注：本文的作者是 Ed Schofield。原文的地址为： Cheat Sheet: Writing Python 2-3 compatible code
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 future
import past # pip install past
import six # pip install six
以下的脚本也适用于 pip 安装：
futurize # pip install futurize
pasteurize # pip install pasteurize
查看 http://python-future.org 和 https://pythonhosted.org/six/ 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_function
print('Hello', file=sys.stderr)
# Python 2 only:
print 'Hello',
# Python 2 and 3:
from __future__ import print_function
print('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 1
from six import reraise as raise_
# or
from future.utils import raise_
traceback = sys.exc_info()[2]
raise_(ValueError, "dodgy value", traceback)
# Python 2 and 3: option 2
from future.utils import raise_with_traceback
raise_with_traceback(ValueError("dodgy value"))
异常链 (PEP 3134):
# Setup:
class DatabaseError(Exception):
pass
# Python 3 only
class 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_from
class 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_div
a = 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 only
k = 9223372036854775808L
# Python 2 and 3:
k = 9223372036854775808
# Python 2 only
bigint = 1L
# Python 2 and 3
from future.builtins import int
bigint = 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 1
from future.builtins import int # subclass of long on Py2
if isinstance(x, int): # matches both int and long on Py2
...
# Python 2 and 3: option 2
from past.builtins import long
if 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):
pass
class 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
# or
from future.utils import with_metaclass
class 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 only
s1 = '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 3
s1 = 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 3
from __future__ import unicode_literals # at top of module
s1 = 'The Zen of Python'
s2 = 'きたないのよりきれいな方がいい\n'
查看 http://python-future.org/unicode_literals.html 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 only
s = 'This must be a byte-string'
# Python 2 and 3
s = 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 bytes
for 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, chr
for 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 1
from past.builtins import basestring # pip install future
a = 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 str
a = 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 1
from future.builtins import str
templates = [u"blog/blog_post_detail_%s.html" % str(slug)]
# Python 2 and 3: alternative 2
from future.builtins import str as text
templates = [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 strings
from 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 3
for value in heights.values(): # extra memory overhead on Py2
...
# Python 2 and 3: option 1
from future.builtins import dict
heights = dict(Fred=175, Anne=166, Joe=192)
for key in heights.values(): # efficient on Py2 and Py3
...
# Python 2 and 3: option 2
from future.builtins import itervalues
# or
from six import itervalues
for key in itervalues(heights):
...
迭代字典元素：
# Python 2 only:
for (key, value) in heights.iteritems():
...
# Python 2 and 3: option 1
for (key, value) in heights.items(): # inefficient on Py2
...
# Python 2 and 3: option 2
from future.builtins import iteritems
# or
from six import iteritems
for (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 1
valuelist = list(heights.values()) # inefficient on Py2
# Python 2 and 3: option 2
from future.builtins import dict
heights = dict(Fred=175, Anne=166, Joe=192)
valuelist = list(heights.values())
# Python 2 and 3: option 3
from future.utils import listvalues
valuelist = listvalues(heights)
# Python 2 and 3: option 4
from future.utils import itervalues
# or
from six import itervalues
valuelist = list(itervalues(heights))
字典元素作为一个列表：
# Python 2 and 3: option 1
itemlist = list(heights.items()) # inefficient on Py2
# Python 2 and 3: option 2
from future.utils import listitems
itemlist = listitems(heights)
# Python 2 and 3: option 3
from future.utils import iteritems
# or
from six import iteritems
itemlist = list(iteritems(heights))
自定义类的行为
自定义迭代器：
# Python 2 only
class Upper(object):
def __init__(self, iterable):
self._iter = iter(iterable)
def next(self): # Py2-style
return self._iter.next().upper()
def __iter__(self):
return self
itr = Upper('hello')
assert itr.next() == 'H' # Py2-style
assert list(itr) == list('ELLO')
# Python 2 and 3: option 1
from future.builtins import object
class 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 self
itr = Upper('hello')
assert next(itr) == 'H' # compatible style
assert list(itr) == list('ELLO')
# Python 2 and 3: option 2
from future.utils import implements_iterator
@implements_iterator
class 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 self
itr = 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_compatible
class 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 object
class 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-compatible
from future.builtins import range
for i in range(10**8):
...
