Enable live patching of messages ?

[asmodehn]

The current import system, when importing a message, looks for all its dependencies and generate ROS code for it.

This works fine, but it also means that a python code, cannot override or patch the generated ROS code.

It would be useful to be able to patch/extend a ROS message with some python code without modifying the generated code (usecase in pyros-msgs at the moment), at import time. But I am not sure it is even possible, while ROS message dependencies are other messages (directly, not the python source), and the python default import takes care of all cases (one cannot have an extension to python import system after FileFinder - currently, not a .py file will fail to import, without testing next importer in hooks).

Anyway it might be something worth keeping in mind, although for now we ll implement some workaround in pyros-msgs.

[asmodehn]

I am not sure fit his is a mismatching features (using the namespace package of filefinder2),
or if it is a bug.

When a main package has a subpackage that is a namespace, does the __init__ module of the main package is called before the namespace package (PEP420) gets imported ? If yes, than this doesn't happen here (and prevent patching of generated messages that should normally work). So it is a bug...
If not then ... not sure...

[asmodehn]

Currently this is broken : tests in #13

We also need to test the namespace package case and compare py2 (filefinder2) and py3, and find out if it is because namespace packages or just a bug in our implementation...

[asmodehn]

Actually not : fixing the dependent path makes it work : https://travis-ci.org/pyros-dev/rosimport/builds/271884845

[asmodehn]

But this works only because we "import" the generated class (which patches it) and then we instantiate that class when building the message. if neither the importing or the instantiation is done, then the patch will not be applied, which is logical.

If we don't import the dependent message definition, the ros generated message code will do the import. but then it breaks:

>         self.test_ros_msg_patch = provider.msg.TestRosMsgPatch()
E         AttributeError: 'dict' object has no attribute 'TestRosMsgPatch'

/tmp/rosimport/3750/consumer/msg/_TestRosMsgPatchDeps.py:40: AttributeError

There is probably a bug hiding around that corner...
Maybe some tests that are NOT doing the import (and let message generated code do it) are missing ?