Python 2.7.x 和 3.x 版本的重要区别

本文转载自：http://www.techug.com/the-difference-of-python2-and-python3

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。
<h2>__future__模块</h2>
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。
<pre class="lang:default decode:true ">from __future__ import division</pre>
下表列出了__future__中其他可导入的特性：
<table border="1" cellpadding="10">
<tbody>
<tr>
<th>特性</th>
<th>可选版本</th>
<th>强制版本</th>
<th>效果</th>
</tr>
<tr>
<td>nested_scopes</td>
<td>2.1.0b1</td>
<td>2.2</td>
<td><a href="http://www.python.org/dev/peps/pep-0227" target="_blank" rel="nofollow"><strong>PEP 227</strong></a>:<em>Statically Nested Scopes</em></td>
</tr>
<tr>
<td>generators</td>
<td>2.2.0a1</td>
<td>2.3</td>
<td><a href="http://www.python.org/dev/peps/pep-0255" target="_blank" rel="nofollow"><strong>PEP 255</strong></a>:<em>Simple Generators</em></td>
</tr>
<tr>
<td>division</td>
<td>2.2.0a2</td>
<td>3.0</td>
<td><a href="http://www.python.org/dev/peps/pep-0238" target="_blank" rel="nofollow"><strong>PEP 238</strong></a>:<em>Changing the Division Operator</em></td>
</tr>
<tr>
<td>absolute_import</td>
<td>2.5.0a1</td>
<td>3.0</td>
<td><a href="http://www.python.org/dev/peps/pep-0328" target="_blank" rel="nofollow"><strong>PEP 328</strong></a>:<em>Imports: Multi-Line and Absolute/Relative</em></td>
</tr>
<tr>
<td>with_statement</td>
<td>2.5.0a1</td>
<td>2.6</td>
<td><a href="http://www.python.org/dev/peps/pep-0343" target="_blank" rel="nofollow"><strong>PEP 343</strong></a>:<em>The “with” Statement</em></td>
</tr>
<tr>
<td>print_function</td>
<td>2.6.0a2</td>
<td>3.0</td>
<td><a href="http://www.python.org/dev/peps/pep-3105" target="_blank" rel="nofollow"><strong>PEP 3105</strong></a>:<em>Make print a function</em></td>
</tr>
<tr>
<td>unicode_literals</td>
<td>2.6.0a2</td>
<td>3.0</td>
<td><a href="http://www.python.org/dev/peps/pep-3112" target="_blank" rel="nofollow"><strong>PEP 3112</strong></a>:<em>Bytes literals in Python 3000</em></td>
</tr>
</tbody>
</table>
（来源： <a href="https://docs.python.org/2/library/__future__.html#module-__future__" target="_blank" rel="nofollow">https://docs.python.org/2/library/<strong>future</strong>.html</a>)

<strong>示例：</strong>
<pre class="lang:default decode:true ">from platform import python_version</pre>
<h2>print函数</h2>
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。

<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()
print 'Hello, World!'
print('Hello, World!')
print "text", ; print 'print more text on the same line'</pre>
<pre class="lang:default decode:true ">Python 2.7.6
Hello, World!
Hello, World!
text print more text on the same line</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())
print('Hello, World!')

print("some text,", end="") 
print(' print more text on the same line')</pre>
<pre class="lang:default decode:true ">Python 3.4.1
Hello, World!
some text, print more text on the same line</pre>
<pre class="lang:default decode:true ">print 'Hello, World!'</pre>
<pre class="lang:default decode:true ">File "<ipython-input-3-139a7c5835bd>", line 1
print 'Hello, World!'
^
SyntaxError: invalid syntax</pre>
<strong>注意：</strong>

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。
<pre class="lang:default decode:true ">print 'Python', python_version()
print('a', 'b')
print 'a', 'b'</pre>
<pre class="lang:default decode:true ">Python 2.7.7
('a', 'b')
a b</pre>
<h2>整数除法</h2>
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）。

<strong>Python 2</strong>
<pre class="lang:default decode:true ">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</pre>
<pre class="lang:default decode:true ">Python 2.7.6
3 / 2 = 1
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">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)</pre>
<pre class="lang:default decode:true ">Python 3.4.1
3 / 2 = 1.5
3 // 2 = 1
3 / 2.0 = 1.5
3 // 2.0 = 1.0</pre>
<h2>Unicode</h2>
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。

