python便利贴：python2-3兼容的代码

2024-01-19 13:58
文章标签 python 代码 兼容 python2 便利

本文主要是介绍python便利贴：python2-3兼容的代码，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

 

Table of Contents

支持包: python-future and six

关键语法

raising exceptions

defining exception and exception chaining

catching exceptions

Division

Long integers

Octal constants

Backtick repr

Unicode String/Byte String/BaseString

Imports relative to a package

Dictionaries

xrange vs range

map

raw_input & input

More

 

版权所有：013-2016 Python Charmers Pty Ltd, Australia.

作者： Ed Schofield.

许可：Creative Commons Attribution.

pdf版本： http://python-future.org/compatible_idioms.pdf

视频版本： http://www.youtube.com/watch?v=KOqk8j11aAI&t=10m14s

这个笔记列举了写面向未来的代码（即同时支持python2和3）的一些惯例

对小版本：

  • Python 2: 2.6+
  • Python 3: 3.3+

支持包: python-future and six

python-future是python2和python3之间缺少的兼容层， 其允许你使用一个单一的，干净的python 3.x-compatible基础代码去同时支持python 2和3。这样产生的开销是最小的（minimal overhead）。

我们使用pip install future可以import如下包:

import future
import builtins
import past
import futurize             
import pasteurize

 

目的相同共同相似的包还有python six，可以通过pip install six来安装，安装后，可import如下包：

import six

关键语法

  • print

python2 only:

[wlin@wlin rc_ci]$ ipython2
Python 2.7.5 (default, May 31 2018, 09:41:32) 
In [2]: print 'Hello' 
Hello

for both:

[wlin@wlin python_compatible]$ cat print.py 
from __future__ import print_function
print('hello','world')
[wlin@wlin python_compatible]$ python2 print.py 
hello world
[wlin@wlin python_compatible]$ python3 print.py 
hello world

 

  • raising exceptions

python2 only:

[wlin@wlin python_compatible]$ ipython2 
Python 2.7.5 (default, May 31 2018, 09:41:32) In [1]: raise ValueError, "dodgy value"
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-1-640428d25114> in <module>()
----> 1 raise ValueError, "dodgy value"ValueError: dodgy valueIn [3]: import sysIn [4]: traceback = sys.exc_info()[2]In [5]: raise ValueError, "dodgy value", traceback
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-5-91d00a6d59b2> in <module>()
----> 1 raise ValueError, "dodgy value", tracebackValueError: dodgy value

for both:

carawangs-MacBook-Pro:python_compatible carawang$ cat raise_error.py 
raise ValueError('dodgy value')#from future.utils import raise_with_traceback
#raise_with_traceback(ValueError("dodgy value"))
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 raise_error.py 
Traceback (most recent call last):File "raise_error.py", line 1, in <module>raise ValueError('dodgy value')
ValueError: dodgy value
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 raise_error.py 
Traceback (most recent call last):File "raise_error.py", line 1, in <module>raise ValueError('dodgy value')
ValueError: dodgy valuecarawangs-MacBook-Pro:python_compatible carawang$ cat raise_error.py 
#raise ValueError('dodgy value')from future.utils import raise_with_traceback
raise_with_traceback(ValueError("dodgy value"))
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 raise_error.py 
Traceback (most recent call last):File "raise_error.py", line 4, in <module>raise_with_traceback(ValueError("dodgy value"))File "<string>", line 7, in raise_with_traceback
ValueError: dodgy value
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 raise_error.py 
Traceback (most recent call last):File "raise_error.py", line 4, in <module>raise_with_traceback(ValueError("dodgy value"))File "/usr/local/lib/python3.7/site-packages/future/utils/__init__.py", line 446, in raise_with_tracebackraise exc.with_traceback(traceback)
ValueError: dodgy value

