Important differences between Python 2.7.x and 3.x versions

This article mainly introduces a summary of the important differences between Python 2.7.x and 3.x versions. Friends who need it can refer to it

Many Python beginners will ask: Which version of Python should I learn. For this question, my answer is usually "First choose a Python tutorial that suits you best. Whichever version of Python is used in the tutorial, you use that version. When you are almost done, you can study the differences between different versions."

But if you want to develop a new project in Python, how do you choose the Python version? I can responsibly say that most Python libraries support both Python 2.7.x and 3.x versions, so no matter which version you choose, it is fine. But in order to avoid some common pitfalls in certain versions when using Python, or when you need to port a Python project, it is still necessary to understand the main differences between the two common versions of Python.

Directory

• Use __future__ module

• print function

• Integer division

• ##Unicode

• xrange

• Trigger exception

• Handle exception

• next() function and .next() method

• For loop variable and global namespace leak

• Compare unordered types

• ##Use input() to parse input content

• Returns an iterable instead of a list

• More articles about Python 2 and Python 3

__future__ module

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Python 3.x introduces some keywords that are incompatible with Python 2 and features, which in Python 2 can be imported via the built-in __future__ module. If you want code written in a Python 2 environment to also run in Python 3.x, it is recommended to use the __future__ module. For example, if you want to have the integer division behavior of Python 3.x in Python 2, you can import the corresponding module with the following statement.

from __future__ import pision

The following table lists other importable features in __future__:

Featuresnested_scopes PEP 227##generators2.2.0a12.3:pision2.2.0a23.0:2.5 .0a1PEP 3282.5.0a1PEP 3432.6.0a2PEP 31052.6.0a2PEP 3112

