22.11 Mapping for Interfaces

The mapping of Slice interfaces revolves around the idea that, to invoke a remote operation, you call a method on a local class instance that represents the remote object. This makes the mapping easy and intuitive to use because, for all intents and purposes (apart from error semantics), making a remote procedure call is no different from making a local procedure call.

22.11.1 Proxy Classes

On the client side, Slice interfaces map to Python classes with methods that corre­spond to the operations on those interfaces. Consider the following simple interface:

interface Simple {
    void op();
};

The Python mapping generates the following definition for use by the client:

class SimplePrx(Ice.ObjectPrx):
    def op(self, _ctx=None):
        # ...

    # ...

In the client’s address space, an instance of SimplePrx is the local ambassador for a remote instance of the Simple interface in a server and is known as a proxy instance. All the details about the server-side object, such as its address, what protocol to use, and its object identity are encapsulated in that instance.

Note that SimplePrx inherits from Ice.ObjectPrx. This reflects the fact that all Ice interfaces implicitly inherit from Ice::Object.

For each operation in the interface, the proxy class has a method of the same name. In the preceding example, we find that the operation op has been mapped to the method op. Note that op accepts an optional trailing parameter _ctx repre­senting the operation context. This parameter is a Python dictionary for use by the Ice run time to store information about how to deliver a request. You normally do not need to use it. (We examine the context parameter in detail in Chapter 32. The parameter is also used by IceStorm—see Chapter 44.)

Proxy instances are always created on behalf of the client by the Ice run time, so client code never has any need to instantiate a proxy directly.

A value of None denotes the null proxy. The null proxy is a dedicated value that indicates that a proxy points “nowhere” (denotes no object).

22.11.2 The Ice.ObjectPrx Class

All Ice objects have Object as the ultimate ancestor type, so all proxies inherit from Ice.ObjectPrx. ObjectPrx provides a number of methods:

class ObjectPrx(object):
    def equals(self, other):
    def ice_getIdentity(self):
    def ice_isA(self, id):
    def ice_id(self):
    def ice_ping(self):
    # ...

The methods behave as follows:

• equals

This operation compares two proxies for equality. Note that all aspects of proxies are compared by this operation, such as the communication endpoints for the proxy. This means that, in general, if two proxies compare unequal, that does not imply that they denote different objects. For example, if two proxies denote the same Ice object via different transport endpoints, equals returns false even though the proxies denote the same object.

• ice_getIdentity

This method returns the identity of the object denoted by the proxy. The iden­tity of an Ice object has the following Slice type:

module Ice {
    struct Identity {
        string name;
        string category;
    };
};

To see whether two proxies denote the same object, first obtain the identity for each object and then compare the identities:

proxy1 = ...
proxy2 = ...
id1 = proxy1.ice_getIdentity()
id2 = proxy2.ice_getIdentity()

if id1 == id2:
    # proxy1 and proxy2 denote the same object
else:
    # proxy1 and proxy2 denote different objects

• ice_isA

This method determines whether the object denoted by the proxy supports a specific interface. The argument to ice_isA is a type ID (see Section 4.13). For example, to see whether a proxy of type ObjectPrx denotes a Printer object, we can write:

proxy = ...
if proxy != None and proxy.ice_isA("::Printer"):
    # proxy denotes a Printer object
else:
    # proxy denotes some other type of object

Note that we are testing whether the proxy is None before attempting to invoke the ice_isA method. This avoids getting a run-time error if the proxy is None.

• ice_id

This method returns the type ID of the object denoted by the proxy. Note that the type returned is the type of the actual object, which may be more derived than the static type of the proxy. For example, if we have a proxy of type BasePrx, with a static type ID of ::Base, the return value of ice_id might be "::Base", or it might be something more derived, such as "::Derived".

• ice_ping

This method provides a basic reachability test for the object. If the object can physically be contacted (that is, the object exists and its server is running and reachable), the call completes normally; otherwise, it throws an exception that indicates why the object could not be reached, such as ObjectNotExistException or ConnectTimeoutException.

Note that there are other methods in ObjectPrx, not shown here. These methods provide different ways to dispatch a call. (We discuss these methods in Chapter 32.)

22.11.3 Casting Proxies

The Python mapping for a proxy also generates two static methods:

class SimplePrx(Ice.ObjectPrx):
    # ...

    def checkedCast(proxy, facet=''):
        # ...
    checkedCast = staticmethod(checkedCast)

    def uncheckedCast(proxy, facet=''):
        # ...
    uncheckedCast = staticmethod(uncheckedCast)

Both the checkedCast and uncheckedCast methods implement a down-cast: if the passed proxy is a proxy for an object of type Simple, or a proxy for an object with a type derived from Simple, the cast returns a reference to a proxy of type SimplePrx; otherwise, if the passed proxy denotes an object of a different type (or if the passed proxy is None), the cast returns None.

The method names checkedCast and uncheckedCast are reserved for use in proxies. If a Slice interface defines an operation with either of those names, the mapping escapes the name in the generated proxy by prepending an under­score. For example, an interface that defines an operation named checkedCast is mapped to a proxy with a method named _checkedCast.

