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Preparing for the future - Python 3 is here to ...

Preparing for the future - Python 3 is here to stay

A look at the history and current adoption of Python 2 and 3, major differences between the two version, hurdles to porting and finally a set of tips and recommendations for writing code to run on both Python 2 and 3.

This talk was presented at the Amsterdam Python meetup group on 2015-03-24 (http://www.meetup.com/Amsterdam-Python-Meetup-Group/events/220704819/).

Nick Groenen

March 24, 2015
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  1. ➔ Python 3.0 final was released on December 3rd, 2008.

    ➔ The final 2.x version (2.7) release came out mid-2010 ➔ Python 2.7 support extended by 5 years, to 2020 A little bit of history
  2. ➔ Cleans up some warts in the language ➔ More

    readable, consistent & explicit Intentionally backward-incompatible
  3. Python 2 Unicode vs. 8-bit ASCII str type Separate unicode

    type No byte type Python 3 Text vs. data str is unicode text byte and bytearray for encoded text
  4. Python 2 allows mixing of str and unicode objects >>>

    u'This is unicode.' + " This isn't" u"This is unicode. This isn't"
  5. Python 3 does not >>> "This is unicode." + b'

    These are bytes.' TypeError: Can't convert 'bytes' object to str implicitly >>> "This is unicode." + b' These are bytes.'.decode('us-ascii') 'This is unicode. These are bytes.'
  6. Renames cookielib Cookie htmlentitydefs HTMLParser httplib BaseHTTPServer CGIHTTPServer SimpleHTTPServer pipes.quote

    SocketServer UserDict.UserDict UserList.UserList UserString.UserString xmlrpclib SimpleXMLRPCServer http.cookiejar http.cookies html.entities html.parser http.client http.server http.server http.server shlex.quote socketserver collections.UserDict collections.UserList collections.UserString xmlrpc.client xmlrpc.server
  7. Generators ➔ zip() ➔ map() ➔ filter() ➔ dictionary's .keys()

    method ➔ dictionary's .values() method ➔ dictionary's .items() method
  8. Python 2 3/2 = 1 3.0/2.0 = 1.5 Integer division

    Python 3 3/2 = 1.5 3.0/2.0 = 1.5 3//2 = 1
  9. Comparing unorderable types [1, 2] > 'foo' = False (1,

    2) > 'foo' = True [1, 2] > (1, 2) = False
  10. Comparing unorderable types [1, 2] > 'foo' = False (1,

    2) > 'foo' = True [1, 2] > (1, 2) = False TypeError: unorderable types: list() > str()
  11. Improved variable unpacking a, b, c = range(3) a =

    0 b = 1 c = 2 a, *rest, b = range(10) a = 0 rest = [2, 3, 4, 5, 6, 7, 8, 9] b = 9
  12. Keyword-only arguments >>> def f(a, b, *args, option=True): ... print(repr(args))

    ... print(repr(option)) ... >>> f(1, 2, False) (False,) True
  13. Keyword-only arguments >>> def f(a, b, *args, option=True): ... print(repr(args))

    ... print(repr(option)) ... >>> f(1, 2, False) (False,) True >>> f(1, 2, option=False) () False
  14. Keyword-only arguments >>> def f(a, b, *, option=True): ... print(repr(option))

    ... >>> f(1, 2, 3) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: f() takes 2 positional arguments but 3 were given
  15. Keyword-only arguments >>> def f(a, b, *, option=True): ... print(repr(option))

    ... >>> f(1, 2, 3) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: f() takes 2 positional arguments but 3 were given >>> f(1, 2, option=False) False
  16. Chained exceptions >>> import socket >>> >>> class IPLookupError(Exception): ...

    """Unable to resolve hostname to IP address""" ... >>> >>> def hostname_to_ip(hostname): ... try: ... return socket.gethostbyname(hostname) ... except socket.gaierror: ... raise IPLookupError("Unable to resolve '%s'" % hostname) ...
  17. Chained exceptions >>> print(hostname_to_ip("nick.groenen.me")) 104.28.24.37 >>> print(hostname_to_ip("nosuchhost")) Traceback (most recent

    call last): File "<stdin>", line 1, in <module> File "<stdin>", line 5, in hostname_to_ip __main__.IPLookupError: Unable to resolve 'nosuchhost'
  18. Chained exceptions >>> print(hostname_to_ip("nick.groenen.me")) 104.28.24.37 >>> print(hostname_to_ip("nosuchhost")) Traceback (most recent

    call last): File "<stdin>", line 1, in <module> File "<stdin>", line 5, in hostname_to_ip __main__.IPLookupError: Unable to resolve 'nosuchhost' Where did socket.gaierror go?!?
  19. Chained exceptions >>> print(hostname_to_ip("nosuchhost")) Traceback (most recent call last): File

