Migrate to Python 3 using the six library. Zurich Python User Group

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Six o‘clock
Last call to migrate to Python 3
César C. Desales

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Goal
Show how to write code that runs on Python 2
and Python 3 using the six library

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Not about Python 3 awesomeness!
• Such powerful
• Much awesome
• Wow syntax
• So modern

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Python 2 on Google image search

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Python 3 is still a luxury
• Mostly for new code
• Millions of Python 2 legacy LOC

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The clock is ticking

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To port or not to port? How?
• No port
• Port to Python 3 and ditch support for 2
• Cowboy style
• 2to3 / modernizer
• Support both 2 and 3

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From 2 to 3
Porting Strategy: Cowboy style
• Read “What’s new?” and migrate by hand

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Strategy 2: automatically move on
• Port with a tool, e.g. 2to3 / modernizer
- Works better with clean and tested code
- Works only forward

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Why not just move on to Python 3?
• Python 2 only environment
• Python 2 only dependencies
• You must have backwards compatibility
• You want to fiddle/learn

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Using six:
Which python am I running? – 2.7
>>> import six!
>>> if six.PY2:!
... print('Still stuck there?')!
... elif six.PY3:!
... print('Nice,Python 3')!
...!
Still stuck there?!

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Which python am I running? – 3.4
>>> if six.PY2:!
... print('Still stuck there?')!
... elif six.PY3:!
... print('Nice, Python 3')!
...!
Nice, Python 3!

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six – string compatibility – 2.7
>>> six.b('payload')!
'payload'!
>>> six.u('foo')!
u'foo’!
Binary strings are still just strings in 2.7

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six – string compatibility – 3.4
>>> six.b("payload")!
b'payload'!
>>> six.u('foo')!
'foo’!

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String types – 2.7
>>> ss = 'value’!
>>> isinstance(ss, basetring)!
True!
!
>>> import six!
>>> six.string_types!
(<type 'basestring'>,)!
All strings are instances of basestring

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String types – 3.4
>>> ss = 'value'!
>>> isinstance(ss, str)!
True!
!
>>> import six!
>>> six.string_types!
(<class 'str'>,)!
basestring is gone. Strings are of str type

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String types - six
>>> ss = 'value’!
>>> isinstance(ss, six.string_types)!
True!

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cStringIO – 2.7
>>> import cStringIO!
>>> !
>>> output = cStringIO.StringIO()!
>>> output.write('First line.n')!
>>> output.getvalue()!
'First line.n'!
>>> output.close()!
cStringIO = StringIO written in C

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cStringIO – 3.4
>>> import io!
>>> !
>>> output = io.StringIO()!
>>> output.write('First line.n')!
12!
>>> output.getvalue()!
'First line.n'!
>>> output.close()!
cStringIO and StringIO => io

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cStringIO – six
>>> try:
... from cStringIO import StringIO!
... except ImportError:!
... from six import StringIO!
...!
>>> output = StringIO()!
>>> output.write('First line.n')!
>>> output.getvalue()!
'First line.n'!
>>> output.close()!

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cStringIO with bytes - 2.7 and 3.4
from StringIO import StringIO!
from cStringIO import StringIO!
from io import StringIO!
!
from io import BytesIO!

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cStringIO with bytes – six on 2.7
>>> try:
... from cStringIO import StringIO as BufferIO!
... except ImportError:!
... from six import BytesIO as BufferIO!
...!
>>> buf = BufferIO(b"abcdef")!
>>> buf.read(6)!
'abcdef'!
*A prefix of 'b' or 'B' is ignored in Python 2

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cStringIO with bytes – 3.4
>>> try:
... from cStringIO import StringIO as BufferIO!
... except ImportError:!
... from six import BytesIO as BufferIO!
...!
>>> buf = BufferIO(b"abcdef")!
>>> buf.read(6)!
b'abcdef'!

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urlencode – 2.7 and 3.4
>>> import urllib!
>>> !
>>> urllib.urlencode({'foo':'bar'})!
'foo=bar'!
>>> import urllib.parse!
>>> !
>>> urllib.parse.urlencode({'foo':'bar'})!
'foo=bar'!

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urlencode – six
>>> import six.moves.urllib.parse!
>>> !
>>> six.moves.urllib.parse.urlencode({'foo':'bar'})!
'foo=bar’!

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urlparse - 2.7 and 3.4
>>> import urlparse!
>>> urlparse.urlparse('http://www.cwi.nl:80/%7Eguido/Python.html')!
ParseResult(scheme='http', netloc='www.cwi.nl:80', path=…)!
>>> import urllib.parse!
>>> !
>>> urllib.parse.urlparse('http://www.cwi.nl:80/%7Eguido/Python.html')!
ParseResult(scheme='http', netloc='www.cwi.nl:80', path=…)!

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urlparse - six
>>> import six.moves.urllib.parse as url_parse!
>>> !
>>> url_parse.urlparse('http://www.cwi.nl:80/%7Eguido/Python.html')!
ParseResult(scheme='http', netloc='www.cwi.nl:80', path=…)!
!

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cPickle – 2.7
>>> import cPickle!
>>> cPickle.dumps("value")!
"S'value'np1n.”!

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cPickle – pickle – 3.4
>>> import pickle!
>>> !
>>> pickle.dumps("value")!
b'x80x03Xx05x00x00x00valueqx00.'!
pickle is written in C anyway in Python 3

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cPickle – six in 2.7
>>> import six.moves.cPickle as cPickle!
>>>!
>>> cPickle.dumps("value")!
"S'value'np1n.”!

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cPickle – six in 3.4
>>> import six.moves.cPickle as cPickle!
>>> cPickle.dumps("value")!
b'x80x03Xx05x00x00x00valueqx00.’!

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itertools.ifilter – 2.7
>>> from itertools import ifilter!
>>> result = ifilter(the_filter, data)!
ifilter = filter with iterators

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itertools.ifilter – 3.4
>>> result = filter(the_filter, data)!

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itertools.ifilter – six
>>> import six.moves!
>>> result = six.moves.filter(the_filter, data)!

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itertools.izip – 2.7
>>> import itertools!
>>> for val1, val2 in itertools.izip([1,2], [3, 4]):!
... print(’%s %s’ % (val1, val2))!
...!
1 3!
2 4!
izip = zip with iterators

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itertools.izip – zip – 3.4
>>> for val1, val2 in zip([1,2], [3, 4]):!
... print(’%s %s’ % (val1, val2))!
...!
1 3!
2 4!

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itertools.izip – zip – six
>>> import six.moves!
>>> for val1, val2 in six.moves.zip([1,2], [3, 4]):!
... print(’%s %s’ % (val1, val2))!
...!
1 3!
2 4!

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iteritems – 2.7
>>> for key, value in {'foo':'bar'}. iteritems():!
... print(’%s %s’ % (key, value))!
...!
foo bar!

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iteritems – items – 3.4
>>> for key, value in {'foo':'bar'}.items():!
... print(’%s %s’ % (key, value))!
...!
foo bar!

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iteritems – six
>>> import six!
>>> for key, value in six.iteritems({'foo':'bar'}):!
... print(’%s %s’ % (key, value))!
...!
foo bar!

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Recommendations
• No more Python 2 only commits policy
• Check Python 2/3 in code review
• Migration in your development
roadmap => 2020