<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()
Python 2.7.6
print type(unicode('this is like a python3 str type'))
<type 'unicode'>
print type(b'byte type does not exist')
<type 'str'>
print 'they are really' + b' the same'
they are really the same
print type(bytearray(b'bytearray oddly does exist though'))
<type 'bytearray'></pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())
print('strings are now utf-8 u03BCnicou0394é!')
Python 3.4.1
strings are now utf-8 μnicoΔé!
print('Python', python_version(), end="")
print(' has', type(b' bytes for storing data'))
Python 3.4.1 has <class 'bytes'>
print('and Python', python_version(), end="")
print(' also has', type(bytearray(b'bytearrays')))
and Python 3.4.1 also has <class 'bytearray'>
'note that we cannot add a string' + b'bytes for data'
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-13-d3e8942ccf81> in <module>()
----> 1 'note that we cannot add a string' + b'bytes for data'

TypeError: Can't convert 'bytes' object to str implicitly</pre>
<h2>xrange</h2>
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）。
<pre class="lang:default decode:true ">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</pre>
<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()

print 'ntiming range()'
%timeit test_range(n)

print 'nntiming xrange()'
%timeit test_xrange(n)</pre>
<pre class="lang:default decode:true ">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</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())

print('ntiming range()')
%timeit test_range(n)
Python 3.4.1

timing range()
1000 loops, best of 3: 520 µs per loop
print(xrange(10))
---------------------------------------------------------------------------
NameError Traceback (most recent call last)
in ()
----> 1 print(xrange(10))

NameError: name 'xrange' is not defined</pre>
<h2>Python 3中的range对象中的__contains__方法</h2>
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。
<pre class="lang:default decode:true ">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 3.4.1
1 loops, best of 3: 742 ms per loop
1000000 loops, best of 3: 1.19 µs per loop</pre>
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。
<pre class="lang:default decode:true ">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 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</pre>
下面的代码证明了Python 2.x中没有__contain__方法：
<pre class="lang:default decode:true ">print('Python', python_version())
range.__contains__
Python 3.4.1
<slot wrapper '__contains__' of 'range' objects
print('Python', python_version())
range.__contains__
Python 2.7.7
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-7-05327350dafb> in <module>()
1 print 'Python', python_version()
----> 2 range.__contains__

AttributeError: 'builtin_function_or_method' object has no attribute '__contains__'
print('Python', python_version())
xrange.__contains__
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__'</pre>
<strong>关于Python 2中xrange()与Python 3中range()之间的速度差异的一点说明：</strong>

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。
<pre class="lang:default decode:true ">def test_while():
    i = 0
    while i < 20000:
        i += 1
    return
print('Python', python_version())
%timeit test_while()
Python 3.4.1
%timeit test_while()
100 loops, best of 3: 2.68 ms per loop
print 'Python', python_version()
%timeit test_while()
Python 2.7.6
1000 loops, best of 3: 1.72 ms per loop</pre>
<h2>触发异常</h2>
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）：

<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()
Python 2.7.6
raise IOError, "file error"
---------------------------------------------------------------------------
IOError Traceback (most recent call last)
<ipython-input-8-25f049caebb0> in <module>()
----> 1 raise IOError, "file error"

IOError: file error
raise IOError("file error")
---------------------------------------------------------------------------
IOError Traceback (most recent call last)
<ipython-input-9-6f1c43f525b2> in <module>()
----> 1 raise IOError("file error")

IOError: file error</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())
Python 3.4.1
raise IOError, "file error"
File "<ipython-input-10-25f049caebb0>", line 1
raise IOError, "file error"
^
SyntaxError: invalid syntax
The proper way to raise an exception in Python 3:
print('Python', python_version())
raise IOError("file error")
Python 3.4.1

---------------------------------------------------------------------------
OSError Traceback (most recent call last)
<ipython-input-11-c350544d15da> in <module>()
1 print('Python', python_version())
----> 2 raise IOError("file error")

OSError: file error</pre>
<h2>异常处理</h2>
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。

<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()
try:
    let_us_cause_a_NameError
except NameError, err:
    print err, '--> our error message'
Python 2.7.6
name 'let_us_cause_a_NameError' is not defined --> our error message</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())
try:
    let_us_cause_a_NameError
except NameError as err:
    print(err, '--> our error message')
Python 3.4.1
name 'let_us_cause_a_NameError' is not defined --> our error message</pre>
<h2>next()函数和.next()方法</h2>
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）。

