Modernize Python 2 code to get ready for Python 3

[cclauss]

Make the minimal, safe changes required to convert the repo's code to be syntax compatible with both Python 2 and Python 3. There may be other changes required to complete a port to Python 3 but this PR is a minimal, safe first step.

Run: futurize --stage1 -w **/*.py

See Stage 1: "safe" fixes http://python-future.org/automatic_conversion.html#stage-1-safe-fixes

[dirk-thomas]

This is not necessary in this case (see http://wiki.ros.org/python_2_and_3_compatible_code#Print_is_now_a_function).

Same below for many other cases.

[dirk-thomas]

The extra parenthesis don't seem to make any sense. Why are they being introduced?

[cclauss]

The import keeps Python 2 developers from breaking Python 3 code because when the import is used then print without the parens is a syntax error in both Python 2 and Python 3. The import also deals gracefully with commas inside a print statement thus the double parens for trying to preserve the output...

$ python2

Python 2.7.14 ....
>>> print 0, 1
0 1
>>> print(0, 1)
(0, 1)
>>> print((0, 1))
(0, 1)
>>> from __future__ import print_function
>>> print 0, 1
  File "<stdin>", line 1
    print 0, 1
          ^
SyntaxError: invalid syntax
>>> print(0, 1)
0 1
>>> print((0, 1))
(0, 1)

[dirk-thomas]

Why is this needed here?

Same question below.

[cclauss]

https://www.python.org/dev/peps/pep-0328/#rationale-for-absolute-imports

[dirk-thomas]

The import keeps Python 2 developers from breaking Python 3 code because when the import is used then print without the parens is a syntax error in both Python 2 and Python 3. The import also deals gracefully with commas inside a print statement thus the double parens for trying to preserve the output...

I understand. The wiki page I referenced mentions that. I just don't think it should be used when not necessary.

Wasn't there one occasion where print is called with multiple arguments? In that the future import would make sense. But I can't see it anywhere anymore since you force pushed.

[dirk-thomas]

Please move the check into a unit test so that it is being run locally too.

[cclauss]

Wasn't there one occasion where print is called with multiple arguments?
https://github.com/ros/catkin/blob/kinetic-devel/test/local_tests/test_with_mock_workspace.py#L77

[dirk-thomas]

https://github.com/ros/catkin/blob/kinetic-devel/test/local_tests/test_with_mock_workspace.py#L77

For that line future import would make sense. I doubt the intention in Python 2 was to print parenthesis around the arguments.

[cclauss]

I used str.format() instead of the print_function import to make the intention explicit.

[dirk-thomas]

Looks good to me now. Thank you for creating the patch and iterating on it.