§5控制結構

2009-8-17

磁針石：xurongzhong#gmail.com

§5.1關于print和import更多的東東

打印多個值：

>>>

print 'Age:', 42

Age: 42

輸出時會有空格分隔。為了避免空格，可以使用“+".

在腳本中，這樣就不會換行：

print

'Hello,',

print 'world!'

輸出Hello,

world!.

import的格式：

import

somemodule

from

somemodule import somefunction

from

somemodule import somefunction, anotherfunction, yetanotherfunction

from

somemodule import *

import

math as foobar

§5.2賦值技巧

>>> x, y, z = 1, 2, 3

>>> print x, y, z

1 2 3

>>> x, y = y, x

>>> print x, y, z

2 1 3

>>> values = 1, 2, 3

>>> values

(1, 2, 3)

>>> x, y, z = values

>>> x

1

注意這種情況要求左右的值個數相同。Python 3.0中可以：a, b, rest* = [1, 2, 3, 4]。

x

= y = somefunction()

