Python 3 Compatibility (PyCon 2009)

Python 2.6 and 3.0 Compatibility Lennart Regebro http://regebro.wordpress.com/ regebro@gmail.com PyCon 2009, Chicago

Python 3.0 is here!

Python 3 is incompatible

Python 3 is incompatible O RLY?

Python 3 is incompatible Ya, really! O RLY?

Strategies to deal with incompatibility

For applications: Just port to Python 3

2to3 helps you port

Tests are really helpful

Libraries need to support both Python 2 and Python 3

For most libraries: Develop in Python 2 and run 2to3 for Python 3 support

2to3 supported development has a significant startup cost

For stable libraries: Just port to Python 3

Platform Extensions/ Plugins/etc.

No Plone user will switch to Python 3 until important extensions are available

Nobody will make the extensions support Python 3 until they themselves move to Python 3

Dead lock

The Zope experience

Zope 3: A complete break

Zope 3.1 etc: No break

Wanted: Gradual upgrade path

First support 2.5 and 2.6, then support 2.6 and 3.0, finally dropping 2.x completely

Python 3 is incompatible Oh, right, I forgot already.

Python 2.6 introduces quite a lot of forward compatibility!

Python 2.5: except Exception, e: Python 3.0: except Exception as e:

Python 2.6: Both works!

Python 2.5: 3/2 == 1 Python 3.0: 3/2 == 1.5

Solutions: 3//2 == 1 from __future__ import division 3/2 == 1.5

Python2.5: print >> file, “bla”, “bla”, “bla”, Python3.0: print(“bla”, “bla”, “bla”, file=file, ending='')

Python 2.6: from __future__ import print_function

Python2.5: u”Üniçodê” Python3.0: “Üniçodê”

Python 2.6: from __future__ import unicode_literals

from __future__ import unicode_literals try: str = unicode except NameError: pass isinstance(type(“Üniçodê”), str)

Python2.5: open('filename', 'r').read() 'A stringn' open('filename', 'rb').read() 'A stringn'

Python3.0: open('filename', 'r').read() 'A stringn' open('filename', 'rb').read() b'A stringn'

Python 2.6: b“A string”[5] == “i” Python 3.0: b“A string”[5] == 105

Solutions for 2.6 and 3.0: bytearray(b”A list of bytes”) bytearray(open(file, “rb”).read())

Python 2.6: >>> bytes(a_byte_array) 'Binary data' Python 3.0: >>> bytes(a_byte_array) b'Binary data'

Python2.5: open(“unicodefile”).read() 'xc3x9cnixc3xa7odxc3xaan' Python3.0: 'Üniçodên'

Solution for 2.6 and 3.0: infile =open(“unicodefile”, “rb”) uni = infile.read().decode(enc)

try: from cStringIO import StringIO except ImportError: from io import StringIO

Python2.5: dict.items()/dict.keys()/dict.values() return lists Python3.0: dict.items()/dict.keys()/dict.values() return views

Python 2.5: foo = bar.keys() foo.sort() Python 2.4 - 3.0: foo = sorted(bar.keys())

Removed: dict.iteritems() dict.itervalues() dict.iterkeys()

Python 2.6 solution: try: iter = d.iteritems() except AttributeError: iter = d.items()

Aaaahhhhh........ Now meta classes make sense

infile = open('something', 'rb') bdata = infile.read() python2.6: pass python 3.0: sdata = bdata.decode()

Python 2.x and 3.0: sdata = str(bdata.decode('ascii'))

Running on both 2.x and 3.0 will mean some ugly hacks no matter if you use 2to3 or not

Using 2to3: Easy to support 2.3, 2.4, 2.5 Few contortions Some setup cost Single branch of code

Separate branches: Easy to support 2.3, 2.4, 2.5 No contortions No setup cost You have to fix all bugs twice

2.6 and 3.0 support without 2to3: More contortions Low setup cost Single branch of code But, no support for < 2.6

Supporting Python 2.5 or lower and 3.0 without 2to3: Contortion fest! High hack value Lot's of work

So what if you decide to go for 2.6 and 3.0 without 2.6? 1. 2to3 2. Make it run under 3.0 3. Backport to Python 2.6

2.6 compatible 2to3?

Preparing yourself for 3.0

Already in 2.6: String exceptions are gone “as” and “with” are keywords

Python 2.3: alist.sort(cmp=func) Python 2.4 and later: alist = sorted(alist, key=func)

Use // when you want integer division. Use from __future__ import division already now

Mark binary files with “rb” or “wb”

http://code.google.com/p/python-incompatibility regebro@gmail.com

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Python 3 Compatibility (PyCon 2009)

by Lennart Regebro on Nov 03, 2009

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