<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()
my_generator = (letter for letter in 'abcdefg')
next(my_generator)
my_generator.next()
Python 2.7.6
'b'</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())
my_generator = (letter for letter in 'abcdefg')
next(my_generator)
Python 3.4.1
'a'
my_generator.next()
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-14-125f388bb61b> in <module>()
----> 1 my_generator.next()

AttributeError: 'generator' object has no attribute 'next'</pre>
<h2>For循环变量与全局命名空间泄漏</h2>
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。”

<strong>Python 2</strong>
<pre class="lang:default decode:true">print 'Python', python_version()

i = 1
print 'before: i =', i

print 'comprehension: ', [i for i in range(5)]

print 'after: i =', i
Python 2.7.6
before: i = 1
comprehension: [0, 1, 2, 3, 4]
after: i = 4</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())

i = 1
print('before: i =', i)

print('comprehension:', [i for i in range(5)])

print('after: i =', i)
Python 3.4.1
before: i = 1
comprehension: [0, 1, 2, 3, 4]
after: i = 1</pre>
<h2>比较无序类型</h2>
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。

<strong>Python 2</strong>
<pre class="lang:default decode:true ">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 2.7.6
[1, 2] > 'foo' = False
(1, 2) > 'foo' = True
[1, 2] > (1, 2) = False</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">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 3.4.1
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-16-a9031729f4a0> in <module>()
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()</pre>
<h2>通过input()解析用户的输入</h2>
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()。

<strong>Python 2</strong>
<pre class="lang:default decode:true">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)
<type 'int'>

>>> my_input = raw_input('enter a number: ')

enter a number: 123

>>> type(my_input)
<type 'str'></pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">Python 3.4.1
[GCC 4.2.1 (Apple Inc. build 5577)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

>>> my_input = input('enter a number: ')
enter a number: 123
>>> type(my_input)
<class 'str'></pre>
返回可迭代对象，而不是列表

在xrange一节中可以看到，某些函数和方法在Python中返回的是可迭代对象，而不像在Python 2中返回列表。

由于通常对这些对象只遍历一次，所以这种方式会节省很多内存。然而，如果通过生成器来多次迭代这些对象，效率就不高了。

此时我们的确需要列表对象，可以通过list()函数简单的将可迭代对象转成列表。

<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()

print range(3)
print type(range(3))
Python 2.7.6
[0, 1, 2]
<type 'list'></pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())
print(range(3))
print(type(range(3)))
print(list(range(3)))
Python 3.4.1
range(0, 3)
<class 'range'>
[0, 1, 2]</pre>
下面列出了Python 3中其他不再返回列表的常用函数和方法：
<ol>
	<li>zip()</li>
	<li>map()</li>
	<li>filter()</li>
	<li>字典的.key()方法</li>
	<li>字典的.value()方法</li>
	<li>字典的.item()方法</li>
</ol>
<h2>更多关于Python 2和Python 3的文章</h2>
下面列出了其他一些可以进一步了解Python 2和Python 3的优秀文章，

//迁移到 Python 3
<ol>
	<li><a title="Should I use Python 2 or Python 3 for my development activity?" href="https://wiki.python.org/moin/Python2orPython3" target="_blank" rel="nofollow">Should I use Python 2 or Python 3 for my development activity?</a></li>
	<li><a title="What’s New In Python 3.0" href="https://docs.python.org/3.0/whatsnew/3.0.html" target="_blank" rel="nofollow">What’s New In Python 3.0</a></li>
	<li><a title="Porting to Python 3" href="http://python3porting.com/differences.html" target="_blank" rel="nofollow">Porting to Python 3</a></li>
	<li><a title="Porting Python 2 Code to Python 3" href="https://docs.python.org/3/howto/pyporting.html" target="_blank" rel="nofollow">Porting Python 2 Code to Python 3</a></li>
	<li><a title="How keep Python 3 moving forward" href="http://nothingbutsnark.svbtle.com/my-view-on-the-current-state-of-python-3" target="_blank" rel="nofollow">How keep Python 3 moving forward</a></li>
</ol>

本文转载自：http://www.techug.com/the-difference-of-python2-and-python3

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。

print("some text,", end="") 
print(' print more text on the same line')</pre>
<pre class="lang:default decode:true ">Python 3.4.1
Hello, World!
some text, print more text on the same line</pre>
<pre class="lang:default decode:true ">print 'Hello, World!'</pre>
<pre class="lang:default decode:true ">File "&lt;ipython-input-3-139a7c5835bd&gt;", line 1
print 'Hello, World!'
^
SyntaxError: invalid syntax</pre>
<strong>注意：</strong>

