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Basic Python Mapping for Operations
As we saw in the Python mapping for interfaces, for each operation on an interface, the proxy class contains a corresponding method with the same name. To invoke an operation, you call it via the proxy. For example, here is part of the definitions for our file system:
name operation returns a value of type
string. Given a proxy to an object of type
Node, the client can invoke the operation as follows:
idempotent Operations in Python
You can add an
idempotent qualifier to a Slice operation. As far as the signature for the corresponding proxy method is concerned,
idempotent has no effect. For example, consider the following interface:
The proxy class for this is:
idempotent affects an aspect of call dispatch, not interface, it makes sense for the two methods to look the same.
Passing Parameters in Python
In-Parameters in Python
All parameters are passed by reference in the Python mapping; it is guaranteed that the value of a parameter will not be changed by the invocation.
Here is an interface with operations that pass parameters of various types from client to server:
The Slice compiler generates the following proxy for this definition:
Given a proxy to a
ClientToServer interface, the client code can pass parameters as in the following example:
Out-Parameters in Python
As in Java, Python functions do not support reference arguments. That is, it is not possible to pass an uninitialized variable to a Python function in order to have its value initialized by the function. The Java mapping overcomes this limitation with the use of holder classes that represent each
out parameter. The Python mapping takes a different approach, one that is more natural for Python users.
The semantics of
out parameters in the Python mapping depend on whether the operation returns one value or multiple values. An operation returns multiple values when it has declared multiple
out parameters, or when it has declared a non-
void return type and at least one
If an operation returns multiple values, the client receives them in the form of a result tuple. A non-
void return value, if any, is always the first element in the result tuple, followed by the
out parameters in the order of declaration.
If an operation returns only one value, the client receives the value itself.
Here again are the same Slice definitions we saw earlier, but this time with all parameters being passed in the
The Python mapping generates the following code for this definition:
Given a proxy to a
ServerToClient interface, the client code can receive the results as in the following example:
The operations have no
in parameters, therefore no arguments are passed to the proxy methods. Since
op2 return multiple values, their result tuples are unpacked into separate values, whereas the return value of
op3 requires no unpacking.
Parameter Type Mismatches in Python
Although the Python compiler cannot check the types of arguments passed to a function, the Ice run time does perform validation on the arguments to a proxy invocation and reports any type mismatches as a
Null Parameters in Python
Some Slice types naturally have "empty" or "not there" semantics. Specifically, sequences, dictionaries, and strings all can be
None, but the corresponding Slice types do not have the concept of a null value. To make life with these types easier, whenever you pass
None as a parameter or return value of type sequence, dictionary, or string, the Ice run time automatically sends an empty sequence, dictionary, or string to the receiver.
This behavior is useful as a convenience feature: especially for deeply-nested data types, members that are sequences, dictionaries, or strings automatically arrive as an empty value at the receiving end. This saves you having to explicitly initialize, for example, every string element in a large sequence before sending the sequence in order to avoid a run-time error. Note that using null parameters in this way does not create null semantics for Slice sequences, dictionaries, or strings. As far as the object model is concerned, these do not exist (only empty sequences, dictionaries, and strings do). For example, it makes no difference to the receiver whether you send a string as
None or as an empty string: either way, the receiver sees an empty string.
Optional Parameters in Python
Optional parameters use the same mapping as required parameters. The only difference is that
Ice.Unset can be passed as the value of an optional parameter or return value. Consider the following operation:
A client can invoke this operation as shown below:
A well-behaved program must always test an optional parameter prior to using its value. Keep in mind that the
Ice.Unset marker value has different semantics than
None is a legal value for certain Slice types, the Ice run time requires a separate marker value so that it can determine whether an optional parameter is set. An optional parameter set to
None is considered to be set. If you need to distinguish between an unset parameter and a parameter set to
None, you can do so as follows:
Exception Handling in Python
Slice exceptions are thrown as Python exceptions, so you can simply enclose one or more operation invocations in a
Typically, you will catch only a few exceptions of specific interest around an operation invocation; other exceptions, such as unexpected run-time errors, will usually be handled by exception handlers higher in the hierarchy. For example: