[SPARK-3458] enable python "with" statements for SparkContext

[mattf]

allow for best practice code,

try:
  sc = SparkContext()
  app(sc)
finally:
  sc.stop()

to be written using a "with" statement,

with SparkContext() as sc:
  app(sc)

[mattf]

@davies i'd appreciate your input on this

[SparkQA]

QA tests have started for PR 2335 at commit 5b4e37c.

• This patch merges cleanly.

[davies]

self.assertIsNone(xxx) is better than self.assertEqual(xxx, None)

[mattf]

accordning to https://docs.python.org/2/library/unittest.html#assert-methods assertIsNone and assertIsNotNone are new to python 2.7 and my (potentially incorrect) understanding is we want to target 2.6.

thoughts?

[davies]

Sorry, I missed that, we definitely need to target Python2.6.

[mattf]

side note: i can't even find a box w/ py2.6 (it's so old!)

[JoshRosen]

Yeah, I know :( . Unfortunately, Python 2.6 is still the default Python version on a lot of Linux distributions, so we've been trying to maintain backwards-compatibility in order to ensure a good out-of-the-box experience for those users.

[davies]

@mattf It's cool to have this, just one minor comment about tests.

[SparkQA]

QA tests have finished for PR 2335 at commit 5b4e37c.

• This patch fails unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[mattf]

This patch fails unit tests.

failures are unrelated to this patch

[davies]

Jenkins, retest this please.

[davies]

I'm OK to merge this.

[SparkQA]

QA tests have started for PR 2335 at commit 5b4e37c.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2335 at commit 5b4e37c.

• This patch fails unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[mattf]

This patch fails unit tests.

still not this patch's fault

[rxin]

Jenkins, retest this please.

[SparkQA]

QA tests have started for PR 2335 at commit 5b4e37c.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2335 at commit 5b4e37c.

• This patch passes unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

Merging in master. Thanks!