# Python 2 and 3: backward-compatible
from past.builtins import xrange
for i in xrange(10**8):
...
range：
# Python 2 only
mylist = range(5)
assert mylist == [0, 1, 2, 3, 4]
# Python 2 and 3: forward-compatible: option 1
mylist = list(range(5)) # copies memory on Py2
assert mylist == [0, 1, 2, 3, 4]
# Python 2 and 3: forward-compatible: option 2
from future.builtins import range
mylist = list(range(5))
assert mylist == [0, 1, 2, 3, 4]
# Python 2 and 3: option 3
from future.utils import lrange
mylist = lrange(5)
assert mylist == [0, 1, 2, 3, 4]
# Python 2 and 3: backward compatible
from past.builtins import range
mylist = 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 Py2
mynewlist = list(map(f, myoldlist))
assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 2
from future.builtins import map
mynewlist = list(map(f, myoldlist))
assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 3
try:
import itertools.imap as map
except ImportError:
pass
mynewlist = list(map(f, myoldlist)) # inefficient on Py2
assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 4
from future.utils import lmap
mynewlist = lmap(f, myoldlist)
assert mynewlist == [f(x) for x in myoldlist]
# Python 2 and 3: option 5
from past.builtins import map
mynewlist = map(f, myoldlist)
assert mynewlist == [f(x) for x in myoldlist]
imap：
# Python 2 only:
from itertools import imap
myiter = imap(func, myoldlist)
assert isinstance(myiter, iter)
# Python 3 only:
myiter = map(func, myoldlist)
assert isinstance(myiter, iter)
# Python 2 and 3: option 1
from future.builtins import map
myiter = map(func, myoldlist)
assert isinstance(myiter, iter)
# Python 2 and 3: option 2
try:
import itertools.imap as map
except ImportError:
pass
myiter = 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 only
f = open('myfile.txt')
data = f.read() # as a byte string
text = data.decode('utf-8')
# Python 2 and 3: alternative 1
from io import open
f = open('myfile.txt', 'rb')
data = f.read() # as bytes
text = data.decode('utf-8') # unicode, not bytes
# Python 2 and 3: alternative 2
from io import open
f = 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 reduce
assert 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 input
name = 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 3
from future.builtins import input
eval(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 open
f = open(pathname, 'rb') # if f.read() should return bytes
# or
f = open(pathname, 'rt') # if f.read() should return unicode text
execfile()
# Python 2 only:
execfile('myfile.py')
# Python 2 and 3: alternative 1
from past.builtins import execfile
execfile('myfile.py')
# Python 2 and 3: alternative 2
exec(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 chr
assert chr(8364) == '€'
intern()
# Python 2 only:
intern('mystring')
# Python 3 only:
from sys import intern
intern('mystring')
# Python 2 and 3: alternative 1
from past.builtins import intern
intern('mystring')
# Python 2 and 3: alternative 2
try:
from sys import intern
except ImportError:
pass
intern('mystring')
apply()
args = ('a', 'b')
kwargs = {'kwarg1': True}
# Python 2 only:
apply(f, args, kwargs)
# Python 2 and 3: alternative 1
f(*args, **kwargs)
# Python 2 and 3: alternative 2
from past.builtins import apply
apply(f, args, kwargs)
chr()
# Python 2 only:
assert chr(64) == b'@'
assert chr(200) == b'\xc8'
# Python 3 only: option 1
assert chr(64).encode('latin-1') == b'@'
assert chr(0xc8).encode('latin-1') == b'\xc8'
# Python 2 and 3: option 1
from future.builtins import chr
assert chr(64).encode('latin-1') == b'@'
assert chr(0xc8).encode('latin-1') == b'\xc8'
# Python 3 only: option 2
assert bytes([64]) == b'@'
assert bytes([0xc8]) == b'\xc8'
# Python 2 and 3: option 2
from future.builtins import bytes
assert 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 1
from past.builtins import cmp
assert cmp('a', 'b') < 0 and cmp('b', 'a') > 0 and cmp('c', 'c') == 0
# Python 2 and 3: alternative 2
cmp = 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 3
from imp import reload
reload(mymodule)
标准库
StringIO 模块
# Python 2 only
from StringIO import StringIO
from cStringIO import StringIO
# Python 2 and 3
from io import BytesIO
# and refactor StringIO() calls to BytesIO() if passing byte-strings
http 模块
# Python 2 only:
import httplib
import Cookie
import cookielib
import BaseHTTPServer
import SimpleHTTPServer
import CGIHttpServer
# Python 2 and 3:
from future.standard_library import hooks
with 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 http://python-future.org/standard_library_imports.html#list-of-standard-library-modules ）
# Python 2 only:
from urlparse import urlparse
from urllib import urlencode
# Python 3 only:
from urllib.parse import urlparse, urlencode
# Python 2 and 3: alternative 1
from future.standard_library import hooks
with hooks():
from urllib.parse import urlparse, urlencode
# Python 2 and 3: alternative 2
from future.moves.urllib.parse import urlparse, urlencode
# or
from six.moves.urllib.parse import urlparse, urlencode
# Python 2 and 3: alternative 3
try:
from urllib.parse import urlparse, urlencode
except ImportError:
from urlparse import urlparse
from urllib import urlencode