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IPEP 4: Python 3 Compatibility

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IPEP-4: Python 3 Compatibility

Author: Bradley Froehle <brad.froehle@gmail.com>
Status: Deferred
Created: 2012-10-01
Updated: 2013-01-10
Issue: ipython/ipython#2440

Abstract

IPython currently natively supports Python 2.6 and 2.7. In addition, Python 3.2 and 3.3 are supported by running the source code through the automated 2to3 fixer. With the reintroduction of unicode literals in Python 3.3 it is possible to create a unified code base which could run in Python 2.6, 2.7 and 3.3 without the use of 2to3.

Python 3.2 support would be maintained by continuing to use the 2to3 fixer.

Motivation

Python 3 is the future of Python. While currently only a minority of users use Python 3 as their default version of Python, there is a growing push to provide native Python 3 support.

Most major scientific computing packages, with the notable exception of matplotlib support Python 3 in their latest stable release.

The IPython development experience for users currently using Python 3 is sub-optimal, requiring either developing off of a branch which has been converted using 2to3 and backporting any fixes, or running python setup.py build after every source code change.

Python 3.3 reintroduced unicode literals (u"...") which eliminated a major hurdle in creating a Python 2 & 3 jointly compatible sourcecode.

Lastly, it would restore our "instant running" promise:

$ python3.3 ipython.py

Required Code Changes

The following changes reference the six module, a Python 2 and 3 compatibility library, and which currenly forms the basis of the IPython.utils.py3compat module. Instead of requiring the six module, any necessary features could be included in the py3compat module.

__future__ Imports

As they are the defaults in Python 3k, the absolute_import, division and print_function futures should be included in all modules. The unicode_literals future may optionally be included, but it is not strictly required since the u"..." syntax is supported in Python 3.3.

It is suggested that each module begin as:

"""One-liner

Long description.
"""
from __future__ import absolute_import, division, print_function
#-----------------------------------------------------------------------------
#       Copyright (C) <year> The IPython Development Team
#
#  Distributed under the terms of the BSD License.  The full license is in
#  the file COPYING, distributed as part of this software.
#-----------------------------------------------------------------------------

Syntax Changes

import
Unqualified import statements are no longer relative by default. All unqualified relative imports will need to be converted using the 2to3 -f import fixer.
print

The print statement was replaced with a print(...) function in Python 3. Replace:

print "Hello",
print 1, 2, 3

with:

from __future__ import print_function
print("Hello", end=' ')
print(1, 2, 3)

This can be completely automated using the 2to3 -f print fixer.

exec

The exec statement was replaced with an exec(...) function in Python 3. Replace:

exec code in globals, locals

With:

import six
six.exec_(code, globals, locals)``
long

The long and int types were unified in Python 3. Replace:

0L

With:

six.long(0)
reraise

The syntax for raising an exception with a specified traceback was changed in Python 3. Replace:

# Python 2
raise E, V, T

# Python 3
raise E(V).with_traceback(T)

with:

import six
six.reraise(E, V, T)
raw unicode
Replace ru"..." with u""+r"...", if necessary.

Attribute Name Changes

funcattrs

The attribute name for function code was changed from func_code to __code__. Replace:

f.func_code

with:

import six
getattr(f, six._func_code)

or:

import six
six.get_function_code(f)

Similar changes are required for func_name, func_defaults, and func_globals.

metaclass

The __metaclass__ = meta syntax was replaced by a metaclass keyword argument. Replace:

# Python 2
class A(bases):
    __metaclass__ = meta

# Python 3
class A(bases, metaclass=meta):
    pass

with:

class A(meta("_A", bases, {})):
    pass

or:

import six
class A(six.with_metaclass(meta, bases)):
    pass
dict

Run the 2to3 -f dict automated fixer. This means replacing:

for k, v in d.iteritems():
    pass

with:

for k, v in d.items():
    pass

If this has a noticable performance impact, use:

import six
for k, v, in six.iteritems(d):
    pass
string_types

Replace:

isinstance(..., basestring)

with:

import six
isinstance(..., six.string_types)
text_type

Replace:

unicode

with:

import six
six.text_type

Module Name Changes

Many module name changes are easily handled using the six.moves functionality.

Existing Import New Import
from StringIO import StringIO from six import StringIO
import pickle or import cPickle from six.moves import pickle
import ConfigParser from six.moves import configparser
import copy_reg from six.moves import copyreg
import __builtin__ as builtin_mod from six.moves import builtins as builtin_mod

Additional moves which will need to be added in our six.moves implementation.

Existing Import New Import
from urllib2 import urlopen from six.moves import urlopen
from urllib import urlretrieve from six.moves import urlretrieve
from urlparse import urlparse from six.moves import urlparse
from os import getcwdu from six.moves import getcwdu

Automation

Many of these changes can be applied automatically, by using the existing or new 2to3 fixers.

For example, an automated fixer for the long task could be implemented as:

from lib2to3.pgen2 import token
from lib2to3.fixer_base import BaseFix
from lib2to3.fixer_util import Call, Name, Number, touch_import

class FixLongLiterals(BaseFix):
    """0L -> six.long(0)"""
    _accept_type = token.NUMBER

    def match(self, node):
        # Override
        return node.value[-1] in u"Ll"

    def transform(self, node, results):
        val = node.value
        if val[-1] in u'Ll':
            val = val[:-1]

        touch_import(None, u'six', node)
        return Call(Name(u'six.long'), [Number(val)], prefix=node.prefix)

Alternatives

Runtime 2to3 Conversion

A runtime 2to3 converter could be utilized to automatically convert the IPython source to a Python 3 compatible syntax on import. For example, rt2to3, could be installed and then activated by creating a ipython3dev.pth file (in your Python 3 path) which contains:

/home/user/projects/ipython
import rt2to3; rt2to3.Runtime2to3Installer(nofix=['apply', 'except', 'has_key', 'next', 'repr', 'tuple_params']).install('/home/user/projects/ipython')

The list of excluded fixers is taken from setup.py.

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