需要经常看看的python3代码

文中代码来自 https://wiki.python.org/moin/CategoryDocumentation

或者参考 https://github.com/fingerecho/interview_python转到

“可更改”（mutable）与“不可更改”（immutable）

#!/usr/bin/python3

a=1<<32;print(id(a))
print(id(1<<32))
def ch_(a:int):
    a=1<<32+1;print(id(a))
    print(id(1<<32+1))
    return a
ch_(a)
print(id(a))
print(a==1<<32)
a=ch_(a)
print(a==1<<32)

b=();print(id(b))
print(id(()))
def ch_t(b:tuple):
    b=('hello','world');print(id(b))
    return b
ch_t(b)
print(id(b))
print(b==())
b=ch_t(b)
print(b==())

c=[];print(id(c))
print(id([]))
def ch_l(c:list):
    c=['hello','world'];print(id(c))
    print(id(['hello','world']))
    return c
ch_l(c)
print(id(c))
print(c==[])
c=ch_l(c)
print(c==[])

d={};print(id(d))
print(id({}))
def ch_d(d:dict):
    d={"hello":"world"};print(id(d))
    print(id({"hello":"world"}))
    return d
ch_d(d)
print(id(d))
print(d=={})
d=ch_d(d)
print(d=={})

结论:strings, tuples, 和numbers是不可更改的对象，而 list, dict, set等则是可以修改的对象

当一个引用传递给函数的时候,函数自动复制一份引用,这个函数里的引用和外边的引用没有关系，但是引用指向的内存却没有变化

另外需要注意的是 tuple 只有两个函数 index 和 count ，正是因为它是imutable的

官方提供的unittest 和 doctest 代码

########################################
import unittest
def median(pool):
    copy = sorted(pool)
    size = len(copy)
    if size % 2 == 1:
        return copy[(size - 1) / 2]
    else:
        return (copy[size/2 - 1] + copy[size/2]) / 2
class TestMedian(unittest.TestCase):
    def testMedian(self):
        self.failUnlessEqual(median([2, 9, 9, 7, 9, 2, 4, 5, 8]), 7)
if __name__ == '__main__':
    unittest.main()
########################################
def median(pool):
    '''Statistical median to demonstrate doctest.
    >>> median([2, 9, 9, 7, 9, 2, 4, 5, 8])
    7
    '''
    copy = sorted(pool)
    size = len(copy)
    if size % 2 == 1:
        return copy[(size - 1) / 2]
    else:
        return (copy[size/2 - 1] + copy[size/2]) / 2
if __name__ == '__main__':
    import doctest
    doctest.testmod()
#########################################  

python 中的 group_by ，位于 itertools

from itertools import groupby
lines = '''
This is the
first paragraph.

This is the second.
'''.splitlines()
# Use itertools.groupby and bool to return groups of
# consecutive lines that either have content or don't.
for has_chars, frags in groupby(lines, bool):
    if has_chars:
        print ' '.join(frags)
# PRINTS:
# This is the first paragraph.
# This is the second.
###########################################

python 中 csv 文件的操作

import csv

# write stocks data as comma-separated values
writer = csv.writer(open('stocks.csv', 'wb', buffering=0))
writer.writerows([
    ('GOOG', 'Google, Inc.', 505.24, 0.47, 0.09),
    ('YHOO', 'Yahoo! Inc.', 27.38, 0.33, 1.22),
    ('CNET', 'CNET Networks, Inc.', 8.62, -0.13, -1.49)
])

# read stocks data, print status messages
stocks = csv.reader(open('stocks.csv', 'rb'))
status_labels = {-1: 'down', 0: 'unchanged', 1: 'up'}
for ticker, name, price, change, pct in stocks:
    status = status_labels[cmp(float(change), 0.0)]
    print '%s is %s (%s%%)' % (name, status, pct)
#################################################

装饰器装饰器是一个很著名的设计模式，经常被用于有切面需求的场景，较为经典的有插入日志、性能测试、事务处理等。装饰器是解决这类问题的绝佳设计，有了装饰器，我们就可以抽离出大量函数中与函数功能本身无关的雷同代码并继续重用。概括的讲，装饰器的作用就是为已经存在的对象添加额外的功能。

######################################
def makebold(fn):
    def wrapped():
        return "<b>" + fn() + "</b>"
    return wrapped

def makeitalic(fn):
    def wrapped():
        return "<i>" + fn() + "</i>"
    return wrapped

@makebold
@makeitalic
def hello():
    return "hello world"

print hello() 
## returns "<b><i>hello world</i></b>"
#######################################

参考： https://stackoverflow.com/questions/739654/how-to-make-a-chain-of-function-decorators

传送门

官方定义：https://docs.python.org/3/glossary.html#term-decorator

传送门

python函数式编程

############
>>>a = [1,2,3,4,5,6,7]
>>>b = filter(lambda x: x > 5, a)
>>>print b
>>>[6,7]
#######################
>>> a = map(lambda x:x*2,[1,2,3])
>>> list(a)
[2, 4, 6]
#######################
>>> reduce(lambda x,y:x*y,range(1,4))
6
############
  

python中的深浅拷贝

很早之前的一篇文章：https://blog.csdn.net/fyping_1/article/details/53548136

传送门

import copy
a = [1, 2, 3, 4, ['a', 'b']]  #原始对象

b = a  #赋值，传对象的引用
c = copy.copy(a)  #对象拷贝，浅拷贝
d = copy.deepcopy(a)  #对象拷贝，深拷贝

a.append(5)  #修改对象a
a[4].append('c')  #修改对象a中的['a', 'b']数组对象

print 'a = ', a
print 'b = ', b
print 'c = ', c
print 'd = ', d

输出结果：
a =  [1, 2, 3, 4, ['a', 'b', 'c'], 5]
b =  [1, 2, 3, 4, ['a', 'b', 'c'], 5]
c =  [1, 2, 3, 4, ['a', 'b', 'c']]
d =  [1, 2, 3, 4, ['a', 'b']]