Where/mask methods for Series
add where and mask methods for Series, analagous to DataFrame methods added for 0.9.1
passes all tests

where is equivalent to: `s[cond].reindex_like(s).fillna(other)`

```
In [7]: s = pd.Series(np.random.rand(5))

In [8]: s
Out[8]: 
0    0.638664
1    0.574688
2    0.460510
3    0.641840
4    0.044129

In [10]: s[0:2] = -s[0:2]

In [11]: s
Out[11]: 
0   -0.638664
1   -0.574688
2    0.460510
3    0.641840
4    0.044129
```

boolean selection

```
In [12]: s[s>0]
Out[12]: 
2    0.460510
3    0.641840
4    0.044129

In [13]: s.where(s>0)
Out[13]: 
0         NaN
1         NaN
2    0.460510
3    0.641840
4    0.044129

In [14]: s.where(s>0,-s)
Out[14]: 
0    0.638664
1    0.574688
2    0.460510
3    0.641840
4    0.044129

In [15]: s.mask(s<=0)
Out[15]: 
0         NaN
1         NaN
2    0.460510
3    0.641840
4    0.044129

```

support setting as well (though not used anywhere explicity)

```
In [16]: s2 = s.copy()

In [17]: s2.where(s2>0,inplace=True)
Out[17]: 
0         NaN
1         NaN
2    0.460510
3    0.641840
4    0.044129

In [18]: s2
Out[18]: 
0         NaN
1         NaN
2    0.460510
3    0.641840
4    0.044129

```

I cherry-picked this.
I changed the implementation of where a little bit as _set_value would have failed for non-scalar other.
I also added a few more test cases.

