dictionary_switches / html / _sources / Fronting.txt

Fronting for an Implementation

Both *Proxy* and *State* provide a surrogate class that you use in your code;
the real class that does the work is hidden behind this surrogate class. When
you call a method in the surrogate, it simply turns around and calls the method
in the implementing class. These two patterns are so similar that the *Proxy* is
simply a special case of *State*. One is tempted to just lump the two together
into a pattern called *Surrogate*, but the term "proxy" has a long-standing and
specialized meaning, which probably explains the reason for the two different

The basic idea is simple: from a base class, the surrogate is derived along with
the class or classes that provide the actual implementation:

..  image:: _images/surrogate.*

When a surrogate object is created, it is given an implementation to which to
send all of the method calls.

Structurally, the difference between *Proxy* and *State* is simple: a *Proxy*
has only one implementation, while *State* has more than one. The application of
the patterns is considered (in *Design Patterns*) to be distinct: *Proxy* is
used to control access to its implementation, while *State* allows you to change
the implementation dynamically. However, if you expand your notion of
"controlling access to implementation" then the two fit neatly together.


If we implement *Proxy* by following the above diagram, it looks like this::

    # fronting/
    # Simple demonstration of the Proxy pattern.

    class Implementation:
        def f(self):
        def g(self):
        def h(self):

    class Proxy:
        def __init__(self):
            self.__implementation = Implementation()
        # Pass method calls to the implementation:
        def f(self): self.__implementation.f()
        def g(self): self.__implementation.g()
        def h(self): self.__implementation.h()

    p = Proxy()
    p.f(); p.g(); p.h()

It isn't necessary that **Implementation** have the same interface as **Proxy**;
as long as **Proxy** is somehow "speaking for" the class that it is referring
method calls to then the basic idea is satisfied (note that this statement is at
odds with the definition for Proxy in GoF). However, it is convenient to have a
common interface so that **Implementation** is forced to fulfill all the methods
that **Proxy** needs to call.

Of course, in Python we have a delegation mechanism built in, so it makes the
**Proxy** even simpler to implement::

    # fronting/
    # Simple demonstration of the Proxy pattern.

    class Implementation2:
        def f(self):
        def g(self):
        def h(self):

    class Proxy2:
        def __init__(self):
            self.__implementation = Implementation2()
        def __getattr__(self, name):
            return getattr(self.__implementation, name)

    p = Proxy2()
    p.f(); p.g(); p.h();

The beauty of using **__getattr__( )** is that **Proxy2** is completely generic,
and not tied to any particular implementation (in Java, a rather complicated
"dynamic proxy" has been invented to accomplish this same thing).


The *State* pattern adds more implementations to *Proxy*, along with a way to
switch from one implementation to another during the lifetime of the surrogate::

    # fronting/
    # Simple demonstration of the State pattern.

    class State_d:
        def __init__(self, imp):
            self.__implementation = imp
        def changeImp(self, newImp):
            self.__implementation = newImp
        # Delegate calls to the implementation:
        def __getattr__(self, name):
            return getattr(self.__implementation, name)

    class Implementation1:
        def f(self):
            print("Fiddle de dum, Fiddle de dee,")
        def g(self):
            print("Eric the half a bee.")
        def h(self):
            print("Ho ho ho, tee hee hee,")

    class Implementation2:
        def f(self):
            print("We're Knights of the Round Table.")
        def g(self):
            print("We dance whene'er we're able.")
        def h(self):
            print("We do routines and chorus scenes")

    def run(b):

    b = State_d(Implementation1())

You can see that the first implementation is used for a bit, then the second
implementation is swapped in and that is used.

The difference between *Proxy* and *State* is in the problems that are solved.
The common uses for *Proxy* as described in *Design Patterns* are:

#.  **Remote proxy**. This proxies for an object in a different address space. A
    remote proxy is created for you automatically by the RMI compiler **rmic**
    as it creates stubs and skeletons.

#.  **Virtual proxy**. This provides "lazy initialization" to create expensive
    objects on demand.

#.  **Protection proxy**. Used when you don't want the client programmer to have
    full access to the proxied object.

#.  **Smart reference**. To add additional actions when the proxied object is
    accessed. For example, or to keep track of the number of references that are
    held for a particular object, in order to implement the *copy-on-write*
    idiom and prevent object aliasing. A simpler example is keeping track of the
    number of calls to a particular method.

You could look at a Python reference as a kind of protection proxy, since it
controls access to the actual object on the heap (and ensures, for example, that
you don't use a **null** reference).

[[ Rewrite this: In *Design Patterns*, *Proxy* and *State* are not seen as
related to each other because the two are given (what I consider arbitrarily)
different structures. *State*, in particular, uses a separate implementation
hierarchy but this seems to me to be unnecessary unless you have decided that
the implementation is not under your control (certainly a possibility, but if
you own all the code there seems to be no reason not to benefit from the
elegance and helpfulness of the single base class). In addition, *Proxy* need
not use the same base class for its implementation, as long as the proxy object
is controlling access to the object it "fronting" for. Regardless of the
specifics, in both *Proxy* and *State* a surrogate is passing method calls
through to an implementation object.]]]