>>I learn so much from this list. I didn't even know this error
existed.

And remember: even if it didn't, you could have created your
own:

Erm, it wasn't me who asked. I just wanted to say that I didn't know
that there is a NotImplementedE rror.

Sorry, I sort of lost track. You can still invent your own
Exceptions anyway. ;) Just don't do what I do and take the
lazy way out:

raise "method not implemented"

That's considered bad form these days.

--
Grant Edwards
gr****@visi.com