Missing interfaces in Python...

**Sybren Stuvel** · Apr 17 '06, 09:05 PM

Re: Missing interfaces in Python...

redefined.horiz ons@gmail.com enlightened us with:[color=blue]
> I see that Python is missing "interfaces ".[/color]

No it isn't. It just hasn't got them.
[color=blue]
> The concept of an interface is a key to good programming design in
> Java, but I've read that they aren't really necessary in Python.[/color]

[color=blue]
> In Java I would accomplish this by defining an IFixable interface
> that would be implemented by both the Car and Bus objects. Mechanic
> objects would work with any object implementing this interface.[/color]

In Python, you would simply call the functions you need. No need to
make things that rigidly defined.

Sybren
--
The problem with the world is stupidity. Not saying there should be a
capital punishment for stupidity, but why don't we just take the
safety labels off of everything and let the problem solve itself?
Frank Zappa

**Jonathan Daugherty** · Apr 17 '06, 09:15 PM

Re: Missing interfaces in Python...

# In Python, you would simply call the functions you need. No need to
# make things that rigidly defined.

Except when you need to handle exceptions when those methods don't
exist. I think interfaces can definitely be useful.

--
Jonathan Daugherty

http://www.parsed.org

**Fredrik Lundh** · Apr 17 '06, 09:35 PM

Re: Missing interfaces in Python...

Jonathan Daugherty wrote_
[color=blue]
> # In Python, you would simply call the functions you need. No need to
> # make things that rigidly defined.
>
> Except when you need to handle exceptions when those methods don't
> exist. I think interfaces can definitely be useful.[/color]

so with interfaces, missing methods will suddenly appear out of thin
air ?

</F>

**Egon Frerich** · Apr 17 '06, 09:55 PM

Re: Missing interfaces in Python...

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Have a look at Zope 3.
(http://www.zope.org/DevHome/Wikis/De...ure/FrontPage).
It has an interface implementation. You can use this implementation with
the apllication server Zope 3 or alone.

Regards,
Egon

redefined.horiz ons@gmail.com schrieb am 17.04.2006 22:39:
[color=blue]
> I'm coming from a Java background, so please don't stone me...
>
> I see that Python is missing "interfaces ". The concept of an interface
> is a key to good programming design in Java, but I've read that they
> aren't really necessary in Python. I am wondering what technique I can
> use in Python to get the same benefits to a program design that I would
> get with interfaces in Java.
>
> For example, if I want to have a program with a Car object, and a Bus
> object. I want both of these objects to present a common group of
> methods that can be used by Mechanic objects, but slightly different
> methods that can be used by Driver objects.
>
> In Java I would accomplish this by defining an IFixable interface that
> would be implemented by both the Car and Bus objects. Mechanic objects
> would work with any object implementing this interface.
>
> How would I approach this problem in Python? I think I would use an
> abstract class instead of an interface for IFixable, since Python
> supports multiple inheritance, but I'm not sure this is correct.
>
> Thanks for any suggestions.
>
> Scott Huey
>[/color]

- --
Egon Frerich, Freudenbergstr. 16, 28213 Bremen

E-Mail: e.frerich@nord-com.net
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**I V** · Apr 17 '06, 09:55 PM

Re: Missing interfaces in Python...

redefined.horiz ons@gmail.com wrote:[color=blue]
> I see that Python is missing "interfaces ". The concept of an interface
> is a key to good programming design in Java, but I've read that they
> aren't really necessary in Python. I am wondering what technique I can
> use in Python to get the same benefits to a program design that I would
> get with interfaces in Java.[/color]

To use interfaces in python, just what you would do in Java, except
don't use interfaces.

To expand on that slightly Zen answer, think about why you use
interfaces in Java. The interface declaration tells the compiler that
your object implements a specific set of functions, or that your
function expects an object that implements these functions. Then, at
run time, the actual type of the object is used to decide what function
to call.

However, python doesn't to any type checking at compile time, so it
just uses the dynamic type of the object to decide what function to
call.
[color=blue]
> How would I approach this problem in Python? I think I would use an
> abstract class instead of an interface for IFixable, since Python
> supports multiple inheritance, but I'm not sure this is correct.[/color]

Concretely:

class Car:
def fix(self):
print "Your car is ready, sir"

class Bus:
def fix(self):
print "Your bus is ready, sir"

class Mechanic:
def repair(self, customer, vehicle):
vehicle.fix()
customer.bill()

class Customer:
def bill(self):
print "Ouch, that was expensive"

me = Customer()
my_bus = Bus()
my_car = Car()
m = Mechanic()

m.repair(me, my_bus)
m.repair(me, my_car)

Which gives the output:

Your bus is ready, sir
Ouch, that was expensive
Your car is ready, sir
Ouch, that was expensive

If you try and repair something that can't be fixed:

m.repair(me, me)

you get:

