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Frequently Asked Questions (FAQ)

Who is this for?

1. People with existing or new Python 3 codebases who wish to provide ongoing Python 2.6 / 2.7 support easily and with little maintenance burden.

2. People who wish to ease and accelerate migration of their Python 2 codebases to Python 3.3+, module by module, without giving up Python 2 compatibility.

Why upgrade to Python 3?

“Python 2 is the next COBOL.”

—Alex Gaynor, at PyCon AU 2013

Python 2.7 is the end of the Python 2 line. (See PEP 404.) The language and standard libraries are improving only in Python 3.x.

Python 3.x is a better language and better set of standard libraries than Python 2.x in many ways. Python 3.x is cleaner, less warty, and easier to learn than Python 2. It has better memory efficiency, easier Unicode handling, and powerful new features like the asyncio module.

Porting philosophy

Why write Python 3-style code?

Here are some quotes:

  • “Django’s developers have found that attempting to write Python 3 code that’s compatible with Python 2 is much more rewarding than the opposite.” from the Django docs.
  • “Thanks to Python 3 being more strict about things than Python 2 (e.g., bytes vs. strings), the source translation [from Python 3 to 2] can be easier and more straightforward than from Python 2 to 3. Plus it gives you more direct experience developing in Python 3 which, since it is the future of Python, is a good thing long-term.” from the official guide “Porting Python 2 Code to Python 3” by Brett Cannon.
  • “Developer energy should be reserved for addressing real technical difficulties associated with the Python 3 transition (like distinguishing their 8-bit text strings from their binary data). They shouldn’t be punished with additional code changes ...” from PEP 414 by Armin Ronacher and Nick Coghlan.

Can’t I just roll my own Py2/3 compatibility layer?

Yes, but using python-future will probably be easier and lead to cleaner code with fewer bugs.

Consider this quote:

“Duplication of effort is wasteful, and replacing the various home-grown approaches with a standard feature usually ends up making things more readable, and interoperable as well.”

—Guido van Rossum (blog post)

future also includes various Py2/3 compatibility tools in future.utils picked from large projects (including IPython, Django, Jinja2, Pandas), which should reduce the burden on every project to roll its own py3k compatibility wrapper module.

What inspired this project?

In our Python training courses, we at Python Charmers faced a dilemma: teach people Python 3, which was future-proof but not as useful to them today because of weaker 3rd-party package support, or teach people Python 2, which was more useful today but would require them to change their code and unlearn various habits soon. We searched for ways to avoid polluting the world with more deprecated code, but didn’t find a good way.

Also, in attempting to help with porting packages such as scikit-learn to Python 3, I (Ed) was dissatisfied with how much code cruft was necessary to introduce to support Python 2 and 3 from a single codebase (the preferred porting option). Since backward-compatibility with Python 2 may be necessary for at least the next 5 years, one of the promised benefits of Python 3 – cleaner code with fewer of Python 2’s warts – was difficult to realize before in practice in a single codebase that supported both platforms.

The goal is to accelerate the uptake of Python 3 and help the strong Python community to remain united around a single version of the language.


How well has it been tested?

future is used by several major projects, including mezzanine and ObsPy. It is also currently being used to help with porting 800,000 lines of Python 2 code in Sage to Python 2/3.

Currently python-future has over 1000 unit tests. Many of these are straight from the Python 3.3 and 3.4 test suites.

In general, the future package itself is in good shape, whereas the futurize script for automatic porting is imperfect; chances are it will require some manual cleanup afterwards. The past package also needs to be expanded.

Is the API stable?

Not yet; future is still in beta. Where possible, we will try not to break anything which was documented and used to work. After version 1.0 is released, the API will not change in backward-incompatible ways until a hypothetical version 2.0.

Relationship between python-future and other compatibility tools

How does this relate to 2to3?

2to3 is a powerful and flexible tool that can produce different styles of Python 3 code. It is, however, primarily designed for one-way porting efforts, for projects that can leave behind Python 2 support.

