该笔记向你展示了编写不会过时的兼容 Python 2 和 Python 3 的代码风格。
它是 Ed Schofield 在 PyCon AU 2014 的演讲，“ Writing 2/3 compatible code ”。
最低版本：
- Python 2: 2.6+
- Python 3: 3.3+
安装
一些下面的 imports 适用于使用 pip 安装在 PyPI 上安装：
import future # pip install futureimport past # pip install pastimport six # pip install six
以下的脚本也适用于 pip 安装：
futurize # pip install futurizepasteurize # pip install pasteurize
查看 和 获取更多消息。
基本语法差异
print：
# Python 2 only:print 'Hello'
# Python 2 and 3:print('Hello') 为了打印出多个 strings。 import print_function 来防止 Py2 把它解释成一个元组。
# Python 2 only:print 'Hello', 'Guido'
# Python 2 and 3:from __future__ import print_function # (at top of module)print('Hello', 'Guido') # Python 2 only:print >> sys.stderr, 'Hello'
# Python 2 and 3:from __future__ import print_functionprint('Hello', file=sys.stderr) # Python 2 only:print 'Hello',
# Python 2 and 3:from __future__ import print_functionprint('Hello', end='') 抛出异常
# Python 2 only:raise ValueError, "dodgy value"
# Python 2 and 3:raise ValueError("dodgy value") 使用 traceback 抛出异常：
# Python 2 only:traceback = sys.exc_info()[2]raise ValueError, "dodgy value", traceback
# Python 3 only:raise ValueError("dodgy value").with_traceback() # Python 2 and 3: option 1from six import reraise as raise_# orfrom future.utils import raise_traceback = sys.exc_info()[2]raise_(ValueError, "dodgy value", traceback)
# Python 2 and 3: option 2from future.utils import raise_with_tracebackraise_with_traceback(ValueError("dodgy value")) 异常链 (PEP 3134):
# Setup:class DatabaseError(Exception): pass
# Python 3 onlyclass FileDatabase: def __init__(self, filename): try: self.file = open(filename) except IOError as exc: raise DatabaseError('failed to open') from exc # Python 2 and 3:from future.utils import raise_fromclass FileDatabase: def __init__(self, filename): try: self.file = open(filename) except IOError as exc: raise_from(DatabaseError('failed to open'), exc) # Testing the above:try: fd = FileDatabase('non_existent_file.txt')except Exception as e: assert isinstance(e.__cause__, IOError) # FileNotFoundError on Py3.3+ inherits from IOError 捕获异常：
# Python 2 only:try: ...except ValueError, e: ...
# Python 2 and 3:try: ...except ValueError as e: ...
除法
整除（rounding down）：
# Python 2 only:assert 2 / 3 == 0
# Python 2 and 3:assert 2 // 3 == 0
“True division” (float division):
# Python 3 only:assert 3 / 2 == 1.5
# Python 2 and 3:from __future__ import division # (at top of module)assert 3 / 2 == 1.5
“Old division” (i.e. compatible with Py2 behaviour):
# Python 2 only:a = b / c # with any types
# Python 2 and 3:from past.utils import old_diva = old_div(b, c) # always same as / on Py2
长整数
短整数在 Python 3 中已经去除了，并且 long 已经变成了 int（没有 L 在 repr 后面）
# Python 2 onlyk = 9223372036854775808L# Python 2 and 3:k = 9223372036854775808
# Python 2 onlybigint = 1L# Python 2 and 3from future.builtins import intbigint = int(1)
为了测试一个值是否是整数（任何类型）：
# Python 2 only:if isinstance(x, (int, long)): ...# Python 3 only:if isinstance(x, int): ...# Python 2 and 3: option 1from future.builtins import int # subclass of long on Py2if isinstance(x, int): # matches both int and long on Py2 ...# Python 2 and 3: option 2from past.builtins import longif isinstance(x, (int, long)): ...
