Python 3 compatibility issues

[cclauss]

flake8 testing of https://github.com/RobotWebTools/rosbridge_suite on Python 3.7.1

$ flake8 . --count --select=E9,F63,F72,F82 --show-source --statistics

./rosbridge_library/src/rosbridge_library/util/cbor.py:152:32: F821 undefined name 'unicode'
        return isinstance(val, unicode)
                               ^
./rosbridge_library/src/rosbridge_library/util/cbor.py:218:36: F821 undefined name 'basestring'
        return isinstance(x, (str, basestring, bytes, unicode))
                                   ^
./rosbridge_library/src/rosbridge_library/util/cbor.py:218:55: F821 undefined name 'unicode'
        return isinstance(x, (str, basestring, bytes, unicode))
                                                      ^
./rosbridge_library/src/rosbridge_library/util/cbor.py:220:36: F821 undefined name 'long'
        return isinstance(x, (int, long))
                                   ^
./rosbridge_library/src/rosbridge_library/util/cbor.py:371:18: F821 undefined name 'xrange'
        for i in xrange(aux):
                 ^
./rosbridge_library/src/rosbridge_library/util/cbor.py:378:18: F821 undefined name 'xrange'
        for i in xrange(aux):
                 ^
./rosbridge_library/src/rosbridge_library/util/__init__.py:16:26: F821 undefined name 'unicode'
    string_types = (str, unicode)
                         ^
./rosbridge_library/src/rosbridge_library/internal/cbor_conversion.py:49:20: F821 undefined name 'unicode'
            slot = unicode(slot)
                   ^
./rosbridge_library/src/rosbridge_library/internal/cbor_conversion.py:53:25: F821 undefined name 'unicode'
            out[slot] = unicode(val) if PYTHON2 else str(val)
                        ^
./rosbridge_library/src/rosbridge_library/internal/message_conversion.py:70:35: F821 undefined name 'long'
    primitive_types = [bool, int, long, float]
                                  ^
./rosbridge_library/src/rosbridge_library/capabilities/subscribe.py:316:21: F821 undefined name 'unicode'
            topic = unicode(topic)
                    ^
./rosbridge_library/test/internal/test_message_conversion.py:21:26: F821 undefined name 'unicode'
    string_types = (str, unicode)
                         ^
./rosbridge_library/test/internal/test_services.py:19:26: F821 undefined name 'unicode'
    string_types = (str, unicode)
                         ^
./rosbridge_library/test/internal/test_services.py:30:53: F821 undefined name 'unicode'
    elif sys.version_info < (3,0) and isinstance(d, unicode):
                                                    ^
./rosbridge_library/test/internal/test_services.py:31:16: F821 undefined name 'unicode'
        return unicode(random.random())
               ^
./rosbridge_library/test/internal/test_cbor_conversion.py:39:55: F821 undefined name 'unicode'
            self.assertEqual(type(extracted['data']), unicode)
                                                      ^
./rosbridge_library/test/internal/test_cbor_conversion.py:152:45: F821 undefined name 'unicode'
                self.assertEqual(type(key), unicode)
                                            ^
./rosapi/src/rosapi/params.py:70:8: F632 use ==/!= to compare str, bytes, and int literals
    if default is not "":
       ^
1     F632 use ==/!= to compare str, bytes, and int literals
17    F821 undefined name 'unicode'
18

E901,E999,F821,F822,F823 are the "showstopper" flake8 issues that can halt the runtime with a SyntaxError, NameError, etc. These 5 are different from most other flake8 issues which are merely "style violations" -- useful for readability but they do not effect runtime safety.

• F821: undefined name name
• F822: undefined name name in __all__
• F823: local variable name referenced before assignment
• E901: SyntaxError or IndentationError
• E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree

[mvollrath]

I've reviewed all of the CBOR script related warnings, they are all in Python 2 compatibility blocks. Do we need to scrub all Python 2 support?

[cclauss]

The linters will get confused by code that does not follow the Python porting best practice use feature detection instead of version detection so if you are sure that the F821 issues are all safe uses done via version detection then those issues can be ignored. The usual approaches are to add the six module and replace unicode with six.text_type or without six, to do it like:

try:
    long
    unicode
except NameError:  # Python 3
    long = int
    unicode = str

[mikepurvis]

Is anyone actively working on this? I'd like a version of rosbridge that is either bilingual or ported to Python 3.

[mvollrath]

I've been using rosbridge with Python 3, but have not tested the latest release with it. It would be prudent to start running the tests in both 2 and 3.

[cclauss]

#420 runs basic linting on both Python 2 and Python 3.

[mikepurvis]

@mvollrath Great, thanks for the confirmation; I've done some basic testing under Python 3 on my system and it does indeed seem to be working well.

[github-actions]

This issue has been marked as stale because it has been open for 180 days with no activity. Please remove the stale label or add a comment to keep it open.

[cclauss]

These issues continue to grow. #592 Demonstrates that we are now at 26 undefined names on Python 3.

Does this codebase still need to support Python 2? It would be helpful if README.md stated what versions of Python are supported.

[mvollrath]

ROS1 melodic requires Python 2 support until May 2023 and noetic requires Python 3 until May 2025. ROS2 branch does not require any Python 2 support.

[amacneil]

• ros2 branch has now been ported to python 3 and warnings fixed (thanks @cclauss!)
• ros1 branch supports both python 2 and 3, but is in maintenance mode and will only receive critical bug fixes