Mode() not compatible with fillna()

[alfonsomhc]

I made an toy dataframe:
df = pandas.DataFrame([[1, 1, 1],[2, 1, 1],[2, 1, 1],[numpy.nan, numpy.nan, numpy.nan]], columns=["a","b","c"])

I try different methods to fill missing values. These work as expected:
df.fillna(df.mean())
df.fillna(df.median())

But this doesnt work:
df.fillna(df.mode())

Inspecting the output from df.mode() I see it has different format than df.mean() and df.median(). As I user I would expect the same behavior for these functions, and be able to fill missing values as described.
Using Pandas 0.15.2

[alfonsomhc]

I have found that if I want to fill NaN with the mode, I need to do this:
df.fillna(df.mode().ix[0])
I would have expected the mean, median and mode to all return the same type of object. As far as I have understood, mean and median return an series (for my example data frame), but the mode returns a dataframe...

[TomAugspurger]

mode can't reduce a DataFrame to a Series because there could be items with the same number of counts

In [16]: df = pd.DataFrame({'A': [1, 2, 1, 2, 1, 2, 3]})

In [17]: df.mode()
Out[17]:
   A
0  1
1  2

[shoyer]

Hmm. If I were designing mode from scratch, I would probably choose to have just use the first such value -- similar to np.argmax. But at this point, we are probably stuck. We could consider adding some sort of keyword argument to change this behavior, but indexing is also pretty easy.

[alfonsomhc]

Thanks for looking into this and also for the explanation. As a user I would like a parameter that controls this behavior, where the default is to return a series (i.e. choose the first mode if many). Whatever you decide, may I suggest that at least the clarification/example given by TomAugspurger is added to the documentation (http://pandas.pydata.org/pandas-docs/dev/generated/pandas.DataFrame.mode.html)? I did read that page before creating this issue, and the reason why a dataframe is returned was not clear to me...

[shoyer]

@alfonsomhc If you'd like to put together a PR with a documentation patch, it would be gratefully accepted.

[alfonsomhc]

I see that the page I referred to is generated by the documentation in file pandas/core/frame.py
Should I just add the note there then?

[alfonsomhc]

I didnt really know how to do the pull request. Hopefully I didnt break anything!

[alfonsomhc]

And now suddenly the issue is closed? Hopefully somebody can verify what I did. In case it wasnt clear enough, it's the first time I contribute to an open source project...

[mroeschke]

Closing as it looks like the proper documentation was added.