Capability of using <, <=, > in the requirements and their combination about catkin_virtualenv

[wkentaro]

And I'd like to know what made you to use multiple requirements.txt files, and their combination:

catkin_virtualenv/catkin_virtualenv/cmake/catkin_generate_virtualenv.cmake

Lines 59 to 62 in 21bd7dc

	add_custom_command(OUTPUT ${generated_requirements}
	COMMAND ${CATKIN_ENV} ${PYTHON_EXECUTABLE} ${catkin_virtualenv_CMAKE_DIR}/combine_requirements.py --requirements-list ${requirements_list} --output-file ${generated_requirements}
	DEPENDS ${requirements_list}
	)

[paulbovbel]

What motivated me to use multiple requirements files was the idea that you could have:

• package A implement library code that depends on external library B
• package C on package A, and inherit package A's requirement of library B transparently

I agree, the current implementation is fairly limited. It would certainly be possible to add support for other combination of constraints here.

• The combinatorial explosion would probably require taking a different approach than currently used
• A cleaner way would probably be to rely on the ability of requirements files to recursively include requirements files, letting pip/setuptools handle the messy resolution.

[paulbovbel]

That was fairly easy, and I got to remove a bunch of code! #15

EDIT: closing the associated PR, not a good strategy. A better requirement-constraint-merging strategy should be implemented in catkin_virtualenv, unless pip changes the way it works with duplicated requirements.