八进制常量
0644 # Python 2 only
0o644 # Python 2 and 3
Backtick repr
`x` # Python 2 only
repr(x) # Python 2 and 3
元类
class BaseForm(object): passclass FormType(type): pass
# Python 2 only:class Form(BaseForm): __metaclass__ = FormType pass
# Python 3 only:class Form(BaseForm, metaclass=FormType): pass
# Python 2 and 3:from six import with_metaclass# orfrom future.utils import with_metaclassclass Form(with_metaclass(FormType, BaseForm)): pass
Strings 和 bytes
Unicode（text） 字符字面量
如果你更新一个已经存在的 Python 2 的代码库，为所有的字符字面量使用 u 做前缀作为 unicode 的标记可能会更好。
# Python 2 onlys1 = 'The Zen of Python's2 = u'きたないのよりきれいな方がいい\n'# Python 2 and 3s1 = u'The Zen of Python's2 = u'きたないのよりきれいな方がいい\n'
futurize 和 python-modernize 工具没有提供一个选项自动完成这个。
如果你为了个项目编写新的代码库，你可以用一个 unicode 字符模块标记所有的字符字面量。
# Python 2 and 3from __future__ import unicode_literals # at top of modules1 = 'The Zen of Python's2 = 'きたないのよりきれいな方がいい\n'
查看 这里获取更多的关于讨论使用哪种风格的讨论。
字节-字符 字面量
# Python 2 onlys = 'This must be a byte-string'# Python 2 and 3s = b'This must be a byte-string'
To loop over a byte-string with possible high-bit characters, obtaining each character as a byte-string of length 1:
# Python 2 only:for bytechar in 'byte-string with high-bit chars like \xf9': ...# Python 3 only:for myint in b'byte-string with high-bit chars like \xf9': bytechar = bytes([myint])# Python 2 and 3:from future.builtins import bytesfor myint in bytes(b'byte-string with high-bit chars like \xf9'): bytechar = bytes([myint])
chr() 和 .encode('latin-1') 其中的任何一个都可以用于把一个 int 转换成一个 1-char byte string
# Python 3 only:for myint in b'byte-string with high-bit chars like \xf9': char = chr(myint) # returns a unicode string bytechar = char.encode('latin-1')# Python 2 and 3:from future.builtins import bytes, chrfor myint in bytes(b'byte-string with high-bit chars like \xf9'): char = chr(myint) # returns a unicode string bytechar = char.encode('latin-1') # forces returning a byte str basestring
# Python 2 only:a = u'abc'b = 'def'assert (isinstance(a, basestring) and isinstance(b, basestring))# Python 2 and 3: alternative 1from past.builtins import basestring # pip install futurea = u'abc'b = b'def'assert (isinstance(a, basestring) and isinstance(b, basestring))
# Python 2 and 3: alternative 2: refactor the code to avoid considering# byte-strings as strings.from future.builtins import stra = u'abc'b = b'def'c = b.decode()assert isinstance(a, str) and isinstance(c, str)# ...
unicode
# Python 2 only:templates = [u"blog/blog_post_detail_%s.html" % unicode(slug)]
# Python 2 and 3: alternative 1from future.builtins import strtemplates = [u"blog/blog_post_detail_%s.html" % str(slug)]
# Python 2 and 3: alternative 2from future.builtins import str as texttemplates = [u"blog/blog_post_detail_%s.html" % text(slug)]
StringIO
# Python 2 only:from StringIO import StringIO# or:from cStringIO import StringIO# Python 2 and 3:from io import BytesIO # for handling byte stringsfrom io import StringIO # for handling unicode strings
Imports relative to a package
假设包的结构是这样的：
mypackage/ __init__.py submodule1.py submodule2.py
submodule1.py 的代码如下：
# Python 2 only:import submodule2
# Python 2 and 3:from . import submodule2
# Python 2 and 3:# To make Py2 code safer (more like Py3) by preventing# implicit relative imports, you can also add this to the top:from __future__ import absolute_import
字典
heights = {'Fred': 175, 'Anne': 166, 'Joe': 192} Iterating through dict keys/values/items
