Python clean, mostly pep8

https://www.python.org/dev/peps/pep-0008/

A few hints:
    + About Spaces:
        - always after a comma
        - never after [, (, {
        - never before ], ), }
        - always around -, +, *, **, %, ==, >=, <=
        - always around = for an assignation
        - never around = for a default value of an argument
        - never at the end of a line
    + One statement by line:
        - nothing after : unless inside []
        - no ;
    + No useless (), like in a simple if or assert
    + Indentation uses four spaces
    + Blanklines:
        - 2 before class or function declaration
        - 1 before class method declaration
        - never more than 2
    + Line length:
        - We said 79. This is far from beeign respected. Maybe 119 is more reasonable
    + Align:
        - at the begining of a line continuation
        - not multiple spaces before = to align multiple assignements

Of course, in our case, it would be tricky to follow some other rules, like:
    N802 function name should be lowercase
    N803 argument name should be lowercase
    N806 variable in function should be lowercase

Other issues may be addressed:
    F401 '<something>' imported but unused
    F403 'from <some_lib> import *' used; unable to detect undefined names

To check all of this:
    flake8 --ignore=N802,N803,N806 --max-line-length=119 <root_folder>

Other things changed:
    - import order (thanks isort)
    - useless elif changed to if
    - useless else removed
    - usage of ValueError, when an argument does not fit the expected type / size
    - issubclass(x.__class__, X) / x.__class__ == X → isinstance(x, X)
    - isinstance accepts a list or a tuple of classes (eg: isinstance(x, (int, float)))
    - print "" → print