读书笔记--《Python基础教程第二版》-- 第五章 条件循环和其他语句

Posted 2020-10-06

5.1 print和import的更多信息

5.1.1 使用独号输出

>>> print ‘Age:‘,42

Age: 42

>>> 1,2,3

(1, 2, 3)

>>> print 1,2,3

1 2 3

>>> print (1,2,3)

(1, 2, 3)

>>> name=‘Gumby‘

>>> greeting=‘Hello‘

>>> salutation=‘Mr.‘

>>> print greeting ,salutation,name

Hello Mr. Gumby

>>> print greeting ,‘,‘,salutation,name

Hello , Mr. Gumby

>>> print greeting+‘,‘,salutation,name

Hello, Mr. Gumby

>>> print ‘Hello‘, # 在结尾处加独号，会打印在同一行

>>> print ‘World‘

Hello World

5.1.2 把某件事作为另一件事导入

import somemodule

from  somemodule import somefunction

from  somemodule import somefunction,anotherfunction

from somemodule import *

多个模块有同样的函数

module1.open()

module2.open()

>>> import math as foobar

>>> foobar.sqrt(4)

2.0

>>> from math import sqrt as foobar

>>> foobar(4)

2.0

from module1 import open as open1

from module2 import open as open2

5.2 赋值魔法

5.2.1 序列解包，将多个值的序列解开，然后放到变量的序列中，序列解包要解开的值和赋值的值的数量完全一致

>>> print x,y,z

1 2 3

>>> x,y=y,x

>>> print x,y,z

2 1 3

>>> scoundrel={‘name‘:‘Robin‘,‘girlfried‘:‘Marion‘}

>>> key,value=scoundrel.popitem()

>>> key

‘girlfried‘

>>> value

5.2.2 链式赋值

x=y=somefunction()

等价于：

y=somefunction()

x=y

注意上面的语句和下面的语句不一定等价：

x=somefunction()

y=somefunction()

5.2.2 增量赋值

>>> x=2

>>> x+=1

>>> x*=2

>>> x

6

>>> fnord=‘foo‘

>>> fnord +=‘bar‘

>>> fnord*=2

>>> fnord

5.3 语句块：缩排的乐趣

  语句块是在条件为真时执行，或者执行多次的一组语句，语句块用：开始，块中每一条语句都有相同的缩进

 

5.4 条件和条件语句

5.4.1 这就是布尔变量的作用

下面的值作为布尔表示试的时候，会被解释器看做假

False、None、0,()、"" ,(),[],{}

True,出去上面为空的，其他的一切都为真

True和Flase只不过是1和0的华丽说法而已

>>> True

True

>>> False

False

>>> True==1

True

>>> False==0

True

>>> True+False+42

43

所有的值都可以用做布尔值，所以不需要对它们进行显示转换

>>> bool(‘I think.therefore I am‘)

True

>>> bool(43)

True

>>> bool(‘‘)

False

>>> bool

>>> []!=""

True

5.4.2 条件执行和if语句

>>> name="Gumby"

>>> if name.endswith(‘Gumby‘):

...  print ‘Hello,Mr.Gumby‘

... 

Hello,Mr.Gumby

5.4.3 else 子句

>>> name="Gumby"

>>> if name.endswith(‘Gumby‘):

...  print ‘Hello,Mr.Gumby‘

... else:

...  print ‘Hello,stranger‘

... 

Hello,Mr.Gumby

5.4.4 elif 子句

>>> num=input(‘Enter a number:‘)

Enter a number:4

>>> if num>0:

...  print ‘The number is positive‘

... elif num<0:

...  print ‘The number is negative‘

... else:

...  print ‘The number is zero‘

... 

The number is positive

5.4.5 嵌套代码块

#!/usr/bin/python

name=raw_input(‘What is your name?‘)

if name.endswith(‘Gumby‘):

    if name.startswith(‘Mr.‘):

        print ‘Hello,Mr.Gumby‘

    elif name.startswith(‘Mrs.‘):

        print ‘Hello,Mrs Gumby‘

    else:

        print ‘Hello,Gumby‘

else:

    print ‘Hello,Stranger‘

5.4.6 更复杂的条件

1、比较运算符

x == y

x < y

x > y

x >= y

x <= y

x != y

x is y x和y是同一对象

x is not y 

x in y

x not in  y

0<age<100 几个运算符可以连在一起

2、相等运算符

>>> ‘foo‘==‘foo‘

True

>>> ‘foo‘==‘bar‘

False

3、is：同一性运算符

>>> x = y = [1,2,3]

>>> z=[1,2,3]

>>> x==y

True

>>> x==z

True

>>> x is y

True

>>> x is z

False

>>> x = [1,2,3]

>>> y=[2,4]

>>> x is not y

True

>>> del x[2]

>>> y[1]=1

>>> y.reverse()

>>> x==y

True

>>> x is y

False

尽量将is运算符比较类似数值和字符串这类不可变值

4、in:成员资格运算符

>>> name="allen"

>>> if ‘s‘ in name:

...     print ‘Your name contains the letter "s"‘

... else:

...     print ‘Your name dose not contains the letter "s"‘

... 

