Add property: DataFrame.ndim, Index.ndim, MultiIndex.ndim (#947)

https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.ndim.html#pandas.DataFrame.ndim

```python
>>> df = ks.range(10)
>>> df.ndim
2

>>> df = ks.DataFrame([[1, 2], [4, 5], [7, 8]],
...                   index=['cobra', 'viper', 'sidewinder'],
...                   columns=['max_speed', 'shield'])
>>> df.ndim
2
```

Author: itholic

databricks/koalas/base.py

@@ -363,6 +363,39 @@ def is_monotonic_decreasing(self):
         window = Window.orderBy(monotonically_increasing_id()).rowsBetween(-1, -1)
         return self._with_new_scol((col <= F.lag(col, 1).over(window)) & col.isNotNull()).all()
 
+    @property
+    def ndim(self):
+        """
+        Return an int representing the number of array dimensions.
+
+        Return 1 for Series / Index / MultiIndex.
+
+        Examples
+        --------
+
+        For Series
+
+        >>> s = ks.Series([None, 1, 2, 3, 4], index=[4, 5, 2, 1, 8])
+        >>> s.ndim
+        1
+
+        For Index
+
+        >>> s.index.ndim
+        1
+
+        For MultiIndex
+
+        >>> midx = pd.MultiIndex([['lama', 'cow', 'falcon'],
+        ...                       ['speed', 'weight', 'length']],
+        ...                      [[0, 0, 0, 1, 1, 1, 2, 2, 2],
+        ...                       [1, 1, 1, 1, 1, 2, 1, 2, 2]])
+        >>> s = ks.Series([45, 200, 1.2, 30, 250, 1.5, 320, 1, 0.3], index=midx)
+        >>> s.index.ndim
+        1
+        """
+        return 1
+
     def astype(self, dtype):
         """
         Cast a Koalas object to a specified dtype ``dtype``.

databricks/koalas/frame.py

@@ -377,6 +377,29 @@ def __init__(self, data=None, index=None, columns=None, dtype=None, copy=False):
     def _sdf(self) -> spark.DataFrame:
         return self._internal.sdf
 
+    @property
+    def ndim(self):
+        """
+        Return an int representing the number of array dimensions.
+
+        return 2 for DataFrame.
+
+        Examples
+        --------
+
+        >>> df = ks.DataFrame([[1, 2], [4, 5], [7, 8]],
+        ...                   index=['cobra', 'viper', None],
+        ...                   columns=['max_speed', 'shield'])
+        >>> df
+               max_speed  shield
+        cobra          1       2
+        viper          4       5
+        NaN            7       8
+        >>> df.ndim
+        2
+        """
+        return 2
+
     def _reduce_for_stat_function(self, sfun, name, axis=None, numeric_only=False):
         """
         Applies sfun to each column and returns a pd.Series where the number of rows equal the

databricks/koalas/missing/frame.py

@@ -32,7 +32,6 @@ class _MissingPandasLikeDataFrame(object):
     # Properties
     axes = unsupported_property('axes')
     iat = unsupported_property('iat')
-    ndim = unsupported_property('ndim')
 
     # Deprecated properties
     blocks = unsupported_property('blocks', deprecated=True)

databricks/koalas/missing/indexes.py

@@ -33,7 +33,6 @@ class _MissingPandasLikeIndex(object):
     T = unsupported_property('T')
     has_duplicates = unsupported_property('has_duplicates')
     nbytes = unsupported_property('nbytes')
-    ndim = unsupported_property('ndim')
     nlevels = unsupported_property('nlevels')
     shape = unsupported_property('shape')
 
@@ -134,7 +133,6 @@ class _MissingPandasLikeMultiIndex(object):
     is_all_dates = unsupported_property('is_all_dates')
     levels = unsupported_property('levels')
     levshape = unsupported_property('levshape')
-    ndim = unsupported_property('ndim')
     nlevels = unsupported_property('nlevels')
     shape = unsupported_property('shape')
 

databricks/koalas/series.py

@@ -821,11 +821,6 @@ def shape(self):
         """Return a tuple of the shape of the underlying data."""
         return len(self),
 
-    @property
-    def ndim(self):
-        """Returns number of dimensions of the Series."""
-        return 1
-
     @property
     def name(self) -> Union[str, Tuple[str, ...]]:
         """Return name of the Series."""

docs/source/reference/frame.rst

@@ -27,6 +27,7 @@ Attributes and underlying data
 
    DataFrame.dtypes
    DataFrame.shape
+   DataFrame.ndim
    DataFrame.size
    DataFrame.select_dtypes
 

docs/source/reference/indexing.rst

@@ -21,6 +21,7 @@ Properties
    Index.dtype
    Index.name
    Index.names
+   Index.ndim
    Index.empty
 
 Modifying and computations
@@ -76,6 +77,7 @@ MultiIndex Properties
    :toctree: api/
 
    MultiIndex.names
+   MultiIndex.ndim
 
 MultiIndex Modifying and computations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~