Python 2.7.x 和 3.x 版本的区别
2016-09-10 18:38
459 查看
许多Python初学者都会问：我应该学习哪个版本的Python。对于这个问题，我的回答通常是“先选择一个最适合你的Python教程，教程中使用哪个版本的Python，你就用那个版本。等学得差不多了，再来研究不同版本之间的差别”。
但如果想要用Python开发一个新项目，那么该如何选择Python版本呢？我可以负责任的说，大部分Python库都同时支持Python 2.7.x和3.x版本的，所以不论选择哪个版本都是可以的。但为了在使用Python时避开某些版本中一些常见的陷阱，或需要移植某个Python项目时，依然有必要了解一下Python两个常见版本之间的主要区别。
目录
使用future模块
print函数
整数除法
Unicode
xrange
触发异常
处理异常
next()函数和.next()方法
For循环变量与全局命名空间泄漏
比较无序类型
使用input()解析输入内容
返回可迭代对象，而不是列表
更多关于Python 2和Python 3的文章
future模块
[回到目录]
Python 3.x引入了一些与Python 2不兼容的关键字和特性，在Python 2中，可以通过内置的future模块导入这些新内容。如果你希望在Python 2环境下写的代码也可以在Python 3.x中运行，那么建议使用future模块。例如，如果希望在Python 2中拥有Python 3.x的整数除法行为，可以通过下面的语句导入相应的模块。
Python
from future import division
1
from future import division
下表列出了future中其他可导入的特性：
特性 可选版本 强制版本 效果
nested_scopes 2.1.0b1 2.2 PEP 227:
Statically Nested Scopes
generators 2.2.0a1 2.3 PEP 255:
Simple Generators
division 2.2.0a2 3.0 PEP 238:
Changing the Division Operator
absolute_import 2.5.0a1 3.0 PEP 328:
Imports: Multi-Line and Absolute/Relative
with_statement 2.5.0a1 2.6 PEP 343:
The “with” Statement
print_function 2.6.0a2 3.0 PEP 3105:
Make print a function
unicode_literals 2.6.0a2 3.0 PEP 3112:
Bytes literals in Python 3000
（来源： https://docs.python.org/2/library/future.html)
示例：
Python
1
from platform import python_version
print函数
[回到目录]
虽然print语法是Python 3中一个很小的改动，且应该已经广为人知，但依然值得提一下：Python 2中的print语句被Python 3中的print()函数取代，这意味着在Python 3中必须用括号将需要输出的对象括起来。
在Python 2中使用额外的括号也是可以的。但反过来在Python 3中想以Python2的形式不带括号调用print函数时，会触发SyntaxError。
Python 2
Python
print ‘Python’, python_version()
print ‘Hello, World!’
print(‘Hello, World!’)
print “text”, ; print ‘print more text on the same line’
1
2
3
4
print ‘Python’, python_version()
print ‘Hello, World!’
print(‘Hello, World!’)
print “text”, ; print ‘print more text on the same line’
Python
Python 2.7.6
Hello, World!
Hello, World!
text print more text on the same line
1
2
3
4
Python 2.7.6
Hello, World!
Hello, World!
text print more text on the same line
Python 3
Python
print(‘Python’, python_version())
print(‘Hello, World!’)
print(“some text,”, end=”“)
print(’ print more text on the same line’)
1
2
3
4
5
print(‘Python’, python_version())
print(‘Hello, World!’)
print(“some text,”, end=”“)
print(’ print more text on the same line’)
Python
Python 3.4.1
Hello, World!
some text, print more text on the same line
1
2
3
Python 3.4.1
Hello, World!
some text, print more text on the same line
Python
print ‘Hello, World!’
1
print ‘Hello, World!’
Python
File “”, line 1
print ‘Hello, World!’
^
SyntaxError: invalid syntax
1
2
3
4
File “”, line 1
print ‘Hello, World!’
^
SyntaxError: invalid syntax
注意：
在Python中，带不带括号输出”Hello World”都很正常。但如果在圆括号中同时输出多个对象时，就会创建一个元组，这是因为在Python 2中，print是一个语句，而不是函数调用。
Python
print ‘Python’, python_version()
print(‘a’, ‘b’)
print ‘a’, ‘b’
1
2
3
print ‘Python’, python_version()
print(‘a’, ‘b’)
print ‘a’, ‘b’
Python
Python 2.7.7
(‘a’, ‘b’)
a b
1
2
3
Python 2.7.7
(‘a’, ‘b’)
a b
整数除法
[回到目录]
由于人们常常会忽视Python 3在整数除法上的改动（写错了也不会触发Syntax Error），所以在移植代码或在Python 2中执行Python 3的代码时，需要特别注意这个改动。
所以，我还是会在Python 3的脚本中尝试用float(3)/2或 3/2.0代替3/2，以此来避免代码在Python 2环境下可能导致的错误（或与之相反，在Python 2脚本中用from future import division来使用Python 3的除法）。
Python 2
Python
print ‘Python’, python_version()
print ‘3 / 2 =’, 3 / 2
print ‘3 // 2 =’, 3 // 2
print ‘3 / 2.0 =’, 3 / 2.0
print ‘3 // 2.0 =’, 3 // 2.0
1
2
3
4
5
print ‘Python’, python_version()
print ‘3 / 2 =’, 3 / 2
print ‘3 // 2 =’, 3 // 2
print ‘3 / 2.0 =’, 3 / 2.0
print ‘3 // 2.0 =’, 3 // 2.0
Python
Python 2.7.6
3 / 2 = 1
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0
1
2
3
4
5
Python 2.7.6
3 / 2 = 1
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0
Python 3
Python
print(‘Python’, python_version())
print(‘3 / 2 =’, 3 / 2)
print(‘3 // 2 =’, 3 // 2)
print(‘3 / 2.0 =’, 3 / 2.0)
print(‘3 // 2.0 =’, 3 // 2.0)
1
2
3
4
5
print(‘Python’, python_version())
print(‘3 / 2 =’, 3 / 2)
print(‘3 // 2 =’, 3 // 2)
print(‘3 / 2.0 =’, 3 / 2.0)
print(‘3 // 2.0 =’, 3 // 2.0)
Python
Python 3.4.1
3 / 2 = 1.5
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0
1
2
3
4
5
Python 3.4.1
3 / 2 = 1.5