Traceback (most recent call last):
File "test.py", line 30, in ?
m.repair(me, me)
File "test.py", line 14, in repair
vehicle.fix()
AttributeError: Customer instance has no attribute 'fix'

Obviously, you don't want this to happen when people use your program.
Java would check this at compile time, but as python doesn't, you can
write a unit test to check that the object's you want to implement the
relevant functions really do.

def is_fixable(obj) :
try:
obj.fix
except AttributeError:
return False
return True

assert is_fixable(Car( ))
assert is_fixable(Bus( ))
assert not is_fixable(Cust omer())
assert not is_fixable(Mech anic())

**Larry Bates** · Apr 17 '06, 09:55 PM

Re: Missing interfaces in Python...

redefined.horiz ons@gmail.com wrote:[color=blue]
> I'm coming from a Java background, so please don't stone me...
>
> I see that Python is missing "interfaces ". The concept of an interface
> is a key to good programming design in Java, but I've read that they
> aren't really necessary in Python. I am wondering what technique I can
> use in Python to get the same benefits to a program design that I would
> get with interfaces in Java.
>
> For example, if I want to have a program with a Car object, and a Bus
> object. I want both of these objects to present a common group of
> methods that can be used by Mechanic objects, but slightly different
> methods that can be used by Driver objects.
>
> In Java I would accomplish this by defining an IFixable interface that
> would be implemented by both the Car and Bus objects. Mechanic objects
> would work with any object implementing this interface.
>
> How would I approach this problem in Python? I think I would use an
> abstract class instead of an interface for IFixable, since Python
> supports multiple inheritance, but I'm not sure this is correct.
>
> Thanks for any suggestions.
>
> Scott Huey
>[/color]
Just thought I'd put in my 2 cents. You may want to take a look at
Zope 3 (www.zope.com). If I understand what you ware looking for, I
think they have already solved the problem (at least in one way). It
is at least worth a quick review.

You will find that most Python programmers bristle at words like
"missing", "enforcemen t" and "strictly defined the type". Python
programmers just don't work that way. The fact that programmers in
other languages must, is their loss.

-Larry Bates

**I V** · Apr 17 '06, 10:05 PM

Re: Missing interfaces in Python...

Jonathan Daugherty wrote:[color=blue]
> Except when you need to handle exceptions when those methods don't
> exist. I think interfaces can definitely be useful.[/color]

I think I see what you mean, but that's an odd way to put it.
Typically, you aren't going to handle the exceptions produced by type
errors. Of course, you want some way to test that your code doesn't
have type errors. Static type checking is one way of doing the
requisite testing, unit tests are another, but it's the tests that are
useful, not the interfaces per se. Adding interfaces to python, which
doesn't support static type checking, would be useless IMO.

**Roy Smith** · Apr 17 '06, 10:05 PM

Re: Missing interfaces in Python...

<redefined.hori zons@gmail.com> wrote:[color=blue]
> I see that Python is missing "interfaces ". The concept of an interface
> is a key to good programming design in Java, but I've read that they
> aren't really necessary in Python. I am wondering what technique I can
> use in Python to get the same benefits to a program design that I would
> get with interfaces in Java.[/color]

Python is a very dynamic language. Java is a very static language.
What that means is that in Java (like C++), you do a lot of error
checking at compile time. That's what interfaces are all about. In
Python, you do almost no error checking (beyond basic language syntax)
at compile time, and do everything at run time. For the most part,
this means wrapping things in try blocks and catching exceptions.
[color=blue]
>For example, if I want to have a program with a Car object, and a Bus
>object. I want both of these objects to present a common group of
>methods that can be used by Mechanic objects, but slightly different
>methods that can be used by Driver objects.
>
>In Java I would accomplish this by defining an IFixable interface that
>would be implemented by both the Car and Bus objects. Mechanic objects
>would work with any object implementing this interface.
>
>How would I approach this problem in Python? I think I would use an
>abstract class instead of an interface for IFixable, since Python
>supports multiple inheritance, but I'm not sure this is correct.[/color]

Well, let's say your IFixable interface in Java would have included
changeTire(), rechargeBattery (), and adjustBrakes() methods. In
Python, I'd just go ahead and implement those methods for both Car and
Bus classes. All Java's interface mechanism does for you is provide
some compile-time checking that those methods are implemented. In
Python, you would just call those methods when appropriate, and catch
any NameError exception that would happen if it turns out there is no
such method.

Consider that in Python, an object can have methods added to it after
it is created. It's entirely possible that the Car class has no
changeTire() method, but one would be added to each Car instance
sometime before the first place it might be called. Consider
something like:

if self.owner.hasR oadsideAssistan ce():
self.changeTire = callForHelp
elif self.owner.canF ixThings:
self.changeTire = getHandsDirty

Now, calling myCar.changeTir e() could end up calling callForHelp(), or
calling getHandsDirty() , or throwing NameError is no way exists to get
the tire fixed. Maybe that's what makes sense in your application.

