Updated tox file and changed Travis CI to use tox

[jadchaar]

Along with these tox configuration changes, I also updated the isort config to not conflict with Black (which wraps import lines beyond 88 characters and uses trailing commas).

Closes: #589

[jadchaar]

@asottile I took your advice and decided to tweak our tox environment and use it for our Travis builds, but I am not sure why the builds are failing. Am I using tox incorrectly on Travis?

I tried to use pytest as an example and made sure to include the requirements.txt file for modules such as dateutil, but I am still having some issues with the build failing with the error:

ERROR: FAIL could not package project - v = InvocationError(u'/home/travis/virtualenv/python2.7.15/bin/python setup.py sdist --formats=zip --dist-dir /home/travis/build/crsmithdev/arrow/.tox/dist', 1)

Setting the environment variable and running tox --recreate locally works fine. Also, dateutils is in the requirements.txt file as python-dateutils.

[asottile]

the failure actually points out an important point of the packaging (and one of the many motivations for using tox in the first place!): the packaging for arrow is currently broken when installing from source

this line in the setup.py: https://github.com/crsmithdev/arrow/blob/cfafc0e4b2f2fcf2b004cb0ab2a67e2bb5feb56d/setup.py#L4

that attempts to import arrow/__init__.py

which in turn imports most of the arrow packaging

You generally have ~3 approaches here:

1. Read the file in setup.py instead of importing (PRO: no dependencies, CON: slightly janky? ish?)

   and you actually have two sub-approaches here, you can either use the ast or you can just read it and use a regex, the regex usually works fine 🤷‍♂

   version_re = re.compile('(?<=__version__ = ")([^"]+)(?=")')
   with open('arrow/__init__.py') as f:
       version = version_re.search(f.read()).group()

   here's a way with an ast parser

   class Visitor(ast.NodeVisitor):
       def visit_Assign(self, node):
           if (
                   len(node.targets) == 1 and
                   isinstance(node.targets[0], ast.Name) and
                   node.targets[0].id == '__version__' and
                   isinstance(node.value, ast.Str)
           ):
               self.version = node.value.s
   
   with open('arrow/__init__.py', 'rb') as f:
       visitor = Visitor()
       visitor.visit(ast.parse(f.read(), filename='arrow/__init__.py'))
       version = visitor.version

   it's more code, but it's more likely to not break due to regex fragility

2. only define the version in setup.py, and at runtime use importlib_metadata.version(__name__) to retrieve the version

   (this is the way I've used in my projects with success, though it does come with a slight import-time cost) -- for example: https://github.com/pre-commit/pre-commit/blob/a75fe69984b1e1508cb89c41139bb8f384c86a53/pre_commit/constants.py#L21

3. Use setuptools-scm (or similar tool)

I don't know enough about how to set this up, but it's apparently a reasonable option

---

To demonstrate that it's broken from source:

$ pip install arrow --no-binary :all:
Collecting arrow
  Downloading https://files.pythonhosted.org/packages/a0/3b/0b5c7771a619ee4eae1f894669dd5f7c0aae9c92d87891c4c746937f9cc4/arrow-0.14.0.tar.gz (102kB)
     |████████████████████████████████| 112kB 2.4MB/s 
    ERROR: Complete output from command python setup.py egg_info:
    ERROR: Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-pki32a8v/arrow/setup.py", line 4, in <module>
        from arrow import __version__
      File "/tmp/pip-install-pki32a8v/arrow/arrow/__init__.py", line 3, in <module>
        from .api import get, now, utcnow
      File "/tmp/pip-install-pki32a8v/arrow/arrow/api.py", line 10, in <module>
        from arrow.factory import ArrowFactory
      File "/tmp/pip-install-pki32a8v/arrow/arrow/factory.py", line 15, in <module>
        from dateutil import tz as dateutil_tz
    ModuleNotFoundError: No module named 'dateutil'
    ----------------------------------------
ERROR: Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-install-pki32a8v/arrow/

---

cheers! hope this helps :)

[codecov-io]

Codecov Report

Merging #597 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #597   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           7      7           
  Lines        1501   1501           
  Branches      225    225           
=====================================
  Hits         1501   1501

---

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[systemcatch]

@jadchaar does this need to go in 0.14.1 as well or can it wait until the next release?

[jadchaar]

We can wait until the next release since this is more of a backend/housekeeping thing.

[asottile]

otherwise looks good!

[jadchaar]

@systemcatch mind reviewing these changes so we can get them merged in for release 0.14.2? Andrew found another regression with setup.py and locales (#601).

[jadchaar]

Note: I replaced codecs with io in setup.py in the latest commit after some research (namely https://stackoverflow.com/questions/5250744/difference-between-open-and-codecs-open-in-python).