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0
Unicode
[回到目录]
Python 2有基于ASCII的str()类型，其可通过单独的unicode()函数转成unicode类型，但没有byte类型。
而在Python 3中，终于有了Unicode（utf-8）字符串，以及两个字节类：bytes和bytearrays。
Python 2
Python
print ‘Python’, python_version()
1
print ‘Python’, python_version()
Python
Python 2.7.6
1
Python 2.7.6
Python
print type(unicode(‘this is like a python3 str type’))
1
print type(unicode(‘this is like a python3 str type’))
Python
in ()
—-> 1 ‘note that we cannot add a string’ + b’bytes for data’
TypeError: Can’t convert ‘bytes’ object to str implicitly
xrange
[回到目录]
在Python 2.x中，经常会用xrange()创建一个可迭代对象，通常出现在“for循环”或“列表/集合/字典推导式”中。
这种行为与生成器非常相似（如”惰性求值“），但这里的xrange-iterable无尽的，意味着可能在这个xrange上无限迭代。
由于xrange的“惰性求知“特性，如果只需迭代一次（如for循环中），range()通常比xrange()快一些。不过不建议在多次迭代中使用range()，因为range()每次都会在内存中重新生成一个列表。
在Python 3中，range()的实现方式与xrange()函数相同，所以就不存在专用的xrange()（在Python 3中使用xrange()会触发NameError）。
Python
import timeit
n = 10000
def test_range(n):
return for i in range(n):
pass
def test_xrange(n):
for i in xrange(n):
pass
1
2
3
4
5
6
7
8
9
10
import timeit
n = 10000
def test_range(n):
return for i in range(n):
pass
def test_xrange(n):
for i in xrange(n):
pass
Python 2
Python
print ‘Python’, python_version()
print ‘ntiming range()’
%timeit test_range(n)
print ‘nntiming xrange()’
%timeit test_xrange(n)
1
2
3
4
5
6
7
8
print ‘Python’, python_version()
print ‘ntiming range()’
%timeit test_range(n)
print ‘nntiming xrange()’
%timeit test_xrange(n)
Python
Python 2.7.6
timing range()
1000 loops, best of 3: 433 µs per loop
timing xrange()
1000 loops, best of 3: 350 µs per loop
1
2
3
4
5
6
7
Python 2.7.6
timing range()
1000 loops, best of 3: 433 µs per loop
timing xrange()
1000 loops, best of 3: 350 µs per loop
Python 3
Python
print(‘Python’, python_version())
print(‘ntiming range()’)
%timeit test_range(n)
1
2
3
4
print(‘Python’, python_version())
print(‘ntiming range()’)
%timeit test_range(n)
Python
Python 3.4.1
timing range()
1000 loops, best of 3: 520 µs per loop
1
2
3
4
Python 3.4.1
timing range()
1000 loops, best of 3: 520 µs per loop
Python
print(xrange(10))
1
print(xrange(10))
Python
NameError Traceback (most recent call last)
in ()
—-> 1 print(xrange(10))
NameError: name ‘xrange’ is not defined
1
2
3
4
5
in ()
—-> 1 print(xrange(10))
NameError: name ‘xrange’ is not defined
Python 3中的range对象中的contains方法
另一个值得一提的是，在Python 3.x中，range有了一个新的contains方法。contains方法可以有效的加快Python 3.x中整数和布尔型的“查找”速度。
Python
x = 10000000
def val_in_range(x, val):
return val in range(x)
def val_in_xrange(x, val):
return val in xrange(x)
print(‘Python’, python_version())
assert(val_in_range(x, x/2) == True)
assert(val_in_range(x, x//2) == True)
%timeit val_in_range(x, x/2)
%timeit val_in_range(x, x//2)
1
2
3
4
5
6
7
8
9
10
11
12
13
x = 10000000
def val_in_range(x, val):
return val in range(x)
def val_in_xrange(x, val):
return val in xrange(x)
print(‘Python’, python_version())
assert(val_in_range(x, x/2) == True)
assert(val_in_range(x, x//2) == True)
%timeit val_in_range(x, x/2)
%timeit val_in_range(x, x//2)
Python
Python 3.4.1
1 loops, best of 3: 742 ms per loop
1000000 loops, best of 3: 1.19 µs per loop
1
2
3
Python 3.4.1
1 loops, best of 3: 742 ms per loop
1000000 loops, best of 3: 1.19 µs per loop
根据上面的timeit的结果，查找整数比查找浮点数要快大约6万倍。但由于Python 2.x中的range或xrange没有contains方法，所以在Python 2中的整数和浮点数的查找速度差别不大。
Python
print ‘Python’, python_version()
assert(val_in_xrange(x, x/2.0) == True)
assert(val_in_xrange(x, x/2) == True)
assert(val_in_range(x, x/2) == True)
assert(val_in_range(x, x//2) == True)
%timeit val_in_xrange(x, x/2.0)
%timeit val_in_xrange(x, x/2)
%timeit val_in_range(x, x/2.0)
%timeit val_in_range(x, x/2)
1
2
3
4
5
6
7
8
9
10
11
12
13
print ‘Python’, python_version()
assert(val_in_xrange(x, x/2.0) == True)
assert(val_in_xrange(x, x/2) == True)
assert(val_in_range(x, x/2) == True)
assert(val_in_range(x, x//2) == True)
%timeit val_in_xrange(x, x/2.0)
%timeit val_in_xrange(x, x/2)
%timeit val_in_range(x, x/2.0)
%timeit val_in_range(x, x/2)
Python
Python 2.7.7
1 loops, best of 3: 285 ms per loop
1 loops, best of 3: 179 ms per loop
1 loops, best of 3: 658 ms per loop
1 loops, best of 3: 556 ms per loop
