Add examples for importlib in doc

[SaimadhavHeblikar]

BPO	21108
Nosy	@brettcannon, @ncoghlan, @ericsnowcurrently, @vadmium, @raulcd
Files	importlib-example.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-03-18.18:30:02.573>
created_at = <Date 2014-03-31.04:42:24.104>
labels = ['docs']
title = 'Add examples for importlib in doc'
updated_at = <Date 2020-03-18.18:30:02.572>
user = 'https://bugs.python.org/SaimadhavHeblikar'

bugs.python.org fields:

activity = <Date 2020-03-18.18:30:02.572>
actor = 'brett.cannon'
assignee = 'docs@python'
closed = True
closed_date = <Date 2020-03-18.18:30:02.573>
closer = 'brett.cannon'
components = ['Documentation']
creation = <Date 2014-03-31.04:42:24.104>
creator = 'Saimadhav.Heblikar'
dependencies = []
files = ['34675']
hgrepos = []
issue_num = 21108
keywords = ['patch']
message_count = 3.0
messages = ['215220', '215246', '364551']
nosy_count = 7.0
nosy_names = ['brett.cannon', 'ncoghlan', 'docs@python', 'eric.snow', 'martin.panter', 'Saimadhav.Heblikar', 'raulcd']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue21108'
versions = ['Python 3.4', 'Python 3.5']

[SaimadhavHeblikar]

The imp module was the defacto module to access the import internals,upto 3.3
It has been deprecated(pending) in 3.4 in favor of importlib.
There are plenty of examples on how to use imp around the web, but very little (even in doc.python.org), on how to use importlib.

The proposed patch adds couple of examples. These examples are aimed towards people, who were earlier using imp's load_module().

Inspiration for the patch from this SO answer, where user had to go through source code.
http://stackoverflow.com/questions/19009932/import-abitrary-python-source-file-python-3-3

[brettcannon]

While I'm totally happy to add examples on usage, it is probably best to put them at the end of the class-level docs instead of trailing after the last method of the class. That way someone reading about what the class does can also see examples of how to use it.

Another point is that SourceFileLoader.load_module() is documented as deprecated as of Python 3.4 and so promoting it in an example is not good. Now that module creation has been broken out separately this "create a module from a file path" approach is becoming more and more difficult. If we care to continue to support this use case we will probably need to come up with a function(s) to make it easier and toss something up on PyPI that is Python 2.7-3.5 compatible.

[brettcannon]

There are now examples in the importlib docs.