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。

<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()
Python 2.7.6
print type(unicode('this is like a python3 str type'))
&lt;type 'unicode'&gt;
print type(b'byte type does not exist')
&lt;type 'str'&gt;
print 'they are really' + b' the same'
they are really the same
print type(bytearray(b'bytearray oddly does exist though'))
&lt;type 'bytearray'&gt;</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())
print('strings are now utf-8 u03BCnicou0394é!')
Python 3.4.1
strings are now utf-8 μnicoΔé!
print('Python', python_version(), end="")
print(' has', type(b' bytes for storing data'))
Python 3.4.1 has &lt;class 'bytes'&gt;
print('and Python', python_version(), end="")
print(' also has', type(bytearray(b'bytearrays')))
and Python 3.4.1 also has &lt;class 'bytearray'&gt;
'note that we cannot add a string' + b'bytes for data'
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
&lt;ipython-input-13-d3e8942ccf81&gt; in &lt;module&gt;()
----&gt; 1 'note that we cannot add a string' + b'bytes for data'

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。

n = 10000
def test_range(n):
    return for i in range(n):
    pass

def test_xrange(n):
    for i in xrange(n):
    pass</pre>
<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()

print 'ntiming range()'
%timeit test_range(n)

print 'nntiming xrange()'
%timeit test_xrange(n)</pre>
<pre class="lang:default decode:true ">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</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())

print('ntiming range()')
%timeit test_range(n)
Python 3.4.1

def val_in_xrange(x, val):
    return val in xrange(x)

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 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</pre>
下面的代码证明了Python 2.x中没有__contain__方法：
<pre class="lang:default decode:true ">print('Python', python_version())
range.__contains__
Python 3.4.1
&lt;slot wrapper '__contains__' of 'range' objects
print('Python', python_version())
range.__contains__
Python 2.7.7
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
&lt;ipython-input-7-05327350dafb&gt; in &lt;module&gt;()
1 print 'Python', python_version()
----&gt; 2 range.__contains__

AttributeError: 'builtin_function_or_method' object has no attribute '__contains__'
print('Python', python_version())
xrange.__contains__
Python 2.7.7

---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
in ()
1 print 'Python', python_version()
----&gt; 2 xrange.__contains__

AttributeError: type object 'xrange' has no attribute '__contains__'</pre>
<strong>关于Python 2中xrange()与Python 3中range()之间的速度差异的一点说明：</strong>

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。
<pre class="lang:default decode:true ">def test_while():
    i = 0
    while i &lt; 20000:
        i += 1
    return
print('Python', python_version())
%timeit test_while()
Python 3.4.1
%timeit test_while()
100 loops, best of 3: 2.68 ms per loop
print 'Python', python_version()
%timeit test_while()
Python 2.7.6
1000 loops, best of 3: 1.72 ms per loop</pre>
<h2>触发异常</h2>
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）：

<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()
Python 2.7.6
raise IOError, "file error"
---------------------------------------------------------------------------
IOError Traceback (most recent call last)
&lt;ipython-input-8-25f049caebb0&gt; in &lt;module&gt;()
----&gt; 1 raise IOError, "file error"

IOError: file error
raise IOError("file error")
---------------------------------------------------------------------------
IOError Traceback (most recent call last)
&lt;ipython-input-9-6f1c43f525b2&gt; in &lt;module&gt;()
----&gt; 1 raise IOError("file error")

IOError: file error</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())
Python 3.4.1
raise IOError, "file error"
File "&lt;ipython-input-10-25f049caebb0&gt;", line 1
raise IOError, "file error"
^
SyntaxError: invalid syntax
The proper way to raise an exception in Python 3:
print('Python', python_version())
raise IOError("file error")
Python 3.4.1

---------------------------------------------------------------------------
OSError Traceback (most recent call last)
&lt;ipython-input-11-c350544d15da&gt; in &lt;module&gt;()
1 print('Python', python_version())
----&gt; 2 raise IOError("file error")

OSError: file error</pre>
<h2>异常处理</h2>
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。

<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()
try:
    let_us_cause_a_NameError
except NameError, err:
    print err, '--&gt; our error message'
Python 2.7.6
name 'let_us_cause_a_NameError' is not defined --&gt; our error message</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())
try:
    let_us_cause_a_NameError
except NameError as err:
    print(err, '--&gt; our error message')
Python 3.4.1
name 'let_us_cause_a_NameError' is not defined --&gt; our error message</pre>
<h2>next()函数和.next()方法</h2>
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）。