1
2
3
4
5
Python 2.7.7
1 loops, best of 3: 285 ms per loop
1 loops, best of 3: 179 ms per loop
1 loops, best of 3: 658 ms per loop
1 loops, best of 3: 556 ms per loop
下面的代码证明了Python 2.x中没有contain方法：
Python
print(‘Python’, python_version())
range.contains
1
2
print(‘Python’, python_version())
range.contains
Python
Python 3.4.1
in ()
1 print ‘Python’, python_version()
—-> 2 range.contains
AttributeError: ‘builtin_function_or_method’ object has no attribute ‘contains’
1
2
3
4
5
6
7
8
in ()
1 print ‘Python’, python_version()
—-> 2 range.contains
AttributeError: ‘builtin_function_or_method’ object has no attribute ‘contains’
Python
print(‘Python’, python_version())
xrange.contains
1
2
print(‘Python’, python_version())
xrange.contains
Python
Python 2.7.7
AttributeError Traceback (most recent call last)
in ()
1 print ‘Python’, python_version()
—-> 2 xrange.contains
AttributeError: type object ‘xrange’ has no attribute ‘contains’
1
2
3
4
5
6
7
8
9
Python 2.7.7
AttributeError Traceback (most recent call last)
in ()
1 print ‘Python’, python_version()
—-> 2 xrange.contains
AttributeError: type object ‘xrange’ has no attribute ‘contains’
关于Python 2中xrange()与Python 3中range()之间的速度差异的一点说明：
有读者指出了Python 3中的range()和Python 2中xrange()执行速度有差异。由于这两者的实现方式相同，因此理论上执行速度应该也是相同的。这里的速度差别仅仅是因为Python 3的总体速度就比Python 2慢。
Python
def test_while():
i = 0
while i < 20000:
i += 1
return
1
2
3
4
5
def test_while():
i = 0
while i < 20000:
i += 1
return
Python
print(‘Python’, python_version())
%timeit test_while()
1
2
print(‘Python’, python_version())
%timeit test_while()
Python
Python 3.4.1
%timeit test_while()
100 loops, best of 3: 2.68 ms per loop
1
2
3
4
Python 3.4.1
%timeit test_while()
100 loops, best of 3: 2.68 ms per loop
Python
print ‘Python’, python_version()
%timeit test_while()
1
2
print ‘Python’, python_version()
%timeit test_while()
Python
Python 2.7.6
1000 loops, best of 3: 1.72 ms per loop
1
2
Python 2.7.6
1000 loops, best of 3: 1.72 ms per loop
触发异常
[回到目录]
Python 2支持新旧两种异常触发语法，而Python 3只接受带括号的的语法（不然会触发SyntaxError）：
Python 2
Python
print ‘Python’, python_version()
1
print ‘Python’, python_version()
Python
Python 2.7.6
1
Python 2.7.6
Python
raise IOError, “file error”
1
raise IOError, “file error”
Python
IOError Traceback (most recent call last)
in ()
—-> 1 raise IOError, “file error”
IOError: file error
1
2
3
4
5
in ()
—-> 1 raise IOError, “file error”
IOError: file error
Python
raise IOError(“file error”)
1
raise IOError(“file error”)
Python
IOError Traceback (most recent call last)
in ()
—-> 1 raise IOError(“file error”)
IOError: file error
1
2
3
4
5
in ()
—-> 1 raise IOError(“file error”)
IOError: file error
Python 3
Python
print(‘Python’, python_version())
1
print(‘Python’, python_version())
Python
Python 3.4.1
1
Python 3.4.1
Python
raise IOError, “file error”
1
raise IOError, “file error”
Python
File “”, line 1
raise IOError, “file error”
^
SyntaxError: invalid syntax
The proper way to raise an exception in Python 3:
1
2
3
4
5
File “”, line 1
raise IOError, “file error”
^
SyntaxError: invalid syntax
The proper way to raise an exception in Python 3:
Python
print(‘Python’, python_version())
raise IOError(“file error”)
1
2
print(‘Python’, python_version())
raise IOError(“file error”)
Python
Python 3.4.1
OSError Traceback (most recent call last)
in ()
1 print(‘Python’, python_version())
—-> 2 raise IOError(“file error”)
OSError: file error
1
2
3
4
5
6
7
8
9
Python 3.4.1
OSError Traceback (most recent call last)
in ()
1 print(‘Python’, python_version())
—-> 2 raise IOError(“file error”)
OSError: file error
异常处理
[回到目录]
Python 3中的异常处理也发生了一点变化。在Python 3中必须使用“as”关键字。
Python 2
Python
print ‘Python’, python_version()
try:
let_us_cause_a_NameError
except NameError, err:
print err, ‘–> our error message’
1
2
3
4
5
print ‘Python’, python_version()
try:
let_us_cause_a_NameError
except NameError, err:
print err, ‘–> our error message’
Python
Python 2.7.6
name ‘let_us_cause_a_NameError’ is not defined –> our error message
1
2
Python 2.7.6
name ‘let_us_cause_a_NameError’ is not defined –> our error message
Python 3
Python
print(‘Python’, python_version())