Your name dose not contains the letter "s"

5、字符串和序列比较

>>> "alpha" <‘beta‘

True

>>> ord(‘a‘)

97

>>> ord(‘b‘)

98

>>> chr(97)

‘a‘

>>> chr(98)

‘b‘

>>> ‘FnOrd‘.lower()==‘Fnord‘.lower()

True

>>> [1,2]<[2,1]

True

>>> [2,[1,4]] <[2,[1,5]]

True

6、布尔运算符 AND OR not 

>>> number=20

>>> if number <=10 and number>=1:

...     print ‘Great!‘

... else:

...     print ‘Wrong!‘

... 

Wrong

利用短路的特性

name=raw_input(‘Please enter your name:‘) or ‘<unkown>‘

a if b else c 如果b为真，返回a,否则返回c

5.4.7 断言

>>> age =10

>>> assert 0<age<100

>>> age = -1

>>> assert 0<age<100

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

AssertionError

>>> age = -1

>>> assert 0<age<100 , ‘The age must be realistic‘

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

AssertionError: The age must be realistic

5.5 循环

5.5.1 while循环

>>> x =1 

>>> while x <100:

...     print x,

...     x += 1

... 

>>> name=‘‘

>>> while not name:

...    name=raw_input(‘Please enter your name:‘)

... print ‘Hello,%s!‘ %name

while not name.strip()

while not name or name.isspace()

5.5.2 for循环，一般用来指定循环多次次

>>> words=[‘this‘,‘is‘,‘an‘,‘ex‘,‘parrot‘]

>>> for word in words:

...     print word

... 

this

is

an

ex

parrot

>>> range(0,10)

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> range(10)

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> for number in range(1,101):

...      print number,

... 

>>> xrange(1,10)

xrange(1, 10)

>>> for number in xrange(1,10):

...     print number,

... 

1 2 3 4 5 6 7 8 9

5.5.3 循环遍历字典元素

>>> d={‘x‘:1,‘y‘:2,‘z‘:3}

>>> for key in d:

...     print key,‘corresponds to‘,d[key]

... 

y corresponds to 2

x corresponds to 1

z corresponds to 3

>>> for key,value in d.items():

...     print key,‘corresponds to‘,value

... 

y corresponds to 2

x corresponds to 1

z corresponds to 3

5.5.4 一些迭代的工具

1、并行迭代

>>> names=[‘name‘,‘beth‘,‘damon‘]

>>> ages=[12,24,34]

>>> for i in range(len(names)):

...     print names[i],‘is‘,ages[i],‘years old‘

... 

name is 12 years old

beth is 24 years old

damon is 34 years old

>>> zip(names,ages)

[(‘name‘, 12), (‘beth‘, 24), (‘damon‘, 34)]

>>> for name,age in zip(names,ages):

...     print name, ‘is‘ ,age,‘years old‘

... 

name is 12 years old

beth is 24 years old

damon is 34 years old

zip可以应付不等长序列

>>> zip(range(5),xrange(100000),range(4))

[(0, 0, 0), (1, 1, 1), (2, 2, 2), (3, 3, 3)]

2、编号迭代

>>> l=[‘a‘,‘b‘,‘c‘]

>>> for index,value in enumerate(l):

...     print index,‘is‘,value

... 

0 is a

1 is b

3、翻转和排序迭代

reversed和sorted函数，作用于任何序列和可迭代对象上

>>> sorted([4,2,6,8,3])

[2, 3, 4, 6, 8]

>>> sorted(‘Hello,world!‘)

[‘!‘, ‘,‘, ‘H‘, ‘d‘, ‘e‘, ‘l‘, ‘l‘, ‘l‘, ‘o‘, ‘o‘, ‘r‘, ‘w‘]

>>> list(reversed(‘Hello,World‘))

[‘d‘, ‘l‘, ‘r‘, ‘o‘, ‘W‘, ‘,‘, ‘o‘, ‘l‘, ‘l‘, ‘e‘, ‘H‘]

>>> ‘‘.join(reversed(‘Hello,World!‘))

‘!dlroW,olleH‘

5.5.5 跳出循环

1、break

>>> for n in range(99,0,-1):

...   root=sqrt(n)

...   if root==int(root):

...      print n

...      break

... 