迭代字典的键：
# Python 2 only:for key in heights.iterkeys(): ...
# Python 2 and 3:for key in heights: ...
迭代字典的值：
# Python 2 only:for value in heights.itervalues(): ...
# Idiomatic Python 3for value in heights.values(): # extra memory overhead on Py2 ...
# Python 2 and 3: option 1from future.builtins import dictheights = dict(Fred=175, Anne=166, Joe=192)for key in heights.values(): # efficient on Py2 and Py3 ...
# Python 2 and 3: option 2from future.builtins import itervalues# orfrom six import itervaluesfor key in itervalues(heights): ...
迭代字典元素：
# Python 2 only:for (key, value) in heights.iteritems(): ...
# Python 2 and 3: option 1for (key, value) in heights.items(): # inefficient on Py2 ...
# Python 2 and 3: option 2from future.builtins import iteritems# orfrom six import iteritemsfor (key, value) in iteritems(heights): ...
字典的键/值/元素 作为一个列表
字典的键作为一个列表：
# Python 2 only:keylist = heights.keys()assert isinstance(keylist, list)
# Python 2 and 3:keylist = list(heights)assert isinstance(keylist, list)
字典的值作为一个列表：
# Python 2 only:heights = {'Fred': 175, 'Anne': 166, 'Joe': 192}valuelist = heights.values()assert isinstance(valuelist, list) # Python 2 and 3: option 1valuelist = list(heights.values()) # inefficient on Py2
# Python 2 and 3: option 2from future.builtins import dictheights = dict(Fred=175, Anne=166, Joe=192)valuelist = list(heights.values())
# Python 2 and 3: option 3from future.utils import listvaluesvaluelist = listvalues(heights)
# Python 2 and 3: option 4from future.utils import itervalues# orfrom six import itervaluesvaluelist = list(itervalues(heights))
字典元素作为一个列表：
# Python 2 and 3: option 1itemlist = list(heights.items()) # inefficient on Py2
# Python 2 and 3: option 2from future.utils import listitemsitemlist = listitems(heights)
# Python 2 and 3: option 3from future.utils import iteritems# orfrom six import iteritemsitemlist = list(iteritems(heights))
自定义类的行为
自定义迭代器：
# Python 2 onlyclass Upper(object): def __init__(self, iterable): self._iter = iter(iterable) def next(self): # Py2-style return self._iter.next().upper() def __iter__(self): return selfitr = Upper('hello')assert itr.next() == 'H' # Py2-styleassert list(itr) == list('ELLO') # Python 2 and 3: option 1from future.builtins import objectclass Upper(object): def __init__(self, iterable): self._iter = iter(iterable) def __next__(self): # Py3-style iterator interface return next(self._iter).upper() # builtin next() function calls def __iter__(self): return selfitr = Upper('hello')assert next(itr) == 'H' # compatible styleassert list(itr) == list('ELLO') # Python 2 and 3: option 2from future.utils import implements_iterator@implements_iteratorclass Upper(object): def __init__(self, iterable): self._iter = iter(iterable) def __next__(self): # Py3-style iterator interface return next(self._iter).upper() # builtin next() function calls def __iter__(self): return selfitr = Upper('hello')assert next(itr) == 'H'assert list(itr) == list('ELLO') 自定义 __str__ 方法
# Python 2 only:class MyClass(object): def __unicode__(self): return 'Unicode string: \u5b54\u5b50' def __str__(self): return unicode(self).encode('utf-8')a = MyClass()print(a) # prints encoded string # Python 2 and 3:from future.utils import python_2_unicode_compatible@python_2_unicode_compatibleclass MyClass(object): def __str__(self): return u'Unicode string: \u5b54\u5b50'a = MyClass()print(a) # prints string encoded as utf-8 on Py2
Unicode string: 孔子
自定义 __nonzero__ 对比 __bool__ 方法：
# Python 2 only:class AllOrNothing(object): def __init__(self, l): self.l = l def __nonzero__(self): return all(self.l)container = AllOrNothing([0, 100, 200])assert not bool(container)
# Python 2 and 3:from future.builtins import objectclass AllOrNothing(object): def __init__(self, l): self.l = l def __bool__(self): return all(self.l)container = AllOrNothing([0, 100, 200])assert not bool(container)