try:
let_us_cause_a_NameError
except NameError as err:
print(err, ‘–> our error message’)
1
2
3
4
5
print(‘Python’, python_version())
try:
let_us_cause_a_NameError
except NameError as err:
print(err, ‘–> our error message’)
Python
Python 3.4.1
name ‘let_us_cause_a_NameError’ is not defined –> our error message
1
2
Python 3.4.1
name ‘let_us_cause_a_NameError’ is not defined –> our error message
next()函数和.next()方法
[回到目录]
由于会经常用到next()（.next()）函数（方法），所以还要提到另一个语法改动（实现方面也做了改动）：在Python 2.7.5中，函数形式和方法形式都可以使用，而在Python 3中，只能使用next()函数（试图调用.next()方法会触发AttributeError）。
Python 2
Python
print ‘Python’, python_version()
my_generator = (letter for letter in ‘abcdefg’)
next(my_generator)
my_generator.next()
1
2
3
4
print ‘Python’, python_version()
my_generator = (letter for letter in ‘abcdefg’)
next(my_generator)
my_generator.next()
Python
Python 2.7.6
‘b’
1
2
Python 2.7.6
‘b’
Python 3
Python
print(‘Python’, python_version())
my_generator = (letter for letter in ‘abcdefg’)
next(my_generator)
1
2
3
print(‘Python’, python_version())
my_generator = (letter for letter in ‘abcdefg’)
next(my_generator)
Python
Python 3.4.1
‘a’
1
2
Python 3.4.1
‘a’
Python
my_generator.next()
1
my_generator.next()
Python
AttributeError Traceback (most recent call last)
in ()
—-> 1 my_generator.next()
AttributeError: ‘generator’ object has no attribute ‘next’
1
2
3
4
5
in ()
—-> 1 my_generator.next()
AttributeError: ‘generator’ object has no attribute ‘next’
For循环变量与全局命名空间泄漏
[回到目录]
好消息是：在Python 3.x中，for循环中的变量不再会泄漏到全局命名空间中了！
这是Python 3.x中做的一个改动，在“What’s New In Python 3.0”中有如下描述：
“列表推导不再支持[… for var in item1, item2, …]这样的语法，使用[… for var in (item1, item2, …)]代替。还要注意列表推导有不同的语义：现在列表推导更接近list()构造器中的生成器表达式这样的语法糖，特别要注意的是，循环控制变量不会再泄漏到循环周围的空间中了。”
Python 2
Python
print ‘Python’, python_version()
i = 1
print ‘before: i =’, i
print ‘comprehension: ‘, [i for i in range(5)]
print ‘after: i =’, i
1
2
3
4
5
6
7
8
print ‘Python’, python_version()
i = 1
print ‘before: i =’, i
print ‘comprehension: ‘, [i for i in range(5)]
print ‘after: i =’, i
Python
Python 2.7.6
before: i = 1
comprehension: [0, 1, 2, 3, 4]
after: i = 4
1
2
3
4
Python 2.7.6
before: i = 1
comprehension: [0, 1, 2, 3, 4]
after: i = 4
Python 3
Python
print(‘Python’, python_version())
i = 1
print(‘before: i =’, i)
print(‘comprehension:’, [i for i in range(5)])
print(‘after: i =’, i)
1
2
3
4
5
6
7
8
print(‘Python’, python_version())
i = 1
print(‘before: i =’, i)
print(‘comprehension:’, [i for i in range(5)])
print(‘after: i =’, i)
Python
Python 3.4.1
before: i = 1
comprehension: [0, 1, 2, 3, 4]
after: i = 1
1
2
3
4
Python 3.4.1
before: i = 1
comprehension: [0, 1, 2, 3, 4]
after: i = 1
比较无序类型
[回到目录]
Python 3中另一个优秀的改动是，如果我们试图比较无序类型，会触发一个TypeError。
Python 2
Python
print ‘Python’, python_version()
print “[1, 2] > ‘foo’ = “, [1, 2] > ‘foo’
print “(1, 2) > ‘foo’ = “, (1, 2) > ‘foo’
print “[1, 2] > (1, 2) = “, [1, 2] > (1, 2)
1
2
3
4
print ‘Python’, python_version()
print “[1, 2] > ‘foo’ = “, [1, 2] > ‘foo’
print “(1, 2) > ‘foo’ = “, (1, 2) > ‘foo’
print “[1, 2] > (1, 2) = “, [1, 2] > (1, 2)
Python
Python 2.7.6
[1, 2] > ‘foo’ = False
(1, 2) > ‘foo’ = True
[1, 2] > (1, 2) = False
1
2
3
4
Python 2.7.6
[1, 2] > ‘foo’ = False
(1, 2) > ‘foo’ = True
[1, 2] > (1, 2) = False
Python 3
Python
print(‘Python’, python_version())
print(“[1, 2] > ‘foo’ = “, [1, 2] > ‘foo’)
print(“(1, 2) > ‘foo’ = “, (1, 2) > ‘foo’)
print(“[1, 2] > (1, 2) = “, [1, 2] > (1, 2))
1
2
3
4
print(‘Python’, python_version())
print(“[1, 2] > ‘foo’ = “, [1, 2] > ‘foo’)
print(“(1, 2) > ‘foo’ = “, (1, 2) > ‘foo’)
print(“[1, 2] > (1, 2) = “, [1, 2] > (1, 2))
Python
in ()
1 print(‘Python’, python_version())
—-> 2 print(“[1, 2] > ‘foo’ = “, [1, 2] > ‘foo’)
3 print(“(1, 2) > ‘foo’ = “, (1, 2) > ‘foo’)
4 print(“[1, 2] > (1, 2) = “, [1, 2] > (1, 2))
TypeError: unorderable types: list() > str()
1
2
3
4
5
6
7
8
9
通过input()解析用户的输入
[回到目录]
幸运的是，Python 3改进了input()函数，这样该函数就会总是将用户的输入存储为str对象。在Python 2中，为了避免读取非字符串类型会发生的一些危险行为，不得不使用raw_input()代替input()。
Python 2
Python
Python 2.7.6
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type “help”, “copyright”, “credits” or “license” for more information.
