8. Python Generation from IDL

The Python DCPS API supports generation of Python topic classes from IDL. This chapter describes the details of the IDL-Python binding.

8.1. Running IDLPP

8.1.1. Static Generation

The Python topic classes can be generated statically using an IDL file.

Compiling IDL into python code is done using the -l python switch on idlpp:

idlpp -l python idl-file-to-compile.idl

Note

  • A Python package with the same name as the idl file (without the .idl extension) is always created.

  • It defines types not included in an IDL module. IDL modules become Python packages within this base package.

8.1.2. Dynamic Generation

The Python topic classes can be generated dynamically using an IDL file, and the topic name. An api, ddsutil.get_dds_classes_from_idl, is provided to generate the topic classes at runtime from within a Python script.

Please note that dynamic generation will not work with a deployment license as it requires the idlpp application and should only be used during development of Topics.

HelloWorldDataPublisher.py

...
TOPIC_NAME = 'Msg1'
TOPIC_TYPE = 'HelloWorldData::Msg'
IDL_FILE = 'idl/HelloWorldData.idl'

# Create domain participant
dp = DomainParticipant()

# Create publisher
pub = dp.create_publisher()

# Generate python topic classes from IDL file
gen_info = ddsutil.get_dds_classes_from_idl(IDL_FILE, TOPIC_TYPE)

# Type support class
qos = Qos([DurabilityQosPolicy(DDSDurabilityKind.TRANSIENT),
       ReliabilityQosPolicy(DDSReliabilityKind.RELIABLE)])

# Register topic
topic = gen_info.register_topic(dp, TOPIC_NAME, qos)

# Create a writer
writer = pub.create_datawriter(topic)

# Topic data class
idMessage = 1
message1 = 'Hello World'
s = gen_info.topic_data_class(userID = idMessage, message = message1)

# Write data
writer.write(s)

#output to console
print('=== [Publisher] writing a message containing : ')
print('userID  :',  idMessage)
print('message :', message1)
...

8.1.3. Generated Artifacts

The following table defines the Python artifacts generated from IDL concepts:

IDL Concept

Python Concept

Comment

module

package

Folder with module name, plus __init__.py that defines types defined within the module.

enum

class

Defined in __init__.py files.

enum value

enum value

Defined in __init__.py files.

struct

class

Defined in __init__.py files.

field

class property

Defined in __init__.py files.

union

union

Defined in __init__.py files. (Only statically generated supported)

Datatype mappings

The following table shows the Python equivalents to IDL primitive types:

IDL Type

Python Type

boolean

bool

char

str, length==1

octet

int

short

int

ushort

int

long

int

ulong

int

long long

int

ulong long

int

float

float

double

float

string

str

wchar

Unsupported

wstring

Unsupported

any

Unsupported

long double

Unsupported

Implementing Arrays and Sequences in Python

Both IDL arrays and IDL sequences are mapped to Python lists.

The constructors for generated classes always fully allocate any array fields. Sequences are always initialized to the empty list.

8.2. Limitations of Python Support

The IDL-to-Python binding has the following limitations:

  • IDL unions are Supported by statically generated Python, but not by dynamic or over-the-wire.

  • The following IDL data types are not supported: wchar, wstring, any and long double .