Fix for Python 3.10: don't compare to sys.version string

[hugovk]

When dealing with version numbers, it's generally safer to use the sys.version_info tuple that the sys.version string.

>>> sys.version
'3.7.5 (default, Nov  1 2019, 02:16:32) \n[Clang 11.0.0 (clang-1100.0.33.8)]'
>>> sys.version[:3]
'3.7'
>>> fake_version310_info = '3.10.5 (default, Nov  1 2019, 02:16:32) \n[Clang 11.0.0 (clang-1100.0.33.8)]'
>>> fake_version310_info[:3]
'3.1'

When Python 3.10 comes around (probably in May 2020 when Python 3.9 reaches beta), it will claim to be on Python 3.1!

In this case, Python 3.2 is no longer supported, so this check can be removed. (And in fact, it wouldn't have failed until Python 3.20, but it's good to clean up.)

---

Found with https://github.com/asottile/flake8-2020

[emmanuelle]

Thank you @hugovk. I have no idea why we excluded python 3.2 here... any idea @nicolaskruchten @jonmmease ?

[hugovk]

If it helps, it was added in PR #1476.

[nicolaskruchten]

No idea, but probably for a good reason, in which case I'd rather upgrade the check to use the right thing rather than removing it :)

[hugovk]

Updated:

-        if sys.version[:3] != "3.2":
+        if not sys.version_info[:2] == (3, 2):

[nicolaskruchten]

Thanks! I know my feedback is coming across as very-conservative, and I appreciate the followup :)

[nicolaskruchten]

should there be a [:2] in there? None of the changes in https://github.com/plotly/plotly.py/pull/2078/files include this index...

In fact, I think it might be nice to add this commit to #2078 and close this PR so we can merge it all at once, if you don't mind :)

[hugovk]

The [:2] is needed for == comparisons. It can be used but isn't required for >, >=, <, <= etc.

>>> sys.version_info
sys.version_info(major=3, minor=8, micro=1, releaselevel='final', serial=0)
>>> sys.version_info[:2]
(3, 8)
>>> sys.version_info[:2] == (3, 8)
True
>>> sys.version_info == (3, 8)
False
>>> sys.version_info >= (3, 8)
True
>>> sys.version_info[:2] >= (3, 8)
True
>>> sys.version_info >= (3, 9)
False
>>> sys.version_info[:2] >= (3, 9)
False

---

Yep, will combine.

[hugovk]

In fact, I think it might be nice to add this commit to #2078 and close this PR so we can merge it all at once, if you don't mind :)

Added, closing.

[hugovk]

Thanks! I know my feedback is coming across as very-conservative, and I appreciate the followup :)

No problem at all, it's good to be careful with widely used code!

By the way, how about (in another PR) adding python_requires to the setup.py files? That way modern pip (>=9) installs the right version of the package for user's running Python. For example, pip itself has:

    python_requires='>=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*',

Which allows install on Python 2.7 and 3.5+.

https://github.com/pypa/pip/blob/fed360a64e907f3ce8f5d497cc806df2fa9a0799/setup.py#L86