strategy pattern and non-public virtual functions

[pythoncurious@gmail.com]

Hi python experts

In C++ I can do something like this:
class Base {
public:
void f() { this->f_(); }
private:
virtual void f_() = 0;
};

class Derived : public Base {
private:
void f_() { // Do something }
};

int main() {
Derived d;
d.f();
}

The point of this is that the a number of classes will inherit from
Base and only implement a private member function that only will be
accessed from the base class public 'f' function.
The Base::f() can then perform validation of input/return values, add
logging and things like that.
The users of the derived classes are unable to bypass this base class
function.

I've been wanting to do the same thing in python, to make sure that
there is no confusion about what function to call.

Just translating this code to python won't work, due to the name
mangling of private functions:
class B(object):
def f(self):
self.__f()

class D(B):
def __f(self):
pass

d = D()
d.f()

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 3, in f
AttributeError: 'D' object has no attribute '_B__f'

So my questions are:
1. Is there a "pythonic" way to do what I'm trying to do?
2. Should I be doing this at all? Any thoughts?

[Duncan Booth]

Re: strategy pattern and non-public virtual functions

wrote:
[color=blue]
> The users of the derived classes are unable to bypass this base class
> function.[/color]

Just to be clear, the users of the derived C++ classes *are* able to bypass
the base class function and call f_() directly, they just have to be
prepared to twist the rules somewhat. I know: I've been in the situation
where there was absolutely no other way to get the job done other than to
call a private method in a class supplied in a library which I couldn't
change.

To do the same in Python you simply make a rule 'dont call f_ directly' and
you get effectively the same thing: users can call f_ if they really want,
but they are breaking the rules if they do. The difference in Python is
that you don't have to go to such extremes to break the rules.

The Pythonic way to do it:

class B(object):
def f(self):
self._f()

class D(B):
def _f(self):
pass

d = D()
d.f()

Calling a method with a single leading underscore from outside the
implementation of the class is generally accepted in Python to be 'breaking
the rules' and therefore something only to be done by consenting adults in
the full knowledge of the consquences of their actions.

[Martin v. Löwis]

Re: strategy pattern and non-public virtual functions

pythoncurious@g mail.com wrote:[color=blue]
> Just translating this code to python won't work, due to the name
> mangling of private functions:
> class B(object):
> def f(self):
> self.__f()
>
> class D(B):
> def __f(self):
> pass
>
> d = D()
> d.f()
>
> So my questions are:
> 1. Is there a "pythonic" way to do what I'm trying to do?[/color]

Just use a single underscore, i.e. "_f" instead of "__f".

[color=blue]
> 2. Should I be doing this at all? Any thoughts?[/color]

I can't spot the strategy pattern here; neither in the C++ code
nor in the Python code. For this to be the strategy pattern,
you should have two objects: the context, and the strategy
object. So for example, you could have

class Context:
def f(self):
return self.strategy.f ()

class D:
def f(self):
pass

Then, assigning to context.strateg y lets you change the
strategy dynamically.

It's not clear to what you are trying achieve with your
pattern, so it is hard to tell whether you should do this
at all. Most likely, the answer is "no".

If you are really looking for the strategy pattern:
be aware that people often use the strategy pattern
as a work-around for functions not being first-class
objects. In Python, they are, so you can often allow
for arbitrary callables in the strategy pattern.

Regards,
Martin

[Maric Michaud]

Re: strategy pattern and non-public virtual functions

Le Lundi 05 Juin 2006 16:07, pythoncurious@g mail.com a écrit :[color=blue]
> class Base {
>   public:
>     void f() { this->f_(); }
>   private:
>     virtual void f_() = 0;
> };
>
> class Derived : public Base {
>   private:
>     void f_() { // Do something }
> };
>
> int main() {
>     Derived d;
>     d.f();
> }[/color]

This is just polymorphism, not strategy pattern, and I would expect f_ to be
protected here not private.

You want to ensure derived class will use a given method in the Base class,
this could be done explicit with a good naming convention as Duncan said, but
here is a strategy pattern to ensure a sanity check for example :

class strategyBase(ob ject) :
def __call__(self, *sa) : raise NotImplementedE rror('abstract class')

class InputProcessor( object) :

def sanitize(self, *a) :
return a

def f(self, *a) :
sa = self.sanitize(* a)
return self.strategy(* sa)

def __setStrategy(s elf, strat) :
if not isinstance(stra t, strategyBase) :
raise ValueError("str at must be of type strategyBase")
self.__strat = strat

strategy = property(fget=l ambda s : s.__strat, fset=__setStrat egy)

The main purpose of this is to define a common API for all Strategies, and
this is really useful if you intend to manage many of them.

--
_____________

Maric Michaud
_____________

Aristote - www.aristote.info
3 place des tapis
69004 Lyon
Tel: +33 426 880 097

[John J. Lee]

Re: strategy pattern and non-public virtual functions

pythoncurious@g mail.com writes:
[color=blue]
> Hi python experts
>
> In C++ I can do something like this:
> class Base {
> public:
> void f() { this->f_(); }
> private:
> virtual void f_() = 0;
> };
>
> class Derived : public Base {
> private:
> void f_() { // Do something }
> };
>
> int main() {
> Derived d;
> d.f();
> }[/color]
[...]

The others have already answered the Python question and pointed out
this isn't really the Strategy Pattern.

Still, these slides from Alex Martelli specifically on the Template
Method DP in Python are probably very relevant to you:

http://www.aleax.it/Python/os03_template_dp.pdf

John

[Bruno Desthuilliers]

Re: strategy pattern and non-public virtual functions

pythoncurious@g mail.com a écrit :[color=blue]
> Hi python experts
>
> In C++ I can do something like this:
> class Base {
> public:
> void f() { this->f_(); }
> private:
> virtual void f_() = 0;
> };
>
> class Derived : public Base {
> private:
> void f_() { // Do something }
> };
>
> int main() {
> Derived d;
> d.f();
> }[/color]

<ot>
This is eventually the template method pattern, but certainly not the
strategy pattern.
</ot>

[color=blue]
> The point of this is that the a number of classes will inherit from
> Base and only implement a private member function that only will be
> accessed from the base class public 'f' function.
> The Base::f() can then perform validation of input/return values, add
> logging and things like that.[/color]

This is usually done in Python with function decorators. But the
implementer of the derived class

(snip - cf other posts in this thread)
[color=blue]
> So my questions are:
> 1. Is there a "pythonic" way to do what I'm trying to do?
>
> 2. Should I be doing this at all? Any thoughts?[/color]

Doing what ? adding logging, validation etc, or using the template
method pattern ?-)