列表对迭代器
xrange：
# Python 2 only:for i in xrange(10**8): ...
# Python 2 and 3: forward-compatiblefrom future.builtins import rangefor i in range(10**8): ...
# Python 2 and 3: backward-compatiblefrom past.builtins import xrangefor i in xrange(10**8): ...
range：
# Python 2 onlymylist = range(5)assert mylist == [0, 1, 2, 3, 4]
# Python 2 and 3: forward-compatible: option 1mylist = list(range(5)) # copies memory on Py2assert mylist == [0, 1, 2, 3, 4]
# Python 2 and 3: forward-compatible: option 2from future.builtins import rangemylist = list(range(5))assert mylist == [0, 1, 2, 3, 4]
# Python 2 and 3: option 3from future.utils import lrangemylist = lrange(5)assert mylist == [0, 1, 2, 3, 4]
# Python 2 and 3: backward compatiblefrom past.builtins import rangemylist = range(5)assert mylist == [0, 1, 2, 3, 4]
map：
# Python 2 only:mynewlist = map(f, myoldlist)assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 1# Idiomatic Py3, but inefficient on Py2mynewlist = list(map(f, myoldlist))assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 2from future.builtins import mapmynewlist = list(map(f, myoldlist))assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 3try: import itertools.imap as mapexcept ImportError: passmynewlist = list(map(f, myoldlist)) # inefficient on Py2assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 4from future.utils import lmapmynewlist = lmap(f, myoldlist)assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 5from past.builtins import mapmynewlist = map(f, myoldlist)assert mynewlist == [f(x) for x in myoldlist]
imap：
# Python 2 only:from itertools import imapmyiter = imap(func, myoldlist)assert isinstance(myiter, iter)
# Python 3 only:myiter = map(func, myoldlist)assert isinstance(myiter, iter)
# Python 2 and 3: option 1from future.builtins import mapmyiter = map(func, myoldlist)assert isinstance(myiter, iter)
# Python 2 and 3: option 2try: import itertools.imap as mapexcept ImportError: passmyiter = map(func, myoldlist)assert isinstance(myiter, iter)
zip, izip
As above with zip and itertools.izip.
