Enum: The Enum library is not in the standard library with older Python versions. It has been backported and can be installed from PyPi. Install the Enum34 library to use this feature.

set syntax: Newer versions of Python support the single curly braces shorthand syntax to create set objects like{"MSFT"}. In older versions, we will need to explicitly create sets with this equivalent longhand syntax: set(["MSFT"]).

print statement: print is defined as a statement in Python 2.x, so we cannot call it as a function, neither can we mock it out. We can get around this by adding the line from __future__ import print_function to the top of all the files that use print.

builtins: The builtins module is called __builtin__ in Python 2.x. Therefore, we need to use __builtin__.print or __builtin__.open when we want to mock the print or open functions.

yield from expression: This expression is not available in older Python versions. It has to be replaced with an iteration.

mock_open: This mock helper only mocks the read method in the backported version. It doesn't support mocking iteration on the file object. So, we need to change the implementation to not use iteration.