[SPARK-10380][SQL] Fix confusing documentation examples for astype/drop_duplicates.

## What changes were proposed in this pull request?
We have seen users getting confused by the documentation for astype and drop_duplicates, because the examples in them do not use these functions (but do uses their aliases). This patch simply removes all examples for these functions, and say that they are aliases.

## How was this patch tested?
Existing PySpark unit tests.

Closes #11543.

Author: Reynold Xin <rxin@databricks.com>

Closes #11698 from rxin/SPARK-10380.

python/pyspark/__init__.py

@@ -37,6 +37,8 @@ Public classes:
 
 """
 
+import types
+
 from pyspark.conf import SparkConf
 from pyspark.context import SparkContext
 from pyspark.rdd import RDD
@@ -64,6 +66,24 @@ def since(version):
     return deco
 
 
+def copy_func(f, name=None, sinceversion=None, doc=None):
+    """
+    Returns a function with same code, globals, defaults, closure, and
+    name (or provide a new name).
+    """
+    # See
+    # http://stackoverflow.com/questions/6527633/how-can-i-make-a-deepcopy-of-a-function-in-python
+    fn = types.FunctionType(f.__code__, f.__globals__, name or f.__name__, f.__defaults__,
+                            f.__closure__)
+    # in case f was given attrs (note this dict is a shallow copy):
+    fn.__dict__.update(f.__dict__)
+    if doc is not None:
+        fn.__doc__ = doc
+    if sinceversion is not None:
+        fn = since(sinceversion)(fn)
+    return fn
+
+
 # for back compatibility
 from pyspark.sql import SQLContext, HiveContext, Row
 

python/pyspark/sql/column.py

@@ -22,7 +22,7 @@ if sys.version >= '3':
     basestring = str
     long = int
 
-from pyspark import since
+from pyspark import copy_func, since
 from pyspark.context import SparkContext
 from pyspark.rdd import ignore_unicode_prefix
 from pyspark.sql.types import *
@@ -337,7 +337,7 @@ class Column(object):
             raise TypeError("unexpected type: %s" % type(dataType))
         return Column(jc)
 
-    astype = cast
+    astype = copy_func(cast, sinceversion=1.4, doc=":func:`astype` is an alias for :func:`cast`.")
 
     @since(1.3)
     def between(self, lowerBound, upperBound):

python/pyspark/sql/dataframe.py

@@ -26,7 +26,7 @@ if sys.version >= '3':
 else:
     from itertools import imap as map
 
-from pyspark import since
+from pyspark import copy_func, since
 from pyspark.rdd import RDD, _load_from_socket, ignore_unicode_prefix
 from pyspark.serializers import BatchedSerializer, PickleSerializer, UTF8Deserializer
 from pyspark.storagelevel import StorageLevel
@@ -829,8 +829,6 @@ class DataFrame(object):
             raise TypeError("condition should be string or Column")
         return DataFrame(jdf, self.sql_ctx)
 
-    where = filter
-
     @ignore_unicode_prefix
     @since(1.3)
     def groupBy(self, *cols):
@@ -1361,8 +1359,20 @@ class DataFrame(object):
     # Pandas compatibility
     ##########################################################################################
 
-    groupby = groupBy
-    drop_duplicates = dropDuplicates
+    groupby = copy_func(
+        groupBy,
+        sinceversion=1.4,
+        doc=":func:`groupby` is an alias for :func:`groupBy`.")
+
+    drop_duplicates = copy_func(
+        dropDuplicates,
+        sinceversion=1.4,
+        doc=":func:`drop_duplicates` is an alias for :func:`dropDuplicates`.")
+
+    where = copy_func(
+        filter,
+        sinceversion=1.3,
+        doc=":func:`where` is an alias for :func:`filter`.")
 
 
 def _to_scala_map(sc, jm):