  • defining exception and exception chaining

python3 only:

carawangs-MacBook-Pro:python_compatible carawang$ cat exception_class.py 
class DatabaseError(Exception):passclass FileDataBase:def __init__(self, filename):try:self.file = open(filename)except IOError as exc:raise DatabaseError('Failed to open') from excFileDataBase("non_existent_file.txt")carawangs-MacBook-Pro:python_compatible carawang$ vi exception_class.py 
YouCompleteMe unavailable: unable to load Python.
Press ENTER or type command to continue
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 exception_class.py 
Traceback (most recent call last):File "exception_class.py", line 7, in __init__self.file = open(filename)
FileNotFoundError: [Errno 2] No such file or directory: 'non_existent_file.txt'The above exception was the direct cause of the following exception:Traceback (most recent call last):File "exception_class.py", line 11, in <module>FileDataBase("non_existent_file.txt")File "exception_class.py", line 9, in __init__raise DatabaseError('Failed to open') from exc
__main__.DatabaseError: Failed to open
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 exception_class.py File "exception_class.py", line 9raise DatabaseError('Failed to open') from exc^
SyntaxError: invalid syntax

for both:

carawangs-MacBook-Pro:python_compatible carawang$ cat exception_class.py 
from future.utils import  raise_fromclass DatabaseError(Exception):passclass FileDataBase:def __init__(self, filename):try:self.file = open(filename)except IOError as exc:raise_from(DatabaseError('Failed to open'), exc)FileDataBase("non_existent_file.txt")
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 exception_class.py 
Traceback (most recent call last):File "exception_class.py", line 9, in __init__self.file = open(filename)
FileNotFoundError: [Errno 2] No such file or directory: 'non_existent_file.txt'The above exception was the direct cause of the following exception:Traceback (most recent call last):File "exception_class.py", line 13, in <module>FileDataBase("non_existent_file.txt")File "exception_class.py", line 11, in __init__raise_from(DatabaseError('Failed to open'), exc)File "/usr/local/lib/python3.7/site-packages/future/utils/__init__.py", line 403, in raise_fromexec(execstr, myglobals, mylocals)File "<string>", line 1, in <module>
__main__.DatabaseError: Failed to open
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 exception_class.py 
Traceback (most recent call last):File "exception_class.py", line 13, in <module>FileDataBase("non_existent_file.txt")File "exception_class.py", line 11, in __init__raise_from(DatabaseError('Failed to open'), exc)File "/Users/carawang/Library/Python/2.7/lib/python/site-packages/future/utils/__init__.py", line 483, in raise_fromraise e
__main__.DatabaseError: Failed to open

  • catching exceptions

python2 only:

carawangs-MacBook-Pro:python_compatible carawang$ cat catch_exceptions.py 
try:raise ValueError('ValueError: dodgy value!')
except ValueError, e:print(e.message)
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 catch_exceptions.py 
ValueError: dodgy value!
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 catch_exceptions.py File "catch_exceptions.py", line 3except ValueError, e:^
SyntaxError: invalid syntax
for both:

carawangs-MacBook-Pro:python_compatible carawang$ cat catch_exceptions.py 
try:raise ValueError('ValueError: dodgy value!')
except ValueError as e:print(e.args)
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 catch_exceptions.py 
('ValueError: dodgy value!',)
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 catch_exceptions.py 
('ValueError: dodgy value!',)

  • Division

python 2 and python 3 get different results:

carawangs-MacBook-Pro:python_compatible carawang$ cat division.py 
result = [2/3, 3/2]
print(result)
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 division.py 
[0, 1]
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 division.py 
[0.6666666666666666, 1.5]

for both:

carawangs-MacBook-Pro:python_compatible carawang$ cat division.py 
from __future__ import division
from past.utils import old_div
result = [2/3, 3/2]
print(result)
result = [old_div(2, 3), old_div(3, 2)]
print(result)
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 division.py 
[0.6666666666666666, 1.5]
[0, 1]
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 division.py 
[0.6666666666666666, 1.5]
[0, 1]

  • Long integers

Python3  取消了short integers而仅支持int。

for python2 only:

carawangs-MacBook-Pro:python_compatible carawang$ cat integer.py 
k = 123456789999
print(type(k))
lk = 123456789999L
print(type(lk))
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 integer.py 
<type 'int'>
<type 'long'>
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 integer.py File "integer.py", line 3lk = 123456789999L^
SyntaxError: invalid syntax

for both:

carawangs-MacBook-Pro:python_compatible carawang$ cat integer.py 
from builtins import int
from past.builtins import long
if isinstance(1234567890, int):print(type(1234567890))
if isinstance(1, (int, long)):print(type(1))
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 integer.py 
<type 'int'>
<type 'int'>
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 integer.py 
<class 'int'>
<class 'int'>
 