    "<stdin>", line 3, in hostname_to_ip socket.gaierror: [Errno -2] Name or service not known During handling of the above exception, another exception occurred: Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 5, in hostname_to_ip __main__.IPLookupError: Unable to resolve 'nosuchhost'
  20. Chained exceptions >>> def hostname_to_ip(hostname): ... try: ... return socket.gethostbyname(hostname)

    ... except socket.gaierror: ... raise IPLookupError("Unable to resolve '%s'" % hostname) ...
  21. Chained exceptions >>> def hostname_to_ip(hostname): ... try: ... return socket.gethostbyname(hostname)

    ... except socket.gaierror as e: ... raise IPLookupError("Unable to resolve '%s'" % hostname) from e ...
  22. Chained exceptions >>> print(hostname_to_ip("nosuchhost")) Traceback (most recent call last): File

    "<stdin>", line 3, in hostname_to_ip socket.gaierror: [Errno -2] Name or service not known The above exception was the direct cause of the following exception: Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 5, in hostname_to_ip __main__.IPLookupError: Unable to resolve 'nosuchhost'
  23. OSError subclasses try: shutil.copy2(source, dest) except OSError as e: if

    e.errno in [errno.EPERM, errno.EACCES]: sudo(shutil.copy2(source, dest)) else: raise
  24. >>> def f(x: int) -> float: ... pass … >>>

    f.__annotations__ {'return': <class 'float'>, 'x': <class 'int'>} Function annotations
  25. The Absolute Minimum Every Software Developer Absolutely, Positively Must Know

    About Unicode and Character Sets (No Excuses!) http://www.joelonsoftware.com/articles/Unicode.html
  26. ➔ Encoded text comes in (socket, pipe, file, etc) ➔

    .decode() ➔ Unicode internally ➔ .encode() ➔ Encoded text goes out Unicode sandwich
  27. Target Python 2.7 and 3.3+ from __future__ import absolute_import, division,

    print_function PY2 = sys.version_info[0] < 3 PY3 = not PY2 if PY3: from urllib.parse import urljoin from collections import MutableMapping as DictMixin else: from urlparse import urljoin from UserDict import DictMixin
  28. Python-Future from __future__ import ( absolute_import, division, print_function, ) from

    builtins import (bytes, str, open, super, range, zip, round, input, int, pow, object)
  29. Python-Future # Backported Py3 bytes object b = bytes(b'ABCD') assert

    repr(b) == "b'ABCD'" # These raise TypeErrors: # b + u'EFGH' # Backported Py3 str object s = str(u'ABCD') assert s != bytes(b'ABCD') # These raise TypeErrors: # bytes(b'B') in s # input() replaces Py2's raw_input() (with no eval()): name = input('What is your name? ') print('Hello ' + name)
  30. Python-Future # Many Py3 module names are supported directly on

    both Py2.x and 3.x: from http.client import HttpConnection import html.parser import queue import xmlrpc.client # Refactored modules with clashing names on Py2 and Py3 are supported # as follows: from future import standard_library standard_library.install_aliases() # Then, for example: from itertools import filterfalse, zip_longest from urllib.request import urlopen from collections import Counter, OrderedDict # backported to Py2.6 from collections import UserDict, UserList, UserString from subprocess import getoutput, getstatusoutput
  31. 2to3 from setuptools import setup setup( name='your.module', version = '1.0',

    description='A description of your module', packages = ['your', 'you.module'], use_2to3 = True, convert_2to3_doctests = ['src/your/module/README.txt'], )
  32. Resources ➔ Porting to Python 3 - The Book Site

    http://python3porting.com/bookindex.html ➔ Can I use Python 3? https://caniusepython3.com/ ➔ Python 3 Readiness http://py3readiness.org/ ➔ Porting to Python 3 (Django) https://docs.djangoproject.com/en/1.7/topics/python3/ ➔ PortingToPy3k https://wiki.python.org/moin/PortingToPy3k/ ➔ The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!) http://www.joelonsoftware.com/articles/Unicode.html