Optional versions	Forced version	Effect
2.1.0b1	2.2		:Statically Nested Scopes
		PEP 255	Simple Generators
		PEP 238	Changing the pision Operator ##absolute_import
3.0		:	Imports: Multi-Line and Absolute/Relative with_statement
2.6		:	The “with” Statement print_function
3.0		:	Make print a function unicode_literals
3.0		:	Bytes literals in Python 3000 （来源： https://docs.python.org/2/library/future.html) 示例： 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 print &#39;Python&#39;, python_version() print &#39;Hello, World!&#39; print(&#39;Hello, World!&#39;) print "text", ; print &#39;print more text on the same line&#39; Python 2.7.6 Hello, World! Hello, World! text print more text on the same line Python 3 print(&#39;Python&#39;, python_version()) print(&#39;Hello, World!&#39;) print("some text,", end="") print(&#39; print more text on the same line&#39;) Python 3.4.1 Hello, World! some text, print more text on the same line print &#39;Hello, World!&#39; File "", line 1 print &#39;Hello, World!&#39; ^ 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。 print &#39;Python&#39;, python_version() print(&#39;a&#39;, &#39;b&#39;) print &#39;a&#39;, &#39;b&#39; Python 2.7.7 (&#39;a&#39;, &#39;b&#39;) 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 pision来使用Python 3的除法）。 Python 2 print &#39;Python&#39;, python_version() print &#39;3 / 2 =&#39;, 3 / 2 print &#39;3 // 2 =&#39;, 3 // 2 print &#39;3 / 2.0 =&#39;, 3 / 2.0 print &#39;3 // 2.0 =&#39;, 3 // 2.0 Python 2.7.6 3 / 2 = 1 3 // 2 = 1 3 / 2.0 = 1.5 3 // 2.0 = 1.0 Python 3 print(&#39;Python&#39;, python_version()) print(&#39;3 / 2 =&#39;, 3 / 2) print(&#39;3 // 2 =&#39;, 3 // 2) print(&#39;3 / 2.0 =&#39;, 3 / 2.0) print(&#39;3 // 2.0 =&#39;, 3 // 2.0) 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 print &#39;Python&#39;, python_version() Python 2.7.6 print type(unicode(&#39;this is like a python3 str type&#39;)) <type &#39;unicode&#39;> print type(b&#39;byte type does not exist&#39;) <type &#39;str&#39;> print &#39;they are really&#39; + b&#39; the same&#39; they are really the same print type(bytearray(b&#39;bytearray oddly does exist though&#39;)) <type &#39;bytearray&#39;> Python 3 print(&#39;Python&#39;, python_version()) print(&#39;strings are now utf-8 u03BCnicou0394é!&#39;) Python 3.4.1 strings are now utf-8 μnicoΔé! print(&#39;Python&#39;, python_version(), end="") print(&#39; has&#39;, type(b&#39; bytes for storing data&#39;)) Python 3.4.1 has <class &#39;bytes&#39;> print(&#39;and Python&#39;, python_version(), end="") print(&#39; also has&#39;, type(bytearray(b&#39;bytearrays&#39;))) and Python 3.4.1 also has <class &#39;bytearray&#39;> &#39;note that we cannot add a string&#39; + b&#39;bytes for data&#39; --------------------------------------------------------------------------- TypeError Traceback (most recent call last) in () ----> 1 &#39;note that we cannot add a string&#39; + b&#39;bytes for data&#39; 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）。 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 print &#39;Python&#39;, python_version() print 'ntiming range()' %timeit test_range(n) print 'nntiming xrange()' %timeit test_xrange(n) 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 print(&#39;Python&#39;, python_version()) print(&#39;ntiming range()&#39;) %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 &#39;xrange&#39; 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。 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(&#39;Python&#39;, 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 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。 print &#39;Python&#39;, 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 下面的代码证明了Python 2.x中没有__contain__方法： print(&#39;Python&#39;, python_version()) range.__contains__ Python 3.4.1 <slot wrapper &#39;__contains__&#39; of &#39;range&#39; objects print(&#39;Python&#39;, python_version()) range.__contains__ Python 2.7.7 --------------------------------------------------------------------------- AttributeError Traceback (most recent call last) in () 1 print &#39;Python&#39;, python_version() ----> 2 range.__contains__ AttributeError: 'builtin_function_or_method' object has no attribute '__contains__' print(&#39;Python&#39;, python_version()) xrange.__contains__ Python 2.7.7 --------------------------------------------------------------------------- AttributeError Traceback (most recent call last) in () 1 print &#39;Python&#39;, 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。 def test_while(): i = 0 while i < 20000: i += 1 return print(&#39;Python&#39;, python_version()) %timeit test_while() Python 3.4.1 %timeit test_while() 100 loops, best of 3: 2.68 ms per loop print &#39;Python&#39;, python_version() %timeit test_while() 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 print &#39;Python&#39;, python_version() Python 2.7.6 raise IOError, "file error" --------------------------------------------------------------------------- IOError Traceback (most recent call last) in () ----> 1 raise IOError, "file error" IOError: file error raise IOError("file error") --------------------------------------------------------------------------- IOError Traceback (most recent call last) in () ----> 1 raise IOError("file error") IOError: file error Python 3 print(&#39;Python&#39;, python_version()) Python 3.4.1 raise IOError, "file error" File "", line 1 raise IOError, "file error" ^ SyntaxError: invalid syntax The proper way to raise an exception in Python 3: print(&#39;Python&#39;, python_version()) raise IOError("file error") Python 3.4.1 --------------------------------------------------------------------------- OSError Traceback (most recent call last) in () 1 print(&#39;Python&#39;, 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 print &#39;Python&#39;, python_version() try: let_us_cause_a_NameError except NameError, err: print err, '--> our error message' Python 2.7.6 name &#39;let_us_cause_a_NameError&#39; is not defined --> our error message Python 3 print(&#39;Python&#39;, python_version()) try: let_us_cause_a_NameError except NameError as err: print(err, '--> our error message') Python 3.4.1 name &#39;let_us_cause_a_NameError&#39; 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 print &#39;Python&#39;, python_version() my_generator = (letter for letter in 'abcdefg') next(my_generator) my_generator.next() Python 2.7.6 &#39;b&#39; Python 3 print(&#39;Python&#39;, python_version()) my_generator = (letter for letter in 'abcdefg') next(my_generator) Python 3.4.1 &#39;a&#39; my_generator.next() --------------------------------------------------------------------------- AttributeError Traceback (most recent call last) <ipython-input-14-125f388bb61b> in <module>() ----> 1 my_generator.next() AttributeError: &#39;generator&#39; object has no attribute &#39;next&#39; 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 print &#39;Python&#39;, 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 Python 3 print(&#39;Python&#39;, 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 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 print &#39;Python&#39;, 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] > &#39;foo&#39; = False (1, 2) > &#39;foo&#39; = True [1, 2] > (1, 2) = False Python 3 print(&#39;Python&#39;, 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) in () 1 print(&#39;Python&#39;, 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 [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 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) >>> my_input = raw_input('enter a number: ') enter a number: 123 >>> type(my_input) <type &#39;str&#39;> Python 3 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(&#39;enter a number: &#39;) enter a number: 123 >>> type(my_input) <class &#39;str&#39;> 返回可迭代对象，而不是列表 [回到目录] 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 print &#39;Python&#39;, python_version() print range(3) print type(range(3)) Python 2.7.6 [0, 1, 2] <type &#39;list&#39;> Python 3 print(&#39;Python&#39;, python_version()) print(range(3)) print(type(range(3))) print(list(range(3))) Python 3.4.1 range(0, 3) <class &#39;range&#39;> [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 更多关于Python 2和Python 3的文章 [回到目录] 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 Should I use Python 2 or Python 3 for my development activity? What's New In Python 3.0 Porting to Python 3 Porting Python 2 Code to Python 3 How keep Python 3 moving forward // Praise and criticism of Python 3 10 awesome features of Python that you can't use because you refuse to upgrade to Python 3 about all the unicode features of Python that you refuse to know Python 3 is destroying Python Python 3 can revive Python Python 3 is fine ##

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