DOC: Merge FAQ and gotcha

doc/source/10min.rst

@@ -810,4 +810,4 @@ If you are trying an operation and you see an exception like:
 
 See :ref:`Comparisons<basics.compare>` for an explanation and what to do.
 
-See :ref:`Gotchas<gotchas>` as well.
+See :ref:`FAQ<faq>` as well.

doc/source/advanced.rst

@@ -853,3 +853,129 @@ Of course if you need integer based selection, then use ``iloc``
 .. ipython:: python
 
    dfir.iloc[0:5]
+
+Miscellaneous indexing FAQ
+--------------------------
+
+Integer indexing with ix
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Label-based indexing with integer axis labels is a thorny topic. It has been
+discussed heavily on mailing lists and among various members of the scientific
+Python community. In pandas, our general viewpoint is that labels matter more
+than integer locations. Therefore, with an integer axis index *only*
+label-based indexing is possible with the standard tools like ``.ix``. The
+following code will generate exceptions:
+
+.. code-block:: python
+
+   s = pd.Series(range(5))
+   s[-1]
+   df = pd.DataFrame(np.random.randn(5, 4))
+   df
+   df.ix[-2:]
+
+This deliberate decision was made to prevent ambiguities and subtle bugs (many
+users reported finding bugs when the API change was made to stop "falling back"
+on position-based indexing).
+
+Non-monotonic indexes require exact matches
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If the index of a ``Series`` or ``DataFrame`` is monotonically increasing or decreasing, then the bounds
+of a label-based slice can be outside the range of the index, much like slice indexing a
+normal Python ``list``. Monotonicity of an index can be tested with the ``is_monotonic_increasing`` and
+``is_monotonic_decreasing`` attributes.
+
+.. ipython:: python
+
+    df = pd.DataFrame(index=[2,3,3,4,5], columns=['data'], data=range(5))
+    df.index.is_monotonic_increasing
+
+    # no rows 0 or 1, but still returns rows 2, 3 (both of them), and 4:
+    df.loc[0:4, :]
+
+    # slice is are outside the index, so empty DataFrame is returned
+    df.loc[13:15, :]
+
+On the other hand, if the index is not monotonic, then both slice bounds must be
+*unique* members of the index.
+
+.. ipython:: python
+
+    df = pd.DataFrame(index=[2,3,1,4,3,5], columns=['data'], data=range(6))
+    df.index.is_monotonic_increasing
+
+    # OK because 2 and 4 are in the index
+    df.loc[2:4, :]
+
+.. code-block:: python
+
+    # 0 is not in the index
+    In [9]: df.loc[0:4, :]
+    KeyError: 0
+
+    # 3 is not a unique label
+    In [11]: df.loc[2:3, :]
+    KeyError: 'Cannot get right slice bound for non-unique label: 3'
+
+
+Endpoints are inclusive
+~~~~~~~~~~~~~~~~~~~~~~~
+
+Compared with standard Python sequence slicing in which the slice endpoint is
+not inclusive, label-based slicing in pandas **is inclusive**. The primary
+reason for this is that it is often not possible to easily determine the
+"successor" or next element after a particular label in an index. For example,
+consider the following Series:
+
+.. ipython:: python
+
+   s = pd.Series(np.random.randn(6), index=list('abcdef'))
+   s
+
+Suppose we wished to slice from ``c`` to ``e``, using integers this would be
+
+.. ipython:: python
+
+   s[2:5]
+
+However, if you only had ``c`` and ``e``, determining the next element in the
+index can be somewhat complicated. For example, the following does not work:
+
+::
+
+    s.loc['c':'e'+1]
+
+A very common use case is to limit a time series to start and end at two
+specific dates. To enable this, we made the design design to make label-based
+slicing include both endpoints:
+
+.. ipython:: python
+
+    s.loc['c':'e']
+
+This is most definitely a "practicality beats purity" sort of thing, but it is
+something to watch out for if you expect label-based slicing to behave exactly
+in the way that standard Python integer slicing works.
+
+
+Indexing potentially changes underlying Series dtype
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The use of ``reindex_like`` can potentially change the dtype of a ``Series``.
+
+.. ipython:: python
+
+   series = pd.Series([1, 2, 3])
+   x = pd.Series([True])
+   x.dtype
+   x = pd.Series([True]).reindex_like(series)
+   x.dtype
+
+This is because ``reindex_like`` silently inserts ``NaNs`` and the ``dtype``
+changes accordingly.  This can cause some issues when using ``numpy`` ``ufuncs``
+such as ``numpy.logical_and``.
+
+See the `this old issue <https://github.com/pydata/pandas/issues/2388>`__ for a more
+detailed discussion.

doc/source/basics.rst

@@ -287,7 +287,7 @@ To evaluate single-element pandas objects in a boolean context, use the method
 
        ValueError: The truth value of an array is ambiguous. Use a.empty, a.any() or a.all().
 
-See :ref:`gotchas<gotchas.truth>` for a more detailed discussion.
+See :ref:`FAQ<faq.truth>` for a more detailed discussion.
 
 .. _basics.equals:
 
@@ -1849,7 +1849,7 @@ gotchas
 
 Performing selection operations on ``integer`` type data can easily upcast the data to ``floating``.
 The dtype of the input data will be preserved in cases where ``nans`` are not introduced (starting in 0.11.0)
-See also :ref:`integer na gotchas <gotchas.intna>`
+See also :ref:`Support for integer ``NA`` <faq.intna>`
 
 .. ipython:: python