Python 3 object construction: which is the most Pythonic / the accepted way?

Question

Having a background in Java, which is very verbose and strict, I find the ability to mutate Python objects as to give them with fields other than those presented to the construc

Answer 1

The first you describe is very common. Some use the shorter

class Foo:
   def __init__(self, foo, bar):
       self.foo, self.bar = foo, bar

Your second approach isn't common, but a similar version is this:

class Thing:
   def __init__(self, **kwargs):
       self.something = kwargs['something']
       #..

which allows to create objects like

t = Thing(something=1)

This can be further modified to

class Thing:
   def __init__(self, **kwargs):
       self.__dict__.update(kwargs)

allowing

t = Thing(a=1, b=2, c=3)
print t.a, t.b, t.c # prints 1, 2, 3

As Debilski points out in the comments, the last method is a bit unsafe, you can add a list of accepted parameters like this:

class Thing:
    keywords = 'foo', 'bar', 'snafu', 'fnord'
    def __init__(self, **kwargs):
        for kw in self.keywords:
            setattr(self, kw, kwargs[kw])

There are many variations, there is no common standard that I am aware of.