[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.

1 files changed, 20 insertions, 0 deletions

diff --git a/python/pyspark/__init__.py b/python/pyspark/__init__.py
index d530723ca9..111ebaafee 100644
--- a/python/pyspark/__init__.py
+++ b/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