enter a number: 123
type(my_input)
但如果想要用Python开发一个新项目，那么该如何选择Python版本呢？我可以负责任的说，大部分Python库都同时支持Python 2.7.x和3.x版本的，所以不论选择哪个版本都是可以的。但为了在使用Python时避开某些版本中一些常见的陷阱，或需要移植某个Python项目时，依然有必要了解一下Python两个常见版本之间的主要区别。
目录
使用future模块
print函数
整数除法
Unicode
xrange
触发异常
处理异常
next()函数和.next()方法
For循环变量与全局命名空间泄漏
比较无序类型
使用input()解析输入内容
返回可迭代对象，而不是列表
更多关于Python 2和Python 3的文章
future模块
[回到目录]
Python 3.x引入了一些与Python 2不兼容的关键字和特性，在Python 2中，可以通过内置的future模块导入这些新内容。如果你希望在Python 2环境下写的代码也可以在Python 3.x中运行，那么建议使用future模块。例如，如果希望在Python 2中拥有Python 3.x的整数除法行为，可以通过下面的语句导入相应的模块。
Python
from future import division
1
from future import division
下表列出了future中其他可导入的特性：
特性 可选版本 强制版本 效果
nested_scopes 2.1.0b1 2.2 PEP 227:
Statically Nested Scopes
generators 2.2.0a1 2.3 PEP 255:
Simple Generators
division 2.2.0a2 3.0 PEP 238:
Changing the Division Operator
absolute_import 2.5.0a1 3.0 PEP 328:
Imports: Multi-Line and Absolute/Relative
with_statement 2.5.0a1 2.6 PEP 343:
The “with” Statement
print_function 2.6.0a2 3.0 PEP 3105:
Make print a function
unicode_literals 2.6.0a2 3.0 PEP 3112:
Bytes literals in Python 3000
（来源： https://docs.python.org/2/library/future.html)
示例：
Python
from platform import python_version
1
from platform import python_version
print函数
[回到目录]
虽然print语法是Python 3中一个很小的改动，且应该已经广为人知，但依然值得提一下：Python 2中的print语句被Python 3中的print()函数取代，这意味着在Python 3中必须用括号将需要输出的对象括起来。
在Python 2中使用额外的括号也是可以的。但反过来在Python 3中想以Python2的形式不带括号调用print函数时，会触发SyntaxError。
Python 2
Python
print ‘Python’, python_version()
print ‘Hello, World!’
print(‘Hello, World!’)
print “text”, ; print ‘print more text on the same line’
1
2
3
4
print ‘Python’, python_version()
print ‘Hello, World!’
print(‘Hello, World!’)
print “text”, ; print ‘print more text on the same line’
Python
Python 2.7.6
Hello, World!
Hello, World!
text print more text on the same line
1
2
3
4
Python 2.7.6
Hello, World!
Hello, World!
text print more text on the same line
Python 3
Python
print(‘Python’, python_version())
print(‘Hello, World!’)
print(“some text,”, end=”“)
print(’ print more text on the same line’)
1
2
3
4
5
print(‘Python’, python_version())
print(‘Hello, World!’)
print(“some text,”, end=”“)
print(’ print more text on the same line’)
Python
Python 3.4.1
Hello, World!
some text, print more text on the same line
1
2
3
Python 3.4.1
Hello, World!
some text, print more text on the same line
Python
print ‘Hello, World!’
1
print ‘Hello, World!’
Python
File “”, line 1
print ‘Hello, World!’
^
SyntaxError: invalid syntax
1
2
3
4
File “”, line 1
print ‘Hello, World!’
^
SyntaxError: invalid syntax
注意：
在Python中，带不带括号输出”Hello World”都很正常。但如果在圆括号中同时输出多个对象时，就会创建一个元组，这是因为在Python 2中，print是一个语句，而不是函数调用。
Python
print ‘Python’, python_version()
print(‘a’, ‘b’)
print ‘a’, ‘b’
1
2
3
print ‘Python’, python_version()
print(‘a’, ‘b’)
print ‘a’, ‘b’
Python
Python 2.7.7
(‘a’, ‘b’)
a b
1
2
3
Python 2.7.7
(‘a’, ‘b’)
a b
整数除法
[回到目录]
由于人们常常会忽视Python 3在整数除法上的改动（写错了也不会触发Syntax Error），所以在移植代码或在Python 2中执行Python 3的代码时，需要特别注意这个改动。
所以，我还是会在Python 3的脚本中尝试用float(3)/2或 3/2.0代替3/2，以此来避免代码在Python 2环境下可能导致的错误（或与之相反，在Python 2脚本中用from future import division来使用Python 3的除法）。
Python 2
Python
print ‘Python’, python_version()
print ‘3 / 2 =’, 3 / 2
print ‘3 // 2 =’, 3 // 2
print ‘3 / 2.0 =’, 3 / 2.0
print ‘3 // 2.0 =’, 3 // 2.0
1
2
3
4
5
print ‘Python’, python_version()
print ‘3 / 2 =’, 3 / 2
print ‘3 // 2 =’, 3 // 2
print ‘3 / 2.0 =’, 3 / 2.0
print ‘3 // 2.0 =’, 3 // 2.0
Python
Python 2.7.6
3 / 2 = 1
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0
1
2
3
4
5
Python 2.7.6
3 / 2 = 1
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0
Python 3
Python
print(‘Python’, python_version())
print(‘3 / 2 =’, 3 / 2)
print(‘3 // 2 =’, 3 // 2)
print(‘3 / 2.0 =’, 3 / 2.0)
print(‘3 // 2.0 =’, 3 // 2.0)
1
2
3
4
5
print(‘Python’, python_version())
print(‘3 / 2 =’, 3 / 2)
print(‘3 // 2 =’, 3 // 2)
print(‘3 / 2.0 =’, 3 / 2.0)
print(‘3 // 2.0 =’, 3 // 2.0)
Python
Python 3.4.1
3 / 2 = 1.5
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0
1
2
3
4
5
Python 3.4.1
3 / 2 = 1.5
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0
Unicode
[回到目录]
Python 2有基于ASCII的str()类型，其可通过单独的unicode()函数转成unicode类型，但没有byte类型。
而在Python 3中，终于有了Unicode（utf-8）字符串，以及两个字节类：bytes和bytearrays。
Python 2
Python
print ‘Python’, python_version()
1
print ‘Python’, python_version()
Python
Python 2.7.6
1
Python 2.7.6
Python
print type(unicode(‘this is like a python3 str type’))
1
