[SPARK-10417] [SQL] Iterating through Column results in infinite loop

`pyspark.sql.column.Column` object has `__getitem__` method, which makes it iterable for Python. In fact it has `__getitem__` to address the case when the column might be a list or dict, for you to be able to access certain element of it in DF API. The ability to iterate over it is just a side effect that might cause confusion for the people getting familiar with Spark DF (as you might iterate this way on Pandas DF for instance)

Issue reproduction:
```
df = sqlContext.jsonRDD(sc.parallelize(['{"name": "El Magnifico"}']))
for i in df["name"]: print i
```

Author: 0x0FFF <programmerag@gmail.com>

Closes #8574 from 0x0FFF/SPARK-10417.

python/pyspark/sql/column.py

@@ -226,6 +226,9 @@ class Column(object):
             raise AttributeError(item)
         return self.getField(item)
 
+    def __iter__(self):
+        raise TypeError("Column is not iterable")
+
     # string methods
     rlike = _bin_op("rlike")
     like = _bin_op("like")

python/pyspark/sql/tests.py

@@ -1066,6 +1066,15 @@ class SQLTests(ReusedPySparkTestCase):
         keys = self.df.withColumn("key", self.df.key).select("key").collect()
         self.assertEqual([r.key for r in keys], list(range(100)))
 
+    # regression test for SPARK-10417
+    def test_column_iterator(self):
+
+        def foo():
+            for x in self.df.key:
+                break
+
+        self.assertRaises(TypeError, foo)
+
 
 class HiveContextSQLTests(ReusedPySparkTestCase):