Warning:

Starting from version 9.0, the default distributed version of Sage is using Python 3. See Python3-Switch for more information.

Advice on writing Python3-compatible code

To make the transition from python2 to python3 possible, it is required that the code must be as compatible as possible with both Python2 and Python3. Let us describe a few important points.

Do not use xrange

You can use xrange only inside Cython files (*.pyx).

Do not use it.next() to advance iterators

Use instead the syntax "next(it)" which is compatible with both Python2 and Python3.

Do not use .itervalues, .iteritems or .iterkeys

You can either use .values, .items and .keys (that are not iterators in Python2).

Do not use cmp()

The cmp global function is gone in Python3 and must be avoided completely.

SageMath now has a "richcmp" function that can sometimes replace "cmp".

Behaviour of print

The behaviour of print differs in Python 2 and in Python 3.

SageMath code is written in Python3.

In the Python 3 syntax, print is a function.

The easiest way to achieve this compatibility is to always use print(...) with a single string inside the parentheses.

For example, print("He has eaten {} bananas".format(10)) will work in both Python 2 and Python 3 and give the same result.

Here below are some instructions for going from Python 2-only print to Python 3-only print.

To convert print from Python 2 to Python 3, you basically need to add parentheses, and write print("x") instead of print "x".

Here is a conversion table to help you adapt your code in more complex cases:

Python 2

Python 3

print a

print(a)

print a, b, c

print(a, b, c)

print x,

print(x, end=" ")

print >>sys.stderr, x

print(x, file=sys.stderr)

Author/Reviewer's checklist

Check the result of the patchbot Python3 compatibility plugins named "next_method", "oldstyle_print", "python3" and "raise_statements" (for example, see the list on the right at this patchbot report).

Otherwise, check Python 3 syntax errors in every file changed in the current branch: 

git diff --name-only develop your-new-branch | xargs -n 1 python3 -m py_compile

Check Python 3 syntax errors in every Python file of Sage library (see this thread): 

find src/sage -name '*.py' | xargs -n 1 python3 -m py_compile

If possible, include the following imports to the modules you are adding or working on: 

from __future__ import division, absolute_import, print_function

The command pyflakes, when installed with pip3, is also very useful to detect incompatibilities with Python3.