81

2、continue

>>> range(0,10,2)

[0, 2, 4, 6, 8]

2、continue

  for x in seq:

    if condition1:continue

if condition2:continue

if condition3:continue

do_someting()

等价：

 for x in seq:

    if not (condition1 or condition2 or condition3):

do_someting()

3、while True/break

>>> while True:

...     word=raw_input(‘Please enter a word:‘)

...     if not word:break

...     print ‘The word was‘+ word

... 

Please enter a word:hello

The word washello

Please enter a word:nihao

The word wasnihao

Please enter a word:

替代下面的写法

>>> word=‘dumy‘

>>> while word:

...    word=raw_input(‘Please enter a word:‘)

...    print ‘The word was ‘+ word

... 

Please enter a word:hello

The word was hello

Please enter a word:nihao

The word was nihao

Please enter a word:

The word was 

5.5.6 循环中的else子句 ，在没有调用break的时候执行

#!/usr/bin/python

from math import sqrt

for n in range(99,80,-1):

    root=sqrt(n)

    if root==int(root):

      print n

      break

else:

    print "didn‘t find it"

for while 循环中都可以使用break,continue,else子句

5.6 列表推导式 - 轻量级循环

利用其它列表创建新的列表

>>> [x*x for x in range(10)]

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

>>> [x*x for x in range(10) if x % 3 ==0]

[0, 9, 36, 81]

>>> [(x,y) for x in range(3) for y in range(3)]

[(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]

>>> 

>>> girls=[‘alice‘,‘bernice‘,‘clarice‘]

>>> boys=[‘chris‘,‘arnold‘,‘bob‘]

>>> [b+‘+‘+g for b in boys for g in girls if b[0]==g[0] ]

[‘chris+clarice‘, ‘arnold+alice‘, ‘bob+bernice‘]

更优的方案：

>>> girls=[‘alice‘,‘bernice‘,‘clarice‘]

>>> boys=[‘chris‘,‘arnold‘,‘bob‘]

>>> for girl in girls:

...     letterGirls.setdefault(girl[0],[]).append(girl)

... 

>>> print letterGirls

{‘a‘: [‘alice‘], ‘c‘: [‘clarice‘], ‘b‘: [‘bernice‘]}

>>> for girl in girls:

...     letterGirls.setdefault(girl[0],[]).append(girl)

...     print [b+‘+‘+g for b in boys for g in letterGirls[b[0]]]

... 

[‘chris+clarice‘, ‘arnold+alice‘, ‘arnold+alice‘, ‘bob+bernice‘]

[‘chris+clarice‘, ‘arnold+alice‘, ‘arnold+alice‘, ‘bob+bernice‘, ‘bob+bernice‘]

[‘chris+clarice‘, ‘chris+clarice‘, ‘arnold+alice‘, ‘arnold+alice‘, ‘bob+bernice‘, ‘bob+bernice‘]

5.7 三人行

5.7.1 什么都没有发生

>>> pass

#!/usr/bin/python

name=‘Ralap‘

if name==‘Ralap‘:

    print ‘Welcome‘

elif name==‘End‘:

    # not end

    pass

elif name ==‘Bill‘:

    print ‘Accedd Denied‘

5.7.2 使用del删除

del 删除的只是引用名，并不删除内存中的值，值由python自动回收

方法一：

>>> scoudrel={‘age‘:42,‘first name‘:‘Robin‘}

>>> robin=scoudrel

>>> robin

{‘first name‘: ‘Robin‘, ‘age‘: 42}

>>> scoudrel=None

>>> robin

{‘first name‘: ‘Robin‘, ‘age‘: 42}

>>> scoudrel

>>> robin=None

方法二：

>>> x = 1

>>> del x

>>> x

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

NameError: name ‘x‘ is not defined

>>> x=["Hello","world"]

>>> y=x

>>> y[1]=‘Python‘

>>> x

[‘Hello‘, ‘Python‘]

>>> y

[‘Hello‘, ‘Python‘]

>>> del x

>>> y

[‘Hello‘, ‘Python‘]

5.7.3 使用exec和eval 执行和求值字符串

exec 执行一系列的python语句

>>> exec "print ‘Hello,World!‘"

Hello,World!

不能简单使用exec，这样并不安全

>>> from math import sqrt

>>> scope={}

>>> exec ‘sqrt=1‘ in scope

>>> sqrt(4)

2.0

>>> scope[‘sqrt‘]

1

>>> scope.keys()

[‘__builtins__‘, ‘sqrt‘]

eval 会计算python表达式，并返回结果值

>>> eval(‘6+18*2‘)

42

给exe和eval语句提供命名空间时，还可以在命名空间中放置一些值进去

>>> scope={}

>>> scope[‘x‘]=2

>>> scope[‘y‘]=3

>>> eval(‘x*y‘,scope)

6

>>> scope={}

>>> exec ‘x=2‘ in scope

>>> eval(‘x*x‘,scope)

4

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