print type(unicode(‘this is like a python3 str type’))
Python
6
TypeError Traceback (most recent call last)in ()
—-> 1 ‘note that we cannot add a string’ + b’bytes for data’
TypeError: Can’t convert ‘bytes’ object to str implicitly
xrange
[回到目录]
在Python 2.x中，经常会用xrange()创建一个可迭代对象，通常出现在“for循环”或“列表/集合/字典推导式”中。
这种行为与生成器非常相似（如”惰性求值“），但这里的xrange-iterable无尽的，意味着可能在这个xrange上无限迭代。
由于xrange的“惰性求知“特性，如果只需迭代一次（如for循环中），range()通常比xrange()快一些。不过不建议在多次迭代中使用range()，因为range()每次都会在内存中重新生成一个列表。
在Python 3中，range()的实现方式与xrange()函数相同，所以就不存在专用的xrange()（在Python 3中使用xrange()会触发NameError）。
Python
import timeit
n = 10000
def test_range(n):
return for i in range(n):
pass
def test_xrange(n):
for i in xrange(n):
pass
1
2
3
4
5
6
7
8
9
10
import timeit
n = 10000
def test_range(n):
return for i in range(n):
pass
def test_xrange(n):
for i in xrange(n):
pass
Python 2
Python
print ‘Python’, python_version()
print ‘ntiming range()’
%timeit test_range(n)
print ‘nntiming xrange()’
%timeit test_xrange(n)
1
2
3
4
5
6
7
8
print ‘Python’, python_version()
print ‘ntiming range()’
%timeit test_range(n)
print ‘nntiming xrange()’
%timeit test_xrange(n)
Python
Python 2.7.6
timing range()
1000 loops, best of 3: 433 µs per loop
timing xrange()
1000 loops, best of 3: 350 µs per loop
1
2
3
4
5
6
7
Python 2.7.6
timing range()
1000 loops, best of 3: 433 µs per loop
timing xrange()
1000 loops, best of 3: 350 µs per loop
Python 3
Python
print(‘Python’, python_version())
print(‘ntiming range()’)
%timeit test_range(n)
1
2
3
4
print(‘Python’, python_version())
print(‘ntiming range()’)
%timeit test_range(n)
Python
Python 3.4.1
timing range()
1000 loops, best of 3: 520 µs per loop
1
2
3
4
Python 3.4.1
timing range()
1000 loops, best of 3: 520 µs per loop
Python
print(xrange(10))
1
print(xrange(10))
Python
NameError Traceback (most recent call last)
in ()
—-> 1 print(xrange(10))
NameError: name ‘xrange’ is not defined
1
2
3
4
5
6
NameError Traceback (most recent call last)in ()
—-> 1 print(xrange(10))
NameError: name ‘xrange’ is not defined
Python 3中的range对象中的contains方法
另一个值得一提的是，在Python 3.x中，range有了一个新的contains方法。contains方法可以有效的加快Python 3.x中整数和布尔型的“查找”速度。
Python
x = 10000000
def val_in_range(x, val):
return val in range(x)
def val_in_xrange(x, val):
return val in xrange(x)
print(‘Python’, python_version())
assert(val_in_range(x, x/2) == True)
assert(val_in_range(x, x//2) == True)
%timeit val_in_range(x, x/2)
%timeit val_in_range(x, x//2)
1
2
3
4
5
6
7
8
9
10
11
12
13
x = 10000000
def val_in_range(x, val):
return val in range(x)
def val_in_xrange(x, val):
return val in xrange(x)
print(‘Python’, python_version())
assert(val_in_range(x, x/2) == True)
assert(val_in_range(x, x//2) == True)
%timeit val_in_range(x, x/2)
%timeit val_in_range(x, x//2)
Python
Python 3.4.1
1 loops, best of 3: 742 ms per loop
1000000 loops, best of 3: 1.19 µs per loop
1
2
3
Python 3.4.1
1 loops, best of 3: 742 ms per loop
1000000 loops, best of 3: 1.19 µs per loop
根据上面的timeit的结果，查找整数比查找浮点数要快大约6万倍。但由于Python 2.x中的range或xrange没有contains方法，所以在Python 2中的整数和浮点数的查找速度差别不大。
Python
print ‘Python’, python_version()
assert(val_in_xrange(x, x/2.0) == True)
assert(val_in_xrange(x, x/2) == True)
assert(val_in_range(x, x/2) == True)
assert(val_in_range(x, x//2) == True)
%timeit val_in_xrange(x, x/2.0)
%timeit val_in_xrange(x, x/2)
%timeit val_in_range(x, x/2.0)
%timeit val_in_range(x, x/2)
1
2
3
4
5
6
7
8
9
10
11
12
13
print ‘Python’, python_version()
assert(val_in_xrange(x, x/2.0) == True)
assert(val_in_xrange(x, x/2) == True)
assert(val_in_range(x, x/2) == True)
assert(val_in_range(x, x//2) == True)
%timeit val_in_xrange(x, x/2.0)
%timeit val_in_xrange(x, x/2)
%timeit val_in_range(x, x/2.0)
%timeit val_in_range(x, x/2)
Python
Python 2.7.7
1 loops, best of 3: 285 ms per loop
1 loops, best of 3: 179 ms per loop
1 loops, best of 3: 658 ms per loop
1 loops, best of 3: 556 ms per loop
1
2
3
4
5
Python 2.7.7
1 loops, best of 3: 285 ms per loop
1 loops, best of 3: 179 ms per loop
1 loops, best of 3: 658 ms per loop
1 loops, best of 3: 556 ms per loop
下面的代码证明了Python 2.x中没有contain方法：
Python
print(‘Python’, python_version())
range.contains
1
2
print(‘Python’, python_version())
range.contains
Python
Python 3.4.1
Python 2.7.7
AttributeError Traceback (most recent call last)in ()
1 print ‘Python’, python_version()
—-> 2 range.contains
AttributeError: ‘builtin_function_or_method’ object has no attribute ‘contains’
1
2
3
4
5
6
7
8
Python 2.7.7
