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Python 2.7.x 和 3.x 版本的重要区别

2017-01-10 13:35 435 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。

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 "<ipython-input-3-139a7c5835bd>", line 1
print 'Hello, World!'
^
SyntaxError: invalid syntax

1
2
3
4

File
"<ipython-input-3-139a7c5835bd>",
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

<type 'unicode'>

1	<type 'unicode'>

Python

print type(b'byte type does not exist')

1	print type(b'byte type does not exist')

Python

<type 'str'>

1	<type 'str'>

Python

print 'they are really' + b' the same'

1	print 'they are really' + b' the same'

Python

they are really the same

1	they are really the same

Python

print type(bytearray(b'bytearray oddly does exist though'))

1	print type(bytearray(b'bytearray oddly does exist though'))

Python

<type 'bytearray'>

1	<type 'bytearray'>

Python 3

Python

print('Python', python_version())
print('strings are now utf-8 u03BCnicou0394é!')

1 2	print('Python', python_version()) print('strings are now utf-8 u03BCnicou0394é!')

Python

Python 3.4.1
strings are now utf-8 μnicoΔé!

1 2	Python 3.4.1 strings are now utf-8 μnicoΔé!

Python

print('Python', python_version(), end="")
print(' has', type(b' bytes for storing data'))

1 2	print('Python', python_version(), end="") print(' has', type(b' bytes for storing data'))

Python

Python 3.4.1 has <class 'bytes'>

1	Python 3.4.1 has <class 'bytes'>

Python

print('and Python', python_version(), end="")
print(' also has', type(bytearray(b'bytearrays')))

1 2	print('and Python', python_version(), end="") print(' also has', type(bytearray(b'bytearrays')))

Python

and Python 3.4.1 also has <class 'bytearray'>

1	and Python 3.4.1 also has <class 'bytearray'>

Python

'note that we cannot add a string' + b'bytes for data'

1	'note that we cannot add a string' + b'bytes for data'

Python

---------------------------------------------------------------------------
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

1
2
3
4
5
6

---------------------------------------------------------------------------

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

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
<slot wrapper '__contains__' of 'range' objects

1 2	Python 3.4.1 <slot wrapper '__contains__' of 'range' objects

Python

print('Python', python_version())
range.__contains__

1 2	print('Python', python_version()) range.__contains__

Python

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__'

1
2
3
4
5
6
7
8

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__'

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)
<ipython-input-8-25f049caebb0> in <module>()
----> 1 raise IOError, "file error"

IOError: file error

1
2
3
4
5
6

---------------------------------------------------------------------------

IOError Traceback
(most recent
call last)
<ipython-input-8-25f049caebb0>
in <module>()

---->
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)
<ipython-input-9-6f1c43f525b2> in <module>()
----> 1 raise IOError("file error")

IOError: file error

1
2
3
4
5
6

---------------------------------------------------------------------------

IOError Traceback
(most recent
call last)
<ipython-input-9-6f1c43f525b2>
in <module>()

---->
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 "<ipython-input-10-25f049caebb0>", 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
"<ipython-input-10-25f049caebb0>",
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)
<ipython-input-11-c350544d15da> in <module>()
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)
<ipython-input-11-c350544d15da>
in <module>()

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)
<ipython-input-14-125f388bb61b> in <module>()
----> 1 my_generator.next()

AttributeError: 'generator' object has no attribute 'next'

1
2
3
4
5
6

---------------------------------------------------------------------------

AttributeError Traceback
(most recent
call last)
<ipython-input-14-125f388bb61b>
in <module>()

---->
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

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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

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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)
<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()

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>()
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()

通过input()解析用户的输入

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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)
<type 'int'>

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

enter a number: 123

>>> type(my_input)
<type 'str'>

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

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'>

Python 3

Python

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'>

1
2
3
4
5
6
7
8

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'>

返回可迭代对象，而不是列表

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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。

Python 2

Python

print 'Python', python_version()

print range(3)
print type(range(3))

1
2
3
4

print
'Python',
python_version()

print
range(3)

print type(range(3))

Python

Python 2.7.6
[0, 1, 2]
<type 'list'>

1
2
3

Python
2.7.6

[0,
1,
2]
<type
'list'>

Python 3

Python

print('Python', python_version())
print(range(3))
print(type(range(3)))
print(list(range(3)))

1
2
3
4

print('Python',
python_version())

print(range(3))
print(type(range(3)))

print(list(range(3)))

Python

Python 3.4.1
range(0, 3)
<class 'range'>
[0, 1, 2]

1
2
3
4

Python
3.4.1

range(0,
3)
<class
'range'>

[0,
1,
2]

下面列出了Python 3中其他不再返回列表的常用函数和方法：

zip()
map()
filter()
字典的.key()方法
字典的.value()方法
字典的.item()方法

Python 2.7.x 和 3.x 版本的重要区别

目录

__future__模块

print函数

整数除法

Unicode

xrange

Python 3中的range对象中的__contains__方法

触发异常

异常处理

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

通过input()解析用户的输入

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

Python 3中的range对象中的contains方法