python3.0 变化 跟2.x 相比

目标：使用python写出正确语法格式的程序，并让它能高效的跑起来。python的确是一个很好的编写脚本的语言，本身自带了很多好的结构。

1.8//5 output 1 8/5 output 1.6 新增加了//运算符，老实说不怎么喜欢这个改变，个人更倾向于int(8/5)来获得8//5的计算

2.In interactive mode，可以通过_得到上次的屏幕输出

3.print 变成一个函数 print（）

4.一些list操作：

>>> # Replace some items:

... a[0:2] = [1, 12]

>>> a

[1, 12, 123, 1234]

>>> # Remove some:

... a[0:2] = []

>>> a

[123, 1234]

>>> # Insert some:

... a[1:1] = [’bletch’, ’xyzzy’]

>>> a

[123, ’bletch’, ’xyzzy’, 1234]

>>> # Insert (a copy of) itself at the beginning

>>> a[:0] = a

>>> a

[123, ’bletch’, ’xyzzy’, 1234, 123, ’bletch’, ’xyzzy’, 1234]

>>> # Clear the list: replace all items with an empty list

>>> a[:] = []

>>> a

[]

5.The keyword end can be used to avoid the newline after the output, or end the output with a different string：

print(b, end=’ ’)

6.range()返回的是一个可以iterable的对象，这个对象并没有展开为一个list，这样可以节省空间。这些对象可以通过一个迭代器iterator来进行遍历，比如for语句，list（）则是另一个iterator

7.Loop statements may have an else clause; it is executed when the loop terminates through exhaustion of the list(with for) or when the condition becomes false (with while), but not when the loop is terminated by a break statement。有些时候这个else语句的确还是蛮有用的，省去了其他语言需要设置一个flag的代价。

8.*操作符用来unpack list 或者tuple：

>>> args = [3, 6]

>>> list(range(*args)) # call with arguments unpacked from a list

[3, 4, 5]

9.**操作符用来unpack dictionary：

>>> def parrot(voltage, state=’a stiff’, action=’voom’):

... print("-- This parrot wouldn’t", action, end=’ ’)

... print("if you put", voltage, "volts through it.", end=’ ’)

... print("E’s", state, "!")

...

>>> d = {"voltage": "four million", "state": "bleedin’ demised", "action": "VOOM"}

>>> parrot(**d)

10.list comprehension：

>>> [3*x for x in vec if x > 3]

11.创建一个空的set，需要使用set()

12.dict()可以从list直接创建一个dictionary，但是要求list的每一项都是一个key:value的tuple：

>>> dict([(’sape’, 4139), (’guido’, 4127), (’jack’, 4098)])

{’sape’: 4139, ’jack’: 4098, ’guido’: 4127}

13.dict comprehensions：{x: x**2 for x in (2, 4, 6)}

14.another dict参数：dict(sape=4139, guido=4127, jack=4098)

15.loop through a sequence，使用enumerate（）可以同时取得position index和value：

>>> for i, v in enumerate([’tic’, ’tac’, ’toe’]):

16.zip（）函数：

>>> questions = [’name’, ’quest’, ’favorite color’]

>>> answers = [’lancelot’, ’the holy grail’, ’blue’]

>>> for q, a in zip(questions, answers):

... print(’What is your {0}? It is {1}.’.format(q, a))

17.import imp; imp.reload(modulename)可以重新加载模块

18.from module important * 可以引入模块中的所有方法和变量，除了以_开头的

19.The module compileall can create .pyc files (or .pyo files when -O is used) for all modules in a directory.

20.the new built-in vars() function, which returns a dictionary containing all local variables

21.print（）函数提供{key：option}格式化的方式，还有string的format（）来取代以前的%操作，虽然%操作被保留但是最终还是会从语言中被删除，所以以后还是不要再使用。

22.except Exception as e 以前是 except Exception，e，感觉现在的语法更容易接受。

23.The with statement allows objects like files to be used in a way that ensures they are always cleaned up promptly

and correctly.

with open("myfile.txt") as f:

for line in f:

print(line)

After the statement is executed, the file f is always closed, even if a problem was encountered while processing

the lines.

一个很奇怪的语法，没弄懂什么意思。

24.关于scope和namespace，写了一长段的东西，看了半天，还不如看下面的一个例子：

def scope_test():

def do_local():

spam = "local spam"

def do_nonlocal():

nonlocal spam

spam = "nonlocal spam"

def do_global():

global spam

spam = "global spam"

spam = "test spam"

do_local()

print("After local assignment:", spam)

do_nonlocal()

print("After nonlocal assignment:", spam)

do_global()

print("After global assignment:", spam)

scope_test()

print("In global scope:", spam)

output is :

after local assignment: test spam

after nonlocal assignment:nonlocal spam

after global assignment:nonlocal spam

In global scope:global spam

这里的函数里面又定义函数第一次看到，fresh。

25.yield 语句可以帮助创建iterator，它会记录上次访问数据的位置，并会自动产生StopIteration：

def reverse(data):

for index in range(len(data)-1, -1, -1):

yield data[index]

>>> for char in reverse(’golf’):

... print(char)

...