TST: Move explicit connectivity checks to decorator.
Instead, `network` decorator in pandas.util.testing catches `IOError` instead.
You have to opt into failing on tests by setting
`pandas.util.testing._RAISE_NETWORK_ERROR_DEFAULT` to `True`.

Also adds a `with_network_connectivity_check` that can automatically check for a connection.

Fixes #3910.

This version of the fix ignores all IOErrors and assumes there are connectivity problems
with any URLError.

This version (jtratner/pandas@77ede4be0607657d713ad9bd7e445e6f024f5cd8) uses google.com as a proxy for network connectivity like the tests were previously doing.
 @jtratner is there a assert raises warning function? i thought i remember u saying something about it somewhere...
 nvm i added one from [here](http://stackoverflow.com/a/12935176/564538).
 This is what I had before...just be aware that warnings seem kinda finicky (and you might want to remove the `extra_warnings` check).

``` python
@contextmanager
def assert_produces_warning(expected_warning=Warning, filter_level="always"):
    """
    Context manager for running code that expects to raise (or not raise)
    warnings.  Checks that code raises the expected warning and only the
    expected warning. Pass ``False`` or ``None`` to check that it does *not*
    raise a warning. Defaults to ``exception.Warning``, baseclass of all
    Warnings. (basically a wrapper around ``warnings.catch_warnings``).

    >>> import warnings
    >>> with assert_produces_warning():
    ...     warnings.warn(UserWarning())
    ...
    >>> with assert_produces_warning(False):
    ...     warnings.warn(RuntimeWarning())
    ...
    Traceback (most recent call last):
        ...
    AssertionError: Caused unexpected warning(s): ['RuntimeWarning'].
    >>> with assert_produces_warning(UserWarning):
    ...     warnings.warn(RuntimeWarning())
    Traceback (most recent call last):
        ...
    AssertionError: Did not see expected warning of class 'UserWarning'.

    ..warn:: This is *not* thread-safe.
    """
    with warnings.catch_warnings(record=True) as w:
        saw_warning = False
        warnings.simplefilter(filter_level)
        yield w
        extra_warnings = []
        for actual_warning in w:
            if expected_warning and \
                    issubclass(actual_warning.category, expected_warning):
                saw_warning = True
            else:
                extra_warnings.append(actual_warning.category.__name__)
        if expected_warning:
            assert saw_warning, ("Did not see expected warning of class %r."
                                 % expected_warning.__name__)
            assert not extra_warnings, ("Caused unexpected warning(s): %r."
                                    % extra_warnings)
```
 i aspire to document like u
 haha :)

Easy to test the basics when you put it into a doctest...

On Sat, Jun 15, 2013 at 1:44 PM, Phillip Cloud notifications@github.comwrote:

> i aspire to document like u
> 
> —
> Reply to this email directly or view it on GitHubhttps://github.com/pydata/pandas/pull/3914#issuecomment-19500372
> .
 @cpcloud if you use it, nice to stick it in util/testing so it can be used elsewhere.
 yep sure thing
 oh i c u r referring to _these_ doctests
 this would be nice 2 merge i think this might address the hanging of google testing and some of those annoying network errors that have been sporadically popping up on travis. @jreback anything to add?
 @jreback @cpcloud Okay, I added optional `raise_on_error` to network. I decided at this point to also implement a `with_connectivity_check` decorator to abstract out the remaining connectivity checks in the test cases + allow authors to choose to rely on the check for a URLError explicitly.  To do this, I also added an `optional_args` decorator for decorators that lets you easily write decorators with optional arguments.

I hooked `raise_on_error` to a global `_RAISE_NETWORK_ERROR_DEFAULT` so that you can change it in one place and change it wherever else (obviously this doesn't work after the file is imported).

A few points:
1. Very easy to go back to the standard network decorator.
2. Using this format means that what `network` does is much clearer.
3. `with_connectivity_check` could potentially check _FIRST_ and skip the test if it fails (and likely would need to test at end too). It's not particularly difficult to do this...I'm fine either way.
 @jtratner need to rebase 1 more time as we just merged stuff (just release notes conflict)...

otherwise ready 2 go?
 @jreback I'll do it after work - need to remove a test snippet I added to try to get at a weird build failure - check out this build - https://travis-ci.org/jtratner/pandas/builds/8187252 - @cpcloud had the same error on a recent build too, so it's not the decorator (and wouldn't be caught by the decorator anyways). I'm puzzled by what set of circumstances causes that test to fail in that way.
 I think that error might be that you have a 0-len series, which you are then indexing...

(e.g. the call failed but it didn't raise, rather returned something)

```
In [18]: df = DataFrame(columns=list('ABC'),index=date_range('20130101',periods=0))

In [19]: df
Out[19]: 
Empty DataFrame
Columns: [A, B, C]
Index: []

In [20]: df.index
Out[20]: 
<class 'pandas.tseries.index.DatetimeIndex'>
Length: 0, Freq: D, Timezone: None

In [21]: df['A'][-1]
IndexError: index out of bounds

```
 @jreback are you okay with the connectivity decorator? Don't want to add unnecessary complexity, but I think it's useful.
 @jtratner this supersedes #3893, right?

cc @glitpak
 @jtratner the connectivity check is fine
 Yes on your question @jreback
On Jun 18, 2013 6:39 PM, "jreback" notifications@github.com wrote:

> @jtratner https://github.com/jtratner the connectivity check is fine
> 
> —
> Reply to this email directly or view it on GitHubhttps://github.com/pydata/pandas/pull/3914#issuecomment-19648264
> .
 @jreback this is totally ready to go now and now features a check_before_test option that might help @cpcloud ;)
 need rebase (prob release notes conflict)
 i was hoping that one of @gliptak /@jtratner's prs would fix the issue...if not i'll fix it
 @jtratner as an aside this is something that would like to show how to use in a testing page
along with use of assert_catch_warnings
 yes test examples would be nice
 @cpcloud @jreback look at the docstrings
 @jtratner no that's what I meant

what we need is something that says here is the toolbox for testing

eg catch warnings - use this
assert equals - use this

almost like a style guide
 Okay, I can do that. I'm in the midst of adding all the Exception tests
anyways. Would it make sense to add that into the docs directly?

On Tue, Jun 18, 2013 at 9:45 PM, jreback notifications@github.com wrote:

> @jtratner https://github.com/jtratner no that's what I meant
> 
> what we need is something that says here is the toolbox for testing
> 
> eg catch warnings - use this
> assert equals - use this
> 
> almost like a style guide
> 
> —
> Reply to this email directly or view it on GitHubhttps://github.com/pydata/pandas/pull/3914#issuecomment-19656764
> .
 I believe this is all rebased now, I'll work on adding this to the wiki page.
 @jtratner #3893 merged...you're on
 @jreback hey, this has actually been working for a while haha. (rebased it about 10 minutes after you merged haha).

@gliptak if you have a moment - could you take a look at this and see whether any of the functions could either:
1. Do the connectivity check first (i.e., the method doesn't matter if there's no internet connection so we should skip it with a brief call to urlopen rather than waiting for the test case to fail)?
2. Not require a connectivity check and simply suppress URLError or IOError?

Thanks!
 @jtratner thanks for completing the changes. your code certainly makes these `@network` tests more compact :) I'm also unsure which is the best choice (I would have kept `@network`). let's see how this works out in practice.
