Introduction of RangeIndex

`RangeIndex(1, 10, 2)` is a memory saving alternative to
`Index(np.arange(1, 10,2))`: c.f. #939.

This re-implementation is compatible with the current `Index()` api and is a
drop-in replacement for `Int64Index()`. It automatically converts to
Int64Index() when required by operations.

At present only for a minimum number of operations the type is
conserved (e.g. slicing, inner-, left- and right-joins). Most other operations
trigger creation of an equivalent Int64Index (or at least an equivalent numpy
array) and fall back to its implementation.

This PR also extends the functionality of the `Index()` constructor to allow
creation of `RangeIndexes()` with
```
Index(20)
Index(2, 20)
Index(0, 20, 2)
```
in analogy to
```
range(20)
range(2, 20)
range(0, 20, 2)
```

restore Index() fastpath precedence

Various fixes suggested by @jreback and @shoyer

Cache a private Int64Index object the first time it or its values are required.
Restore Index(5) as error. Restore its test. Allow Index(0, 5) and Index(0, 5, 1).
Make RangeIndex immutable. See start, stop, step properties.
In test_constructor(): check class, attributes (possibly including dtype).
In test_copy(): check that copy is not identical (but equal) to the existing.
In test_duplicates(): Assert is_unique and has_duplicates return correct values.

fix slicing

fix view

Set RangeIndex as default index
* enh: set RangeIndex as default index
* fix: pandas.io.packers: encode() and decode() for RangeIndex
* enh: array argument pass-through
* fix: reindex
* fix: use _default_index() in pandas.core.frame.extract_index()
* fix: pandas.core.index.Index._is()
* fix: add RangeIndex to ABCIndexClass
* fix: use _default_index() in _get_names_from_index()
* fix: pytables tests
* fix: MultiIndex.get_level_values()
* fix: RangeIndex._shallow_copy()
* fix: null-size RangeIndex equals() comparison
* enh: make RangeIndex.is_unique immutable

enh: various performance optimizations

 * optimize argsort()
 * optimize tolist()
 * comment clean-up

Author: ARF

pandas/core/api.py

@@ -8,7 +8,7 @@
 from pandas.core.categorical import Categorical
 from pandas.core.groupby import Grouper
 from pandas.core.format import set_eng_float_format
-from pandas.core.index import Index, CategoricalIndex, Int64Index, Float64Index, MultiIndex
+from pandas.core.index import Index, CategoricalIndex, Int64Index, RangeIndex, Float64Index, MultiIndex
 
 from pandas.core.series import Series, TimeSeries
 from pandas.core.frame import DataFrame

pandas/core/common.py

@@ -85,6 +85,7 @@ def _check(cls, inst):
 ABCCategoricalIndex = create_pandas_abc_type("ABCCategoricalIndex", "_typ", ("categoricalindex",))
 ABCIndexClass = create_pandas_abc_type("ABCIndexClass", "_typ", ("index",
                                                                  "int64index",
+                                                                 "rangeindex",
                                                                  "float64index",
                                                                  "multiindex",
                                                                  "datetimeindex",
@@ -1756,10 +1757,8 @@ def is_bool_indexer(key):
 
 
 def _default_index(n):
-    from pandas.core.index import Int64Index
-    values = np.arange(n, dtype=np.int64)
-    result = Int64Index(values,name=None)
-    result.is_unique = True
+    from pandas.core.index import RangeIndex
+    result = RangeIndex(0, int(n), name=None)
     return result
 
 
@@ -2157,6 +2156,11 @@ def is_int64_dtype(arr_or_dtype):
     tipo = _get_dtype_type(arr_or_dtype)
     return issubclass(tipo, np.int64)
 
+def is_int64_dtype(arr_or_dtype):
+    tipo = _get_dtype_type(arr_or_dtype)
+    return issubclass(tipo, np.int64)
+
+
 def is_int_or_datetime_dtype(arr_or_dtype):
     tipo = _get_dtype_type(arr_or_dtype)
     return (issubclass(tipo, np.integer) or

pandas/core/frame.py

@@ -5302,7 +5302,7 @@ def extract_index(data):
                            % (lengths[0], len(index)))
                     raise ValueError(msg)
             else:
-                index = Index(np.arange(lengths[0]))
+                index = _default_index(lengths[0])
 
     return _ensure_index(index)
 
@@ -5519,11 +5519,11 @@ def convert(arr):
 
 
 def _get_names_from_index(data):
-    index = lrange(len(data))
     has_some_name = any([getattr(s, 'name', None) is not None for s in data])
     if not has_some_name:
-        return index
+        return _default_index(len(data))
 
+    index = lrange(len(data))
     count = 0
     for i, s in enumerate(data):
         n = getattr(s, 'name', None)