Thanks for the PR!
 added docs for these in commit: 2d579791d9672410ad4d23f884c457c31cbbe652
 cherry-picked. Thank you!
 Are the `where` and `mask` methods supposed to be included in 0.9.1?  I'm not seeing them, but maybe I've got something setup incorrectly?

``` python
>>> pandas.__version__
'0.9.1'
>>> df['col2'].where
Traceback (most recent call last):
  File "<ipython-input-19-885524901cf2>", line 1, in <module>
    df['col2'].where
AttributeError: 'Series' object has no attribute 'where'
```

``` python
>>> dir(df['col2'])
['T', '_AXIS_ALIASES', '_AXIS_NAMES', '_AXIS_NUMBERS', '__abs__', '__add__', '__and__', '__array__', '__array_finalize__', '__array_interface__', '__array_prepare__', '__array_priority__', '__array_struct__', '__array_wrap__', '__class__', '__contains__', '__copy__', '__deepcopy__', '__delattr__', '__delitem__', '__delslice__', '__dict__', '__div__', '__divmod__', '__doc__', '__eq__', '__float__', '__floordiv__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getslice__', '__gt__', '__hash__', '__hex__', '__iadd__', '__iand__', '__idiv__', '__ifloordiv__', '__ilshift__', '__imod__', '__imul__', '__index__', '__init__', '__int__', '__invert__', '__ior__', '__ipow__', '__irshift__', '__isub__', '__iter__', '__itruediv__', '__ixor__', '__le__', '__len__', '__long__', '__lshift__', '__lt__', '__mod__', '__module__', '__mul__', '__ne__', '__neg__', '__new__', '__nonzero__', '__oct__', '__or__', '__pos__', '__pow__', '__radd__', '__rand__', '__rdiv__', '__rdivmod__', '__reduce__', '__reduce_ex__', '__repr__', '__rfloordiv__', '__rlshift__', '__rmod__', '__rmul__', '__ror__', '__rpow__', '__rrshift__', '__rshift__', '__rsub__', '__rtruediv__', '__rxor__', '__setattr__', '__setitem__', '__setslice__', '__setstate__', '__sizeof__', '__str__', '__sub__', '__subclasshook__', '__truediv__', '__weakref__', '__xor__', '_agg_by_level', '_binop', '_can_hold_na', '_check_bool_indexer', '_constructor', '_get_axis', '_get_axis_name', '_get_axis_number', '_get_repr', '_get_val_at', '_get_values', '_get_values_tuple', '_get_with', '_index', '_ix', '_reindex_indexer', '_repr_footer', '_set_labels', '_set_values', '_set_with', '_tidy_repr', 'abs', 'add', 'align', 'all', 'any', 'append', 'apply', 'argmax', 'argmin', 'argsort', 'asfreq', 'asof', 'astype', 'at_time', 'autocorr', 'base', 'between', 'between_time', 'byteswap', 'choose', 'clip', 'clip_lower', 'clip_upper', 'combine', 'combine_first', 'compress', 'conj', 'conjugate', 'copy', 'corr', 'count', 'cov', 'ctypes', 'cummax', 'cummin', 'cumprod', 'cumsum', 'data', 'describe', 'diagonal', 'diff', 'div', 'dot', 'drop', 'dropna', 'dtype', 'dump', 'dumps', 'fill', 'fillna', 'first', 'first_valid_index', 'flags', 'flat', 'flatten', 'from_array', 'from_csv', 'get', 'get_value', 'getfield', 'groupby', 'head', 'hist', 'idxmax', 'idxmin', 'iget', 'iget_value', 'imag', 'index', 'interpolate', 'irow', 'isin', 'isnull', 'item', 'itemset', 'itemsize', 'iteritems', 'iterkv', 'ix', 'keys', 'kurt', 'last', 'last_valid_index', 'load', 'mad', 'map', 'max', 'mean', 'median', 'min', 'mul', 'name', 'nbytes', 'ndim', 'newbyteorder', 'nonzero', 'notnull', 'nunique', 'order', 'pct_change', 'plot', 'prod', 'ptp', 'put', 'quantile', 'rank', 'ravel', 'real', 'reindex', 'reindex_like', 'rename', 'reorder_levels', 'repeat', 'replace', 'resample', 'reset_index', 'reshape', 'resize', 'round', 'save', 'searchsorted', 'select', 'set_value', 'setasflat', 'setfield', 'setflags', 'shape', 'shift', 'size', 'skew', 'sort', 'sort_index', 'sortlevel', 'squeeze', 'std', 'str', 'strides', 'sub', 'sum', 'swapaxes', 'swaplevel', 'tail', 'take', 'to_csv', 'to_dict', 'to_sparse', 'to_string', 'tofile', 'tolist', 'tostring', 'trace', 'transpose', 'truncate', 'tshift', 'tz_convert', 'tz_localize', 'unique', 'unstack', 'update', 'valid', 'value_counts', 'values', 'var', 'view', 'weekday']

```
 in 0.9.1 they only exist in a DataFrame
0.10.0 will support Series as well

docs will explain the slight difference

eg 

s[cond] will return a subset of rows
but df[cond] returns a same sized frame
if s is a series and df is a frame

On Nov 28, 2012, at 3:22 PM, Luke Lee notifications@github.com wrote:

