ENH: allow dataframe get to take an axis argument

[nbonnotte]

closes #6703

I would have like to have a solution also working for panels, but I've never used them, so I'm not sure how it should behave.

[jreback]

just modify the one in core/generic.py

[nbonnotte]

Ok, I used your suggestion and put the tests in tests/test_generic.py... but it still is the same test. I should probably make more tests, in particular to test what happens for Panel.

(Sorry, Atom couldn't help but by itself remove some spaces that were at the end of their lines)

[nbonnotte]

I don't understand, why is the build failing?

[max-sixty]

If you go to 'Details', you can get the build's output. In this case:

======================================================================
FAIL: test_getitem_get (pandas.tests.test_series.TestSeries)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/pydata/pandas/pandas/tests/test_series.py", line 1339, in test_getitem_get
    self.series.get(-1), self.series.get(self.series.index[-1]))
AssertionError: None != -0.2267325702945841

[nbonnotte]

Uh, yes, thanks.

I think I've identified what is going on, but I'm not sure on how to proceed:

In [56]: pd.Series([0,1], index=[0,1]).get(-1)

In [57]: pd.Series([0,1], index=['a','b']).get(-1)
Out[57]: 1

So, basically, the result of the get method for a series depends on the type of the index... Which is not the case for a dataframe:

In [63]: pd.DataFrame([[0,1],[2,3]], columns=['a','b']).get(-1) is None
Out[63]: True

I could redefine Series.get to supersede NDFrame.get... but I don't know if that's a good solution. Maybe there are other classes inheriting from NDFrame that work like Series?

Here is what I'm proposing: if the solution based on the slice and loc fail, and axis is None, we revert to the old implementation. Thus we maintain a backward compatibility...

[jreback]

not needed, this just generates a ValueError which by definition returns default

[nbonnotte]

That generates a TypeError ("cannot use label indexing with a null key"). But you're right, I can also catch it below...

[shoyer]

I'm not sure this is a good idea. Get is a standard method for mapping types and we already have a lot of indexing methods.

[jreback]

yeh, this is getting into too many edge cases. Lets just keep .get simple.