PERF: remove auto-boxing on isnull/notnull

[jreback]

• API/CLN: provide isnull/notnull on NDFrame objects
• PERF: remove auto-boxing on isnull/notnull

related #5135

[jtratner]

hold on - this is a not-insignificant change here - doesn't this mean you lose indexing on isnull() and notnull()? I think it's fine to have pd.isnull() return unboxed, but NDFrame.isnull() and NDFrame.notnull() should definitely box by default. Maybe it could just short-circuit the constructor instead? The boxing behavior is available on 0.10 and later, so it's not reasonable to change without discussing. You lose a lot of expressiveness this way. Maybe you could add a keyword argument "box=True" to it so that internals can call and not box result.

[jtratner]

(I'm assuming it's available in 0.11 and 0.12, can't test it right now)

[jreback]

@jtratner I preserved the orginal behavior (I had actually changed this in internal refactoring for 0.13). the isnull called directly don't need to actually create a Series (I was going to add a box argument, but its not necessary.

This doesn't affect > 1dim objects anyhow didn't even have df.isnull(); pd.isnull(df) returns the correct (boxed version)
Here's 0.12

In [1]: s = Series([1,np.nan])

In [2]: df = DataFrame(dict(A = s))

In [3]: s.isnull()
Out[3]: 
0    False
1     True
dtype: bool

In [4]: df.isnull()
AttributeError

In [5]: pd.isnull(s)
Out[5]: 
0    False
1     True
dtype: bool

In [6]: pd.isnull(df)
Out[6]: 
       A
0  False
1   True

The difference is:
a Series will not box (this happens to work in 0.12)

In [5]: pd.isnull(s)
Out[5]: array([False,  True], dtype=bool)

I could make this work (but will need a box keyword that is True by default by set False by almost
every routine that used this internally. Which I think is a needless complication.

The API is actually much cleaner as isnull/notnull exist on all objects.

and boolean indexing will work exactly the same. I don't actually think this is an API change at all.

[jreback]

hmm...on second thought it DOES box Series.....ok...easy enough to do