Most Python files should include the following boilerplate at the top of the file, right after the comment header:

from __future__ import (absolute_import, division, print_function)

This uses Python 3 semantics for absolute versus relative imports, division, and print. By doing this, we can write code which is portable between Python 2 and Python 3 by following the Python 3 semantics.


When Python 2 encounters an import of a name in a file like import copy it attempts to load copy.py from the same directory as the file is in. This can cause problems if there is a python file of that name in the directory and also a python module in sys.path with that same name. In that case, Python 2 would load the one in the same directory and there would be no way to load the one on sys.path. Python 3 fixes this by making imports absolute by default. import copy will find copy.py from sys.path. If you want to import copy.py from the same directory, the code needs to be changed to perform a relative import: from . import copy.


In Python 2, the division operator (/) returns integer values when used with integers. If there was a remainder, this part would be left off (aka, floor division). In Python 3, the division operator (/) always returns a floating point number. Code that needs to calculate the integer portion of the quotient needs to switch to using the floor division operator (//) instead.