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 .