Given a proxy of any type, you can use a checkedCast to determine whether the corresponding object supports a given type, for example:

obj = ...       # Get a proxy from somewhere...

simple = SimplePrx.checkedCast(obj)
if simple != None:
    # Object supports the Simple interface...
else:
    # Object is not of type Simple...

Note that a checkedCast contacts the server. This is necessary because only the implementation of a proxy in the server has definite knowledge of the type of an object. As a result, a checkedCast may throw a ConnectTimeoutException or an ObjectNotExistException.

In contrast, an uncheckedCast does not contact the server and uncondi­tionally returns a proxy of the requested type. However, if you do use an uncheckedCast, you must be certain that the proxy really does support the type you are casting to; otherwise, if you get it wrong, you will most likely get a run-time exception when you invoke an operation on the proxy. The most likely error for such a type mismatch is OperationNotExistException. However, other exceptions, such as a marshaling exception are possible as well. And, if the object happens to have an operation with the correct name, but different parameter types, no exception may be reported at all and you simply end up sending the invocation to an object of the wrong type; that object may do rather non-sensical things. To illustrate this, consider the following two interfaces:

interface Process {
    void launch(int stackSize, int dataSize);
};

// ...

interface Rocket {
    void launch(float xCoord, float yCoord);
};

Suppose you expect to receive a proxy for a Process object and use an uncheckedCast to down-cast the proxy:

obj = ...                               # Get proxy...
process = ProcessPrx.uncheckedCast(obj) # No worries...
process.launch(40, 60)                  # Oops...

If the proxy you received actually denotes a Rocket object, the error will go unde­tected by the Ice run time: because int and float have the same size and because the Ice protocol does not tag data with its type on the wire, the implementation of Rocket::launch will simply misinterpret the passed integers as floating-point numbers.

In fairness, this example is somewhat contrived. For such a mistake to go unnoticed at run time, both objects must have an operation with the same name and, in addition, the run-time arguments passed to the operation must have a total marshaled size that matches the number of bytes that are expected by the unmar­shaling code on the server side. In practice, this is extremely rare and an incorrect uncheckedCast typically results in a run-time exception.

22.11.4 Using Proxy Methods

The base proxy class ObjectPrx supports a variety of methods for customizing a proxy (see Section 32.11). Since proxies are immutable, each of these “factory methods” returns a copy of the original proxy that contains the desired modifica­tion. For example, you can obtain a proxy configured with a ten second timeout as shown below:

proxy = communicator.stringToProxy(...)
proxy = proxy.ice_timeout(10000)

A factory method returns a new proxy object if the requested modification differs from the current proxy, otherwise it returns the current proxy. With few excep­tions, factory methods return a proxy of the same type as the current proxy, there­fore it is generally not necessary to repeat a down-cast after using a factory method. The example below demonstrates these semantics:

base = communicator.stringToProxy(...)
hello = Demo.HelloPrx.checkedCast(base)
hello = hello.ice_timeout(10000) # Type is preserved
hello.sayHello()

The only exceptions are the factory methods ice_facet and ice_identity. Calls to either of these methods may produce a proxy for an object of an unrelated type, therefore they return a base proxy that you must subsequently down-cast to an appropriate type.

22.11.5 Object Identity and Proxy Comparison

Proxy objects support comparison using the built-in relational operators as well as the cmp function. Note that proxy comparison uses all of the information in a proxy for the comparison. This means that not only the object identity must match for a comparison to succeed, but other details inside the proxy, such as the protocol and endpoint information, must be the same. In other words, comparison tests for proxy identity, not object identity. A common mistake is to write code along the following lines:

p1 = ...        # Get a proxy...
p2 = ...        # Get another proxy...

if p1 != p2:
    # p1 and p2 denote different objects       # WRONG!
else:
    # p1 and p2 denote the same object         # Correct

Even though p1 and p2 differ, they may denote the same Ice object. This can happen because, for example, both p1 and p2 embed the same object identity, but each uses a different protocol to contact the target object. Similarly, the protocols may be the same, but denote different endpoints (because a single Ice object can be contacted via several different transport endpoints). In other words, if two proxies compare equal, we know that the two proxies denote the same object (because they are identical in all respects); however, if two proxies compare unequal, we know absolutely nothing: the proxies may or may not denote the same object.

To compare the object identities of two proxies, you can use a helper function in the Ice module:

def proxyIdentityCompare(lhs, rhs)
def proxyIdentityAndFacetCompare(lhs, rhs)

proxyIdentityCompare allows you to correctly compare proxies for iden­tity:

p1 = ...        # Get a proxy...
p2 = ...        # Get another proxy...

if Ice.proxyIdentityCompare(p1, p2) != 0:
    # p1 and p2 denote different objects       # Correct
else:
    # p1 and p2 denote the same object         # Correct

The function returns 0 if the identities are equal, −1 if p1 is less than p2, and 1 if p1 is greater than p2. (The comparison uses name as the major sort key and category as the minor sort key.)

The proxyIdentityAndFacetCompare function behaves similarly, but compares both the identity and the facet name (see Chapter 33).