It might be dangerous to have a mutable as value for default param. (http://stackoverflow.com/a/11416002/1170730)

e.g

    def func(example, f={}):
        f[example] = 1
        return f

    func(2)

    {2: 1}
    func(3)
    {2:1, 3:1}

mengxr

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #7058 from MechCoder/pipeline_api_playground and squashes the following commits:

40a5eb2 [MechCoder] copy
95f7ff2 [MechCoder] [SPARK-8679] [PySpark] [MLlib] Default values in Pipeline API should be immutable