> Are the where and mask methods supposed to be included in 0.9.1? I'm not seeing them, but maybe I've got something setup incorrectly?
> 
> > > > pandas.**version**
> > > > '0.9.1'
> > > > df['col2'].where
> > > > Traceback (most recent call last):
> > > >   File "<ipython-input-19-885524901cf2>", line 1, in <module>
> > > >     df['col2'].where
> > > > AttributeError: 'Series' object has no attribute 'where'
> > > > dir(df['col2'])
> > > > ['T', '_AXIS_ALIASES', '_AXIS_NAMES', '_AXIS_NUMBERS', '__abs__', '**add**', '**and**', '**array**', '**array_finalize**', '**array_interface**', '**array_prepare**', '**array_priority**', '**array_struct**', '**array_wrap**', '**class**', '**contains**'
> > > >  , '**copy**', '**deepcopy**', '**delattr**', '**delitem**', '**delslice**', '**dict**', '**div**', '**divmod**', '**doc**', '**eq**', '**float**', '**floordiv**', '**format**', '**ge**', '**getattribute**', '**getitem**', '**getslice**', '**gt**', '**hash**', '**hex**', '**iadd**', '**iand**', '**idiv**', '**ifloordiv**', '**ilshift**', '**imod**', '**imul**', '**index**', '**init**', '**int**', '**invert**', '**ior**', '**ipow**', <
> > > >  span class="s">'**irshift**', '**isub**', '**iter**', '**itruediv**', '**ixor**', '**le**', '**len**', '**long**', '**lshift**', '**lt**', '**mod**', '**module**', '**mul**', '**ne**', '**neg**', '**new**', '**nonzero**', <
> > > >  span class="s">'**oct**', '**or**', '**pos**', '**pow**', '**radd**', '**rand**', '**rdiv**', '**rdivmod**', '**reduce**', '**reduce_ex**', '**repr**', '**rfloordiv**', '**rlshift**', '**rmod**', '**rmul**', '**ror**', '**rpow**',
> > > >   '**rrshift**', '**rshift**', '**rsub**', '**rtruediv**', '**rxor**', '**setattr**', '**setitem**', '**setslice**', '**setstate**', '**sizeof**', '**str**', '**sub**', '**subclasshook**', '**truediv**', '**weakref**', '**xor**', '
> > > >  _agg_by_level', '_binop', '_can_hold_na', '_check_bool_indexer', '_constructor', '_get_axis', '_get_axis_name', '_get_axis_number', '_get_repr', '_get_val_at', '_get_values', '_get_values_tuple', '_get_with', '_index', '_ix', '_reindex_indexer', '_repr_
> > > >  footer', '_set_labels', '_set_values', '_set_with', '_tidy_repr', 'abs', 'add', 'align', 'all', 'any', 'append', 'apply', 'argmax', 'argmin', 'argsort', 'asfreq', 'asof', 'astype', 'at_time', 'autocorr', 'base', 'between', 'between_time', 'byteswap', 'choose', 'clip', 'clip_lower', 'clip_upper', 'combine', 'combine_first', 'compress', 'conj', 'conjugate', 'copy', 'corr', 'count', 'cov', 'ctypes', 'cummax', 'cummin', 'cumprod', 'cumsum', 'data', 'describe', 'diagonal', 'diff', 'div', 'dot', 'drop', 'dropna', 'dtype', 'dump', 'dumps', 'fill', 'fillna', 'first', 'first_valid_index', 'flags', 'flat', 'flatten', 'from_array', 'from_csv', 'get', 'get_value', 'getfield', 'groupby', 'head', 'hist', 'idxmax', 'idxmin', 'iget', 'iget_value', 'imag', 'index', 'interpolate', 'irow', 'isin', 'isnull', 'item', 'itemset', 'itemsize', 'iteritems', 'iterkv', 'ix', 'keys', 'kurt', 'last', 'last_valid_index', 'load', 'mad', 'map', 'max', 'mean', 'median', 'min', 'mul', 'name', 'nbytes', 'ndim', 'newbyteorder', 'nonzero', 'notnull', 'nunique', 'order', 'pct_change', 'plot', 'prod', 'ptp', 'put', 'quantile', 'rank', 'ravel', 'real', 'reindex', 'reindex_like', 'rename', 'reorder_levels', 'repeat', 'replace', 'resample', 'reset_index', 'reshape', 'resize', 'round', 'save', 'searchsorted', 'select', 'set_value', 'setasflat', 'setfield', 'setflags', 'shape', 'shift', 'size', 'skew', 'sort', 'sort_index', 'sortlevel', 'squeeze', 'std', 'str', <
> > > >  span class="s">'strides', 'sub', 'sum', 'swapaxes', 'swaplevel', 'tail', 'take', 'to_csv', 'to_dict', 'to_sparse', 'to_string', 'tofile', 'tolist', 'tostring', 'trace', 'transpose', 'truncate', 'tshift', 'tz_convert', 'tz_localize', 'unique', 'unstack', 'update', 'valid', 'value_counts', 'values', 'var', 'view', 'weekday']
> 
> —
> Reply to this email directly or view it on GitHub.
 @jreback Ah, that clears up the confusion.  Thanks!