  • Octal constants

for python2 only:

carawangs-MacBook-Pro:python_compatible carawang$ cat octal_constant.py 
from decimal import Decimal
print(int(Decimal(0644)))
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 octal_constant.py 
420
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 octal_constant.py File "octal_constant.py", line 2print(int(Decimal(0644)))^
SyntaxError: invalid token
for both:

carawangs-MacBook-Pro:python_compatible carawang$ cat octal_constant.py 
from decimal import Decimal
print(int(Decimal(0o644)))
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 octal_constant.py 
420
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 octal_constant.py 
420

  • Backtick repr

for python2 only:

carawangs-MacBook-Pro:python_compatible carawang$ cat repr.py 
hello = "hello"
print(hello)
print(`hello`)
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 repr.py 
hello
'hello'
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 repr.py File "repr.py", line 3print(`hello`)^
SyntaxError: invalid syntax
for both:
carawangs-MacBook-Pro:python_compatible carawang$ cat repr.py 
hello = "hello"
print(hello)
print(repr(hello))
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 repr.py 
hello
'hello'
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 repr.py 
hello
'hello'

 

  • Unicode String/Byte String/BaseString

我们在此处仅列出兼容的代码

carawangs-MacBook-Pro:python_compatible carawang$ cat string.py 
a_bytestring = b'testing'
a_unicodestring = u'testing'
a_str = 'testing'print(type(a_bytestring))
print(type(a_unicodestring))
print(type(a_str))
# python3 does not support basetring()
#print(isinstance(a_bytestring, basestring))
print(isinstance(a_bytestring, str))
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 string.py 
<type 'str'>
<type 'unicode'>
<type 'str'>
True
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 string.py 
<class 'bytes'>
<class 'str'>
<class 'str'>
False

  • Imports relative to a package

假设包结构为：

carawangs-MacBook-Pro:my_pkg carawang$ tree
.
├── __init__.py
├── sub_m1.py
└── sub_m2.py0 directories, 3 files

for both support:

carawangs-MacBook-Pro:my_pkg carawang$ cat sub_m2.py
TEST = True
carawangs-MacBook-Pro:my_pkg carawang$ cat sub_m1.py 
import sub_m2
print(sub_m2.TEST)
carawangs-MacBook-Pro:my_pkg carawang$ python2.7 sub_m1.py 
True
carawangs-MacBook-Pro:my_pkg carawang$ python3.7 sub_m1.py 
True

for both non-support:

carawangs-MacBook-Pro:my_pkg carawang$ cat sub_m1.py 
from . import sub_m2
print(sub_m2.TEST)
carawangs-MacBook-Pro:my_pkg carawang$ cat sub_m2.py 
TEST = True
carawangs-MacBook-Pro:my_pkg carawang$ python2.7 sub_m1.py 
Traceback (most recent call last):File "sub_m1.py", line 1, in <module>from . import sub_m2
ValueError: Attempted relative import in non-package
carawangs-MacBook-Pro:my_pkg carawang$ python3.7 sub_m1.py 
Traceback (most recent call last):File "sub_m1.py", line 1, in <module>from . import sub_m2
ImportError: cannot import name 'sub_m2' from '__main__' (sub_m1.py)
这个地方应该有问题。。。。。

  • Dictionaries

for python2 only:

carawangs-MacBook-Pro:python_compatible carawang$ cat dict.py 
heights = {'Fred': 175, 'Anne': 166, 'Joe': 192}
for key in heights.iterkeys():print(key)
for value in heights.itervalues():print(value)
for (key, value) in heights.iteritems():print(key, value)
keylist = heights.keys()
valuelist = heights.values()
print(isinstance(keylist, list))
print(keylist)
print(isinstance(valuelist, list))
print(valuelist)
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 dict.py 
Anne
Joe
Fred
166
192
175
('Anne', 166)
('Joe', 192)
('Fred', 175)
True
['Anne', 'Joe', 'Fred']
True
[166, 192, 175]
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 dict.py 
Traceback (most recent call last):File "dict.py", line 2, in <module>for key in heights.iterkeys():
AttributeError: 'dict' object has no attribute 'iterkeys'
carawangs-MacBook-Pro:python_compatible carawang$ cat dict_cp.py 
heights = {'Fred': 175, 'Anne': 166, 'Joe': 192}
#for key in heights.iterkeys():
#    print(key)
#for value in heights.itervalues():
#    print(value)
#for (key, value) in heights.iteritems():
#    print(key, value)
keylist = heights.keys()
valuelist = heights.values()
print(isinstance(keylist, list))
print(keylist)
print(isinstance(valuelist, list))
print(valuelist)
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 dict_cp.py
False
dict_keys(['Fred', 'Anne', 'Joe'])
False
dict_values([175, 166, 192])
for both:

