Add simple attribute access to DataFrame columns

[takluyver]

This one might need some discussion, but I think it makes sense.

If a column name in a dataframe is a valid Python name and doesn't conflict with any DataFrame attributes, this lets you type df.MyColumn than df['MyColumn']. That's much handier for interactive use, especially because IPython will tab complete df.MyColumn.<tab>, while it won't by default tab complete df['MyColumn'].<tab>.

This doesn't interfere with accessing columns through the dictionary interface.

[wesm]

This seems useful-- it might even be possible to get tab completion but the overall computational cost (overriding __getattribute__) might be too great. Merged it into master. Thanks!

[takluyver]

Thanks. I think there's a method by which objects can expose extra things for tab completion to IPython, but I that's a lower priority.

Just looking at the code, I think we'd need to register the DataFrame type with IPython.utils.generic.complete_object (see code here). I'll get round to doing it at some point.

[wesm]

I created an issue about this:

https://github.com/wesm/pandas/issues/230