The example at the top of the 2to3 docs demonstrates this. After transformation by 2to3, looks like this:

def greet(name):
    print("Hello, {0}!".format(name))
print("What's your name?")
name = input()

This is Python 3 code that, although syntactically valid on Python 2, is semantically incorrect. On Python 2, it raises an exception for most inputs; worse, it allows arbitrary code execution by the user for specially crafted inputs because of the eval() executed by Python 2’s input() function.

This is not an isolated example; almost every output of 2to3 will need modification to provide backward compatibility with Python 2. As an alternative, the python-future project provides a script called futurize that is based on lib2to3 but will produce code that is compatible with both platforms (Py2 and Py3).

Can I maintain a Python 2 codebase and use 2to3 to automatically convert to Python 3 in the setup script?

This was originally the approach recommended by Python’s core developers, but it has some large drawbacks:

1. First, your actual working codebase will be stuck with Python 2’s warts and smaller feature set for as long as you need to retain Python 2 compatibility. This may be at least 5 years for many projects, possibly much longer.

2. Second, this approach carries the significant disadvantage that you cannot apply patches submitted by Python 3 users against the auto-generated Python 3 code. (See this talk by Jacob Kaplan-Moss.)

What is the relationship between future and six?

python-future is a higher-level compatibility layer than six that includes more backported functionality from Python 3, more forward-ported functionality from Python 2, and supports cleaner code, but requires more modern Python versions to run.

python-future and six share the same goal of making it possible to write a single-source codebase that works on both Python 2 and Python 3. python-future has the further goal of allowing standard Py3 code to run with almost no modification on both Py3 and Py2. future provides a more complete set of support for Python 3’s features, including backports of Python 3 builtins such as the bytes object (which is very different to Python 2’s str object) and several standard library modules.

python-future supports only Python 2.6+ and Python 3.3+, whereas six supports all versions of Python from 2.4 onwards. (See Which versions of Python does python-future support?.) If you must support older Python versions, six will be esssential for you. However, beware that maintaining single-source compatibility with older Python versions is ugly and not fun.

If you can drop support for older Python versions, python-future leverages some important features introduced into Python 2.6 and 2.7, such as import hooks, and a comprehensive and well-tested set of backported functionality, to allow you to write more idiomatic, maintainable code with fewer compatibility hacks.

What is the relationship between python-future and python-modernize?

python-future contains, in addition to the future compatibility package, a futurize script that is similar to in intent and design. Both are based heavily on 2to3.

Whereas python-modernize converts Py2 code into a common subset of Python 2 and 3, with six as a run-time dependency, futurize converts either Py2 or Py3 code into (almost) standard Python 3 code, with future as a run-time dependency.

Because future provides more backported Py3 behaviours from six, the code resulting from futurize is more likely to work identically on both Py3 and Py2 with less additional manual porting effort.

Platform and version support

Which versions of Python does python-future support?

Python 2.6, 2.7, and 3.3+ only.

Python 2.6 and 2.7 introduced many important forward-compatibility features (such as import hooks, b'...' literals and __future__ definitions) that greatly reduce the maintenance burden for single-source Py2/3 compatible code. future leverages these features and aims to close the remaining gap between Python 3 and 2.6 / 2.7.

Python 3.2 could perhaps be supported too, although the illegal unicode literal u'...' syntax may be inconvenient to work around. The Py3.2 userbase is very small, however. Please let us know via GitHub issue #29 if you would like to see Py3.2 support.

Do you support Pypy?

Yes, except for the standard library import hooks (currently). Feedback and pull requests are welcome!

Do you support IronPython and/or Jython?

Not sure. This would be nice...


Is there a mailing list?

Yes, please ask any questions on the python-porting mailing list.


Can I help?

Yes please :) We welcome bug reports, additional tests, pull requests, and stories of either success or failure with using it. Help with the fixers for the futurize script is particularly welcome.