<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()
my_generator = (letter for letter in 'abcdefg')
next(my_generator)
my_generator.next()
Python 2.7.6
'b'</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())
my_generator = (letter for letter in 'abcdefg')
next(my_generator)
Python 3.4.1
'a'
my_generator.next()
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
&lt;ipython-input-14-125f388bb61b&gt; in &lt;module&gt;()
----&gt; 1 my_generator.next()

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：

<strong>Python 2</strong>
<pre class="lang:default decode:true">print 'Python', python_version()

i = 1
print 'before: i =', i

print 'comprehension: ', [i for i in range(5)]

print 'after: i =', i
Python 2.7.6
before: i = 1
comprehension: [0, 1, 2, 3, 4]
after: i = 4</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())

i = 1
print('before: i =', i)

print('comprehension:', [i for i in range(5)])

<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()
print "[1, 2] &gt; 'foo' = ", [1, 2] &gt; 'foo'
print "(1, 2) &gt; 'foo' = ", (1, 2) &gt; 'foo'
print "[1, 2] &gt; (1, 2) = ", [1, 2] &gt; (1, 2)
Python 2.7.6
[1, 2] &gt; 'foo' = False
(1, 2) &gt; 'foo' = True
[1, 2] &gt; (1, 2) = False</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())
print("[1, 2] &gt; 'foo' = ", [1, 2] &gt; 'foo')
print("(1, 2) &gt; 'foo' = ", (1, 2) &gt; 'foo')
print("[1, 2] &gt; (1, 2) = ", [1, 2] &gt; (1, 2))
Python 3.4.1
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
&lt;ipython-input-16-a9031729f4a0&gt; in &lt;module&gt;()
1 print('Python', python_version())
----&gt; 2 print("[1, 2] &gt; 'foo' = ", [1, 2] &gt; 'foo')
3 print("(1, 2) &gt; 'foo' = ", (1, 2) &gt; 'foo')
4 print("[1, 2] &gt; (1, 2) = ", [1, 2] &gt; (1, 2))
TypeError: unorderable types: list() &gt; str()</pre>
<h2>通过input()解析用户的输入</h2>
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()。

<strong>Python 2</strong>
<pre class="lang:default decode:true">Python 2.7.6
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

&gt;&gt;&gt; my_input = input('enter a number: ')

enter a number: 123

&gt;&gt;&gt; type(my_input)
&lt;type 'int'&gt;

&gt;&gt;&gt; my_input = raw_input('enter a number: ')

enter a number: 123

&gt;&gt;&gt; type(my_input)
&lt;type 'str'&gt;</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">Python 3.4.1
[GCC 4.2.1 (Apple Inc. build 5577)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

&gt;&gt;&gt; my_input = input('enter a number: ')
enter a number: 123
&gt;&gt;&gt; type(my_input)
&lt;class 'str'&gt;</pre>
返回可迭代对象，而不是列表

在xrange一节中可以看到，某些函数和方法在Python中返回的是可迭代对象，而不像在Python 2中返回列表。

由于通常对这些对象只遍历一次，所以这种方式会节省很多内存。然而，如果通过生成器来多次迭代这些对象，效率就不高了。

此时我们的确需要列表对象，可以通过list()函数简单的将可迭代对象转成列表。

<strong>Python 2</strong>
<pre class="lang:default decode:true ">print 'Python', python_version()

print range(3)
print type(range(3))
Python 2.7.6
[0, 1, 2]
&lt;type 'list'&gt;</pre>
<strong>Python 3</strong>
<pre class="lang:default decode:true ">print('Python', python_version())
print(range(3))
print(type(range(3)))
print(list(range(3)))
Python 3.4.1
range(0, 3)
&lt;class 'range'&gt;
[0, 1, 2]</pre>
下面列出了Python 3中其他不再返回列表的常用函数和方法：
<ol>
	<li>zip()</li>
	<li>map()</li>
	<li>filter()</li>
	<li>字典的.key()方法</li>
	<li>字典的.value()方法</li>
	<li>字典的.item()方法</li>
</ol>
<h2>更多关于Python 2和Python 3的文章</h2>
下面列出了其他一些可以进一步了解Python 2和Python 3的优秀文章，