Bug#896406: python-cv-bridge: cv_bridge fails to import

[Helmut Grohne]

Package: python-cv-bridge
Version: 1.12.3+ds-1+b2
Severity: serious
User: helmutg at debian.org
Usertags: python-import

After installing python-cv-bridge importing the module cv_bridge
into a python interpreter fails with the following error:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python2.7/dist-packages/cv_bridge/__init__.py", line 1, in <module>
    from .core import CvBridge, CvBridgeError
  File "/usr/lib/python2.7/dist-packages/cv_bridge/core.py", line 34, in <module>
    import sensor_msgs.msg
ImportError: No module named sensor_msgs.msg

The vast majority of import failures is attributed to missing dependencies.
Often times that manifests as an ImportError or ModuleNotFoundError.
Typically, dependencies should be inserted by dh-python via ${python:Depends}
or ${python3:Depends}. Thus a missing dependency can be caused by incomplete
install_requires in setup.py. Sometimes a missing dependency of a dependency
is the cause, in such cases this bug should be reassigned.

Helmut