AttributeError Traceback (most recent call last)in ()
1 print ‘Python’, python_version()
—-> 2 range.contains
AttributeError: ‘builtin_function_or_method’ object has no attribute ‘contains’
Python
print(‘Python’, python_version())
xrange.contains
1
2
print(‘Python’, python_version())
xrange.contains
Python
Python 2.7.7
AttributeError Traceback (most recent call last)
in ()
1 print ‘Python’, python_version()
—-> 2 xrange.contains
AttributeError: type object ‘xrange’ has no attribute ‘contains’
1
2
3
4
5
6
7
8
9
Python 2.7.7
AttributeError Traceback (most recent call last)
in ()
1 print ‘Python’, python_version()
—-> 2 xrange.contains
AttributeError: type object ‘xrange’ has no attribute ‘contains’
关于Python 2中xrange()与Python 3中range()之间的速度差异的一点说明：
有读者指出了Python 3中的range()和Python 2中xrange()执行速度有差异。由于这两者的实现方式相同，因此理论上执行速度应该也是相同的。这里的速度差别仅仅是因为Python 3的总体速度就比Python 2慢。
Python
def test_while():
i = 0
while i < 20000:
i += 1
return
1
2
3
4
5
def test_while():
i = 0
while i < 20000:
i += 1
return
Python
print(‘Python’, python_version())
%timeit test_while()
1
2
print(‘Python’, python_version())
%timeit test_while()
Python
Python 3.4.1
%timeit test_while()
100 loops, best of 3: 2.68 ms per loop
1
2
3
4
Python 3.4.1
%timeit test_while()
100 loops, best of 3: 2.68 ms per loop
Python
print ‘Python’, python_version()
%timeit test_while()
1
2
print ‘Python’, python_version()
%timeit test_while()
Python
Python 2.7.6
1000 loops, best of 3: 1.72 ms per loop
1
2
Python 2.7.6
1000 loops, best of 3: 1.72 ms per loop
触发异常
[回到目录]
Python 2支持新旧两种异常触发语法，而Python 3只接受带括号的的语法（不然会触发SyntaxError）：
Python 2
Python
print ‘Python’, python_version()
1
print ‘Python’, python_version()
Python
Python 2.7.6
1
Python 2.7.6
Python
raise IOError, “file error”
1
raise IOError, “file error”
Python
IOError Traceback (most recent call last)
in ()
—-> 1 raise IOError, “file error”
IOError: file error
1
2
3
4
5
6
IOError Traceback (most recent call last)in ()
—-> 1 raise IOError, “file error”
IOError: file error
Python
raise IOError(“file error”)
1
raise IOError(“file error”)
Python
IOError Traceback (most recent call last)
in ()
—-> 1 raise IOError(“file error”)
IOError: file error
1
2
3
4
5
6
IOError Traceback (most recent call last)in ()
—-> 1 raise IOError(“file error”)
IOError: file error
Python 3
Python
print(‘Python’, python_version())
1
print(‘Python’, python_version())
Python
Python 3.4.1
1
Python 3.4.1
Python
raise IOError, “file error”
1
raise IOError, “file error”
Python
File “”, line 1
raise IOError, “file error”
^
SyntaxError: invalid syntax
The proper way to raise an exception in Python 3:
1
2
3
4
5
File “”, line 1
raise IOError, “file error”
^
SyntaxError: invalid syntax
The proper way to raise an exception in Python 3:
Python
print(‘Python’, python_version())
raise IOError(“file error”)
1
2
print(‘Python’, python_version())
raise IOError(“file error”)
Python
Python 3.4.1
OSError Traceback (most recent call last)
in ()
1 print(‘Python’, python_version())
—-> 2 raise IOError(“file error”)
OSError: file error
1
2
3
4
5
6
7
8
9
Python 3.4.1
OSError Traceback (most recent call last)
in ()
1 print(‘Python’, python_version())
—-> 2 raise IOError(“file error”)
OSError: file error
异常处理
[回到目录]
Python 3中的异常处理也发生了一点变化。在Python 3中必须使用“as”关键字。
Python 2
Python
print ‘Python’, python_version()
try:
let_us_cause_a_NameError
except NameError, err:
print err, ‘–> our error message’
1
2
3
4
5
print ‘Python’, python_version()
try:
let_us_cause_a_NameError
except NameError, err:
print err, ‘–> our error message’
Python
Python 2.7.6
name ‘let_us_cause_a_NameError’ is not defined –> our error message
1
2
Python 2.7.6
name ‘let_us_cause_a_NameError’ is not defined –> our error message
Python 3
Python
print(‘Python’, python_version())
try:
let_us_cause_a_NameError
except NameError as err:
print(err, ‘–> our error message’)
1
2
3
4
5
print(‘Python’, python_version())
try:
let_us_cause_a_NameError
except NameError as err:
print(err, ‘–> our error message’)
Python
Python 3.4.1
name ‘let_us_cause_a_NameError’ is not defined –> our error message
1
2
Python 3.4.1
name ‘let_us_cause_a_NameError’ is not defined –> our error message
next()函数和.next()方法
[回到目录]
由于会经常用到next()（.next()）函数（方法），所以还要提到另一个语法改动（实现方面也做了改动）：在Python 2.7.5中，函数形式和方法形式都可以使用，而在Python 3中，只能使用next()函数（试图调用.next()方法会触发AttributeError）。
Python 2
Python
print ‘Python’, python_version()
my_generator = (letter for letter in ‘abcdefg’)
