Google App Engine

Descriptor Module Functions

The descriptor.py module contains message definitions and functions for converting Google Protocol RPC definitions into a transmittable message format.

Describing an Enum instance, Enum class, Field class, or Message class generates an appropriate descriptor object. A descriptor object is an object that describes other Google Protocol RPC definitions such as enums, messages, and services. You can use this message to transmit information to clients wishing to know the description of an enum value, enum, field or message without needing to download the source code. This format is also compatible with other, non-Python languages.

The descriptors are modeled to be binary compatible with http://code.google.com/p/protobuf/source/browse/trunk/src/google/protobuf/descriptor.proto

Note! The names of types and fields defined in descriptor.py do not necessarily match those defined in descriptor.proto. Google Protocol RPC is designed this way to make source code files that use these descriptors easier to read. For example, FieldDescriptors need to be prefixed with TYPE in descriptor.proto, but not in descriptor.py.

The following code sample demonstrates use of the descriptor module to describe a class called Pixel using the MessageDescriptor class.

from protorpc import descriptor
from protorpc import messages

class Pixel(messages.Message):
    x = messages.IntegerField(1, required=True)
    y = messages.IntegerField(2, required=True)

    color = messages.BytesField(3)

# Describe Pixel class using message descriptor.  
fields = [
  descriptor.FieldDescriptor(name='x', 
                  number=1,
                  label=descriptor.FieldDescriptor.Label.REQUIRED,
                  variant=descriptor.FieldDescriptor.Variant.INT64),

  descriptor.FieldDescriptor(name='y', 
                  number=2,
                  label=descriptor.FieldDescriptor.Label.REQUIRED,
                  variant=descriptor.FieldDescriptor.Variant.INT64),

  descriptor.FieldDescriptor(name='color',
                  number = 3,
                  label=descriptor.FieldDescriptor.Label.OPTIONAL,
                  variant=descriptor.FieldDescriptor.Variant.BYTES)]

message = descriptor.MessageDescriptor(name='Pixel',
                                       fields=fields)

# Describing is the equivalent of building the above message.
message == definition.describe_message(Pixel)

The protorpc.descriptor package provides the following functions:

describe(value)

Describes any value as a descriptor. You can use this helper function to describe any object with an appropriate descriptor object.

Arguments

value
Value to describe as a descriptor.

Returns a descriptor if the object is describable as a descriptor, else None.

describe_enum(enum_definition)

Builds a descriptor from an Enum class.

Arguments

enum_value
The Enum class to describe.

Returns an initialized EnumDescriptor instance describing the Enum instance.

EnumDescriptor Class fields:

name
Name of the specified Enum without any qualifications.
values
Values defined by the Enum class.
describe_enum_value(enum_value)

Builds a descriptor from an Enum instance.

Arguments

enum_value
The Enum value to describe.

Returns an initialized EnumValueDescriptor instance describing the Enum instance.

EnumValueDescriptor Class fields:

name
Name of the enumeration value.
number
Number of the enumeration value.
describe_field(field_definition)

Builds a descriptor from a Field instance.

Arguments

field_definition
The Field instance to describe.

Returns an initialized FieldDescriptor instance describing the Field instance with a list of Enums and fields.

FieldDescriptor Class Enums:

Variant
Wire format hint subtypes for the specified field.
Label
Values for optional, required, and repeated fields.

FieldDescriptor Class fields:

name
Name of the specified field.
number
Number of the specified field.
variant
Variant of the specified field.
type_name
Type name for the message and enum fields.
default_value
String representation of the default value.
describe_file(module)

Builds a file from a specified Python module.

Arguments

module
The Python to describe.

Returns an initialized FileDescriptor instance describing the module.

FileDescriptor Class fields

package
Fully qualified name of the package that the definitions belong to.
message_types
Message definitions contained in the file.
enum_types
Enum definitions contained in the file.
service_types
Service definitions contained in the file.
describe_file_set(modules)

Builds a file set from the specified Python modules.

Arguments

modules
Iterable of the Python module to describe.

Returns an initialized FileSet instance describing the module.

FileSet Class fields:

files
Files in the file set.
describe_message(message_definition)

Builds a descriptor from a Message class.

Arguments

message_definition
The Message class to describe.

Returns an initialized MessageDescriptor instance describing the Message class.

MessageDescriptor Class fields:

name
Fully qualified name of the package that the definitions belong to.
fields
Fields defined for the message.
message_types
Nested Message classes defined on the message.
enum_types
Nested Enum classes defined on the message.
describe_method(method)

Builds a descriptor from a remote service method.

Arguments

method
The remote service method to describe.

Returns an initialized MethodDescriptor instance describing the remote service method.

MethodDescriptor fields:

name
Name of the service method.
request_type
Fully qualified name or relative name of the request message type.
response_type
Fully qualified or relative name of the response message type.
describe_service(service_class)

Builds a descriptor from a Service class.

Arguments

service_class
The Service class to describe.

Returns an initialized ServiceDescriptor instance describing the service.

ServiceDescriptor fields:

name
Unqualified name of the Service.
methods
Remote methods of the Service.
import_descriptor_loader(definition_name, importer=__import__ )

Finds objects by importing modules as needed. A definition loader is a function that resolves a definition name to a descriptor. The import finder resolves definitions to their names by importing modules when necessary.

Arguments

definition_name
Name of the definition to find.
importer=__import__ )
Import function used for importing new modules.

Returns an appropriate descriptor from any describable type located by name.

Raises a DefinitionNotFoundError when a name does not refer to either a definition or a module.

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