**Terry Reedy** · Apr 17 '06, 11:55 PM

Re: Missing interfaces in Python...

<redefined.hori zons@gmail.com> wrote in message
news:1145306373 .781129.305840@ z34g2000cwc.goo glegroups.com.. .[color=blue]
> I'm coming from a Java background, so please don't stone me...[/color]

Most of us came to Python from some other language background ;-)
[color=blue]
> I see that Python is missing "interfaces ".[/color]

As someone else noted, Python objectively does not have 'interfaces' (or
'protocols') as an object type in the language. (But 'missing' is somewhat
subjective.) On the other hand, the concepts are very much part of the
language. See the article on duck typing, which could be called duck
interfacing, that someone gave a link for.

For example, an iterator (newer definition) is an object with an __iter__()
method returning self and a next() method that returns objects until it
raises StopIteration. An iterable is an object with an __iter__() method
that return an iterator. (Hence, iterators are conveniently iterables
also.) Instead of declaring that a class implements IterableInterfa ce, you
just implement it (and the corresponding iterator class if needed) and use
it anywhere an iterable is expected, which is lots places in the builtins
and standard library.
[color=blue]
> How would I approach this problem in Python? I think I would use an
> abstract class instead of an interface for IFixable, since Python
> supports multiple inheritance, but I'm not sure this is correct.[/color]

I believe this
[color=blue][color=green][color=darkred]
>>> NotImplementedE rror[/color][/color][/color]
<class exceptions.NotI mplementedError at 0x00974810>

was added so that people could go the route of abstract base classes with
stub functions.

Terry Jan Reedy

**Alex Martelli** · Apr 18 '06, 02:25 AM

Re: Missing interfaces in Python...

Jonathan Daugherty <cygnus@cprogra mmer.org> wrote:
[color=blue]
> # enforced by whom, at what point ?
>
> In the case of Java, I think the JVM enforces interface implementation
> (probably at the parser level).[/color]

"parser"... ?! If you have an 'Object o', say one just received as an
argument, and cast it to IBlahble, a la

IBlahble blah = (IBlahble) o;

....what can the parser ever say about it? It's clearly up to the
runtime system to "enforce" whatever -- raising the appropriate
exception if the "actual" (leafmost) class of o does not in fact
implement IBlahble (Java doesn't _really_ do compile-time static typing:
it just forces you to violate the "Don't Repeat Yourself" cardinal rule
by redundantly repeating types, as above, but then in general it checks
things at runtime anyway!).

Alex

**Jonathan Daugherty** · Apr 18 '06, 02:55 AM

Re: Missing interfaces in Python...

# "parser"... ?! If you have an 'Object o', say one just received as an
# argument, and cast it to IBlahble, a la
#
# IBlahble blah = (IBlahble) o;
#
# ...what can the parser ever say about it?

Maybe you didn't read the "I think" in my OP. Anyway, you clearly
know more about (or have more recent experience with) Java than I do.

--
Jonathan Daugherty

http://www.parsed.org

**Alex Martelli** · Apr 18 '06, 04:15 AM

Re: Missing interfaces in Python...

Jonathan Daugherty <cygnus@cprogra mmer.org> wrote:
[color=blue]
> # "parser"... ?! If you have an 'Object o', say one just received as an
> # argument, and cast it to IBlahble, a la
> #
> # IBlahble blah = (IBlahble) o;
> #
> # ...what can the parser ever say about it?
>
> Maybe you didn't read the "I think" in my OP. Anyway, you clearly
> know more about (or have more recent experience with) Java than I do.[/color]

My real-world experience with Java is very dated -- nowadays, I'm told,
the NEED to cast is vastly reduced by Java 1.5's "generics" (I haven't
yet written one line of Java 1.5, not even for "play" purposes, much
less "real world" ones;-). Still, all the casting that used to work
still works, so the purported "compile-time type safety" is worth as
much as the social compact to "don't use any of the container classes
that used to work until 1.4.*, but ONLY the very newest ones in 1.5"!-)
So much for "compiler enforcement", hm?-)

Alex

**Jonathan Daugherty** · Apr 18 '06, 04:35 AM

Re: Missing interfaces in Python...

# My real-world experience with Java is very dated -- nowadays, I'm
# told, the NEED to cast is vastly reduced by Java 1.5's "generics" (I
# haven't yet written one line of Java 1.5, not even for "play"
# purposes, much less "real world" ones;-).

Interesting; thanks.

# So much for "compiler enforcement", hm?-)

Yes, indeed. :)

--
Jonathan Daugherty

http://www.parsed.org

**Pablo Orduña** · Apr 18 '06, 09:05 AM

Re: Missing interfaces in Python...

This article in Guido van Rossum's blog might be interesting for this
thread

Interfaces or Abstract Base Classes?

http://www.artima.com/weblogs/viewpost.jsp?thread=92662

--
Pablo