next(my_generator)
my_generator.next()
1
2
3
4
print ‘Python’, python_version()
my_generator = (letter for letter in ‘abcdefg’)
next(my_generator)
my_generator.next()
Python
Python 2.7.6
‘b’
1
2
Python 2.7.6
‘b’
Python 3
Python
print(‘Python’, python_version())
my_generator = (letter for letter in ‘abcdefg’)
next(my_generator)
1
2
3
print(‘Python’, python_version())
my_generator = (letter for letter in ‘abcdefg’)
next(my_generator)
Python
Python 3.4.1
‘a’
1
2
Python 3.4.1
‘a’
Python
my_generator.next()
1
my_generator.next()
Python
AttributeError Traceback (most recent call last)
in ()
—-> 1 my_generator.next()
AttributeError: ‘generator’ object has no attribute ‘next’
1
2
3
4
5
6
AttributeError Traceback (most recent call last)in ()
—-> 1 my_generator.next()
AttributeError: ‘generator’ object has no attribute ‘next’
For循环变量与全局命名空间泄漏
[回到目录]
好消息是：在Python 3.x中，for循环中的变量不再会泄漏到全局命名空间中了！
这是Python 3.x中做的一个改动，在“What’s New In Python 3.0”中有如下描述：
“列表推导不再支持[… for var in item1, item2, …]这样的语法，使用[… for var in (item1, item2, …)]代替。还要注意列表推导有不同的语义：现在列表推导更接近list()构造器中的生成器表达式这样的语法糖，特别要注意的是，循环控制变量不会再泄漏到循环周围的空间中了。”
Python 2
Python
print ‘Python’, python_version()
i = 1
print ‘before: i =’, i
print ‘comprehension: ‘, [i for i in range(5)]
print ‘after: i =’, i
1
2
3
4
5
6
7
8
print ‘Python’, python_version()
i = 1
print ‘before: i =’, i
print ‘comprehension: ‘, [i for i in range(5)]
print ‘after: i =’, i
Python
Python 2.7.6
before: i = 1
comprehension: [0, 1, 2, 3, 4]
after: i = 4
1
2
3
4
Python 2.7.6
before: i = 1
comprehension: [0, 1, 2, 3, 4]
after: i = 4
Python 3
Python
print(‘Python’, python_version())
i = 1
print(‘before: i =’, i)
print(‘comprehension:’, [i for i in range(5)])
print(‘after: i =’, i)
1
2
3
4
5
6
7
8
print(‘Python’, python_version())
i = 1
print(‘before: i =’, i)
print(‘comprehension:’, [i for i in range(5)])
print(‘after: i =’, i)
Python
Python 3.4.1
before: i = 1
comprehension: [0, 1, 2, 3, 4]
after: i = 1
1
2
3
4
Python 3.4.1
before: i = 1
comprehension: [0, 1, 2, 3, 4]
after: i = 1
比较无序类型
[回到目录]
Python 3中另一个优秀的改动是，如果我们试图比较无序类型，会触发一个TypeError。
Python 2
Python
print ‘Python’, python_version()
print “[1, 2] > ‘foo’ = “, [1, 2] > ‘foo’
print “(1, 2) > ‘foo’ = “, (1, 2) > ‘foo’
print “[1, 2] > (1, 2) = “, [1, 2] > (1, 2)
1
2
3
4
print ‘Python’, python_version()
print “[1, 2] > ‘foo’ = “, [1, 2] > ‘foo’
print “(1, 2) > ‘foo’ = “, (1, 2) > ‘foo’
print “[1, 2] > (1, 2) = “, [1, 2] > (1, 2)
Python
Python 2.7.6
[1, 2] > ‘foo’ = False
(1, 2) > ‘foo’ = True
[1, 2] > (1, 2) = False
1
2
3
4
Python 2.7.6
[1, 2] > ‘foo’ = False
(1, 2) > ‘foo’ = True
[1, 2] > (1, 2) = False
Python 3
Python
print(‘Python’, python_version())
print(“[1, 2] > ‘foo’ = “, [1, 2] > ‘foo’)
print(“(1, 2) > ‘foo’ = “, (1, 2) > ‘foo’)
print(“[1, 2] > (1, 2) = “, [1, 2] > (1, 2))
1
2
3
4
print(‘Python’, python_version())
print(“[1, 2] > ‘foo’ = “, [1, 2] > ‘foo’)
print(“(1, 2) > ‘foo’ = “, (1, 2) > ‘foo’)
print(“[1, 2] > (1, 2) = “, [1, 2] > (1, 2))
Python
Python 3.4.1
TypeError Traceback (most recent call last)in ()
1 print(‘Python’, python_version())
—-> 2 print(“[1, 2] > ‘foo’ = “, [1, 2] > ‘foo’)
3 print(“(1, 2) > ‘foo’ = “, (1, 2) > ‘foo’)
4 print(“[1, 2] > (1, 2) = “, [1, 2] > (1, 2))
TypeError: unorderable types: list() > str()
1
2
3
4
5
6
7
8
9
Python 3.4.1
TypeError Traceback (most recent call last)
<ipython-input-16-a9031729f4a0> in <module>()
print('Python', python_version())
----> 2 print("[1, 2] > 'foo' = ", [1, 2] > 'foo')
print("(1, 2) > 'foo' = ", (1, 2) > 'foo')
print("[1, 2] > (1, 2) = ", [1, 2] > (1, 2))
TypeError: unorderable types: list() > str()通过input()解析用户的输入
[回到目录]
幸运的是，Python 3改进了input()函数，这样该函数就会总是将用户的输入存储为str对象。在Python 2中，为了避免读取非字符串类型会发生的一些危险行为，不得不使用raw_input()代替input()。
Python 2
Python
Python 2.7.6
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type “help”, “copyright”, “credits” or “license” for more information.
>>> my_input = input('enter a number: ')enter a number: 123
type(my_input)
内容来自用户分享和网络整理，不保证内容的准确性，如有侵权内容，可联系管理员处理 
相关文章推荐
- Python动态类型的学习---引用的理解
- Python3写爬虫（四）多线程实现数据爬取
- 垃圾邮件过滤器 python简单实现
- 下载并遍历 names.txt 文件，输出长度最长的回文人名。
- install and upgrade scrapy
- Scrapy的架构介绍
- Centos6 编译安装Python
- 使用Python生成Excel格式的图片
- 让Python文件也可以当bat文件运行
- [Python]推算数独
- Python中zip()函数用法举例
- Python中map()函数浅析
- Python将excel导入到mysql中
- Python在CAM软件Genesis2000中的应用
- 使用Shiboken为C++和Qt库创建Python绑定
- FREEBASIC 编译可被python调用的dll函数示例
- Python 七步捉虫法