filter, ifilter
As above with filter and itertools.ifilter too
其他内建函数
File IO with open()：
# Python 2 onlyf = open('myfile.txt')data = f.read() # as a byte stringtext = data.decode('utf-8')# Python 2 and 3: alternative 1from io import openf = open('myfile.txt', 'rb')data = f.read() # as bytestext = data.decode('utf-8') # unicode, not bytes# Python 2 and 3: alternative 2from io import openf = open('myfile.txt', encoding='utf-8')text = f.read() # unicode, not bytes reduce()
# Python 2 only:assert reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) == 1+2+3+4+5
# Python 2 and 3:from functools import reduceassert reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) == 1+2+3+4+5
raw_input()
# Python 2 only:name = raw_input('What is your name? ')assert isinstance(name, str) # native str # Python 2 and 3:from future.builtins import inputname = input('What is your name? ')assert isinstance(name, str) # native str on Py2 and Py3 input()
# Python 2 only:input("Type something safe please: ") # Python 2 and 3from future.builtins import inputeval(input("Type something safe please: ")) 警告：使用上面任何一个都是不安全的
file()
# Python 2 only:f = file(pathname)
# Python 2 and 3:f = open(pathname)# But preferably, use this:from io import openf = open(pathname, 'rb') # if f.read() should return bytes# orf = open(pathname, 'rt') # if f.read() should return unicode text
execfile()
# Python 2 only:execfile('myfile.py') # Python 2 and 3: alternative 1from past.builtins import execfileexecfile('myfile.py') # Python 2 and 3: alternative 2exec(compile(open('myfile.py').read()))# This can sometimes cause this:# SyntaxError: function ... uses import * and bare exec ...# See https://github.com/PythonCharmers/python-future/issues/37 unichr()
# Python 2 only:assert unichr(8364) == '€'
# Python 3 only:assert chr(8364) == '€'
# Python 2 and 3:from future.builtins import chrassert chr(8364) == '€'
intern()
# Python 2 only:intern('mystring') # Python 3 only:from sys import internintern('mystring') # Python 2 and 3: alternative 1from past.builtins import internintern('mystring') # Python 2 and 3: alternative 2try: from sys import internexcept ImportError: passintern('mystring') apply()
args = ('a', 'b')kwargs = {'kwarg1': True} # Python 2 only:apply(f, args, kwargs)
# Python 2 and 3: alternative 1f(*args, **kwargs)
# Python 2 and 3: alternative 2from past.builtins import applyapply(f, args, kwargs)
chr()
# Python 2 only:assert chr(64) == b'@'assert chr(200) == b'\xc8'
# Python 3 only: option 1assert chr(64).encode('latin-1') == b'@'assert chr(0xc8).encode('latin-1') == b'\xc8' # Python 2 and 3: option 1from future.builtins import chrassert chr(64).encode('latin-1') == b'@'assert chr(0xc8).encode('latin-1') == b'\xc8' # Python 3 only: option 2assert bytes([64]) == b'@'assert bytes([0xc8]) == b'\xc8'
# Python 2 and 3: option 2from future.builtins import bytesassert bytes([64]) == b'@'assert bytes([0xc8]) == b'\xc8'
cmp()
# Python 2 only:assert cmp('a', 'b') < 0 and cmp('b', 'a') > 0 and cmp('c', 'c') == 0 # Python 2 and 3: alternative 1from past.builtins import cmpassert cmp('a', 'b') < 0 and cmp('b', 'a') > 0 and cmp('c', 'c') == 0 # Python 2 and 3: alternative 2cmp = lambda(x, y): (x > y) - (x < y)assert cmp('a', 'b') < 0 and cmp('b', 'a') > 0 and cmp('c', 'c') == 0 reload()
# Python 2 only:reload(mymodule)
# Python 2 and 3from imp import reloadreload(mymodule)
标准库
StringIO 模块
# Python 2 onlyfrom StringIO import StringIOfrom cStringIO import StringIO
# Python 2 and 3from io import BytesIO# and refactor StringIO() calls to BytesIO() if passing byte-strings
http 模块
# Python 2 only:import httplibimport Cookieimport cookielibimport BaseHTTPServerimport SimpleHTTPServerimport CGIHttpServer# Python 2 and 3:from future.standard_library import hookswith hooks(): import http.client import http.cookies import http.cookiejar import http.server
urllib 模块
这个使用 urllib 的示例，相同的模式也适用于其他的已经移除的模块（一个完整的列表 ）
# Python 2 only:from urlparse import urlparsefrom urllib import urlencode
# Python 3 only:from urllib.parse import urlparse, urlencode
# Python 2 and 3: alternative 1from future.standard_library import hookswith hooks(): from urllib.parse import urlparse, urlencode
# Python 2 and 3: alternative 2from future.moves.urllib.parse import urlparse, urlencode# orfrom six.moves.urllib.parse import urlparse, urlencode
# Python 2 and 3: alternative 3try: from urllib.parse import urlparse, urlencodeexcept ImportError: from urlparse import urlparse from urllib import urlencode