carawangs-MacBook-Pro:python_compatible carawang$ cat dict.py 
heights = {'Fred': 175, 'Anne': 166, 'Joe': 192}
for key in heights.keys():print(key)
for value in heights.values():print(value)
for (key, value) in heights.items():print(key, value)
keylist = list(heights)
valuelist = list(heights.values())
print(isinstance(keylist, list))
print(keylist)
print(isinstance(valuelist, list))
print(valuelist)
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 dict.py 
Anne
Joe
Fred
166
192
175
('Anne', 166)
('Joe', 192)
('Fred', 175)
True
['Anne', 'Joe', 'Fred']
True
[166, 192, 175]
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 dict
/usr/local/Cellar/python/3.7.2_1/Frameworks/Python.framework/Versions/3.7/Resources/Python.app/Contents/MacOS/Python: can't open file 'dict': [Errno 2] No such file or directory
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 dict.py 
Fred
Anne
Joe
175
166
192
Fred 175
Anne 166
Joe 192
True
['Fred', 'Anne', 'Joe']
True
[175, 166, 192]

 

  • xrange vs range

for python2 only:

carawangs-MacBook-Pro:python_compatible carawang$ cat range.py 
for i in xrange(5):print(i)
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 range.py 
0
1
2
3
4
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 range.py 
Traceback (most recent call last):File "range.py", line 1, in <module>for i in xrange(5):
NameError: name 'xrange' is not defined

for both:

carawangs-MacBook-Pro:python_compatible carawang$ cat range.py 
for i in range(5):print(i)
range_list = list(range(5))
print(isinstance(range_list, list))
print(range_list)
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 range.py 
0
1
2
3
4
True
[0, 1, 2, 3, 4]
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 range.py 
0
1
2
3
4
True
[0, 1, 2, 3, 4]

  • map

for python2 only:

carawangs-MacBook-Pro:python_compatible carawang$ cat map.py 
def f(x):return int(x)+1
a_list = list(range(5))
b_list = [f(x) for x in a_list]
c_list = map(f, a_list)
print(type(c_list))
print(b_list)
print(c_list)
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 map.py 
<type 'list'>
[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5]
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 map.py 
<class 'map'>
[1, 2, 3, 4, 5]
<map object at 0x106beaac8>

for both:

carawangs-MacBook-Pro:python_compatible carawang$ cat map.py 
def f(x):return int(x)+1
a_list = list(range(5))
b_list = [f(x) for x in a_list]
c_list = list(map(f, a_list))
print(type(c_list))
print(b_list)
print(c_list)
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 map.py 
<type 'list'>
[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5]
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 map.py 
<class 'list'>
[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5]

  • raw_input & input

for python2 only:

carawangs-MacBook-Pro:python_compatible carawang$ cat input.py 
name = raw_input('What is your name?')
print("hello {}".format(name))
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 input.py 
What is your name?cara
hello cara
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 input.py 
Traceback (most recent call last):File "input.py", line 1, in <module>name = raw_input('What is your name?')
NameError: name 'raw_input' is not defined

 

for both:

carawangs-MacBook-Pro:python_compatible carawang$ cat input.py 
from builtins import input
name = input('What is your name?')
print("hello {}".format(name))
carawangs-MacBook-Pro:python_compatible carawang$ python2.7 input.py 
True
What is your name?cara
hello cara
carawangs-MacBook-Pro:python_compatible carawang$ python3.7 input.py 
What is your name?cara
hello cara

  • More

 https://python-future.org/compatible_idioms.html

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