Python – Which Version Requires from __future__ import with_statement?

I would say that no, this is baloney. Even with those imports, there are still significant differences between Python 2 and 3: for example, input() in Python 3 is like raw_input() in Python 2; range() in Python 3 is like xrange() in Python 2. In the case of xrange() you could probably get away with using range() in Python 2 as long as the ranges are small, but if they're large, your program could have very different memory usage under Python 2 and Python 3.

You could add something like this to your code:

try:
    range = xrange
    input = raw_input
except NameError:
    pass

But then you've got to find all those edge cases and fix them up. For example, there are the keys() and values() methods of dict that return iterators in Python 3 but lists in Python 2, so you'd need to write a dict subclass that "fixes" that (and then never use dictionary literals in your code without wrapping them, since those would otherwise be of the built-in dict type).

I suppose that, by using __future__ and various fix-ups, and by limiting yourself to writing code in a subset of Python thus created that will run under both 2.x and 3.x, it might be possible to write code that runs in both versions. Seems like a lot of work, though. There's a reason there's a 2to3 utility...

The import statement always initializes the whole module. The modules are stored in the sys.modules dictionary.

When you use from sys import argv the sys module is not bound locally, only argv is. You cannot use the name sys in your module, because you didn't import that name.

You can only reach the sys module if you imported sys separately:

from sys import argv

script, input = argv

import sys
print sys.path

And you can always access all imported modules by accessing sys.modules:

from sys import modules

print modules['sys'].path

Here I bound the name modules to the sys.modules dictionary, and through that reference, find the sys module, and reference the path attribute.

Demo:

>>> from sys import modules
>>> modules['sys']
<module 'sys' (built-in)>
>>> sys
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'sys' is not defined