Merge pull request #10996 from Winterflower/pandas-reindex-doc

DOC: Improve reindex examples and docstring

Author: jreback

pandas/core/generic.py

@@ -1777,7 +1777,9 @@ def sort_index(self, axis=0, level=None, ascending=True, inplace=False,
             New labels / index to conform to. Preferably an Index object to
             avoid duplicating data
         method : {None, 'backfill'/'bfill', 'pad'/'ffill', 'nearest'}, optional
-            Method to use for filling holes in reindexed DataFrame:
+            method to use for filling holes in reindexed DataFrame.
+            Please note: this is only  applicable to DataFrames/Series with a 
+            monotonically increasing/decreasing index.
               * default: don't fill gaps
               * pad / ffill: propagate last valid observation forward to next valid
               * backfill / bfill: use next valid observation to fill gap
@@ -1801,7 +1803,118 @@ def sort_index(self, axis=0, level=None, ascending=True, inplace=False,
 
         Examples
         --------
-        >>> df.reindex(index=[date1, date2, date3], columns=['A', 'B', 'C'])
+
+        Create a dataframe with some fictional data.
+
+        >>> index = ['Firefox', 'Chrome', 'Safari', 'IE10', 'Konqueror']
+        >>> df = pd.DataFrame({
+        ...      'http_status': [200,200,404,404,301],
+        ...      'response_time': [0.04, 0.02, 0.07, 0.08, 1.0]},
+        ...       index=index)
+        >>> df
+                    http_status  response_time
+        Firefox            200           0.04
+        Chrome             200           0.02
+        Safari             404           0.07
+        IE10               404           0.08
+        Konqueror          301           1.00
+
+        Create a new index and reindex the dataframe. By default
+        values in the new index that do not have corresponding
+        records in the dataframe are assigned ``NaN``. 
+
+        >>> new_index= ['Safari', 'Iceweasel', 'Comodo Dragon', 'IE10',
+        ...             'Chrome']
+        >>> df.reindex(new_index)
+                       http_status  response_time
+        Safari                 404           0.07
+        Iceweasel              NaN            NaN
+        Comodo Dragon          NaN            NaN
+        IE10                   404           0.08
+        Chrome                 200           0.02
+
+        We can fill in the missing values by passing a value to
+        the keyword ``fill_value``. Because the index is not monotonically
+        increasing or decreasing, we cannot use arguments to the keyword  
+        ``method`` to fill the ``NaN`` values. 
+
+        >>> df.reindex(new_index, fill_value=0)
+                       http_status  response_time
+        Safari                 404           0.07
+        Iceweasel                0           0.00
+        Comodo Dragon            0           0.00
+        IE10                   404           0.08
+        Chrome                 200           0.02
+
+        >>> df.reindex(new_index, fill_value='missing')
+                      http_status response_time
+        Safari                404          0.07
+        Iceweasel         missing       missing
+        Comodo Dragon     missing       missing
+        IE10                  404          0.08
+        Chrome                200          0.02
+
+        To further illustrate the filling functionality in 
+        ``reindex``, we will create a dataframe with a 
+        monotonically increasing index (for example, a sequence
+        of dates).
+
+        >>> date_index = pd.date_range('1/1/2010', periods=6, freq='D')
+        >>> df2 = pd.DataFrame({"prices": [100, 101, np.nan, 100, 89, 88]},
+                index=date_index)
+        >>> df2
+                    prices
+        2010-01-01     100
+        2010-01-02     101
+        2010-01-03     NaN
+        2010-01-04     100
+        2010-01-05      89
+        2010-01-06      88
+
+        Suppose we decide to expand the dataframe to cover a wider
+        date range. 
+
+        >>> date_index2 = pd.date_range('12/29/2009', periods=10, freq='D')
+        >>> df2.reindex(date_index2)
+                    prices
+        2009-12-29     NaN
+        2009-12-30     NaN
+        2009-12-31     NaN
+        2010-01-01     100
+        2010-01-02     101
+        2010-01-03     NaN
+        2010-01-04     100
+        2010-01-05      89
+        2010-01-06      88
+        2010-01-07     NaN
+
+        The index entries that did not have a value in the original data frame
+        (for example, '2009-12-29') are by default filled with ``NaN``. 
+        If desired, we can fill in the missing values using one of several
+        options. 
+        
+        For example, to backpropagate the last valid value to fill the ``NaN``
+        values, pass ``bfill`` as an argument to the ``method`` keyword.
+
+        >>> df2.reindex(date_index2, method='bfill')
+                    prices
+        2009-12-29     100
+        2009-12-30     100
+        2009-12-31     100
+        2010-01-01     100
+        2010-01-02     101
+        2010-01-03     NaN
+        2010-01-04     100
+        2010-01-05      89
+        2010-01-06      88
+        2010-01-07     NaN
+
+        Please note that the ``NaN`` value present in the original dataframe
+        (at index value 2010-01-03) will not be filled by any of the 
+        value propagation schemes. This is because filling while reindexing
+        does not look at dataframe values, but only compares the original and
+        desired indexes. If you do want to fill in the ``NaN`` values present
+        in the original dataframe, use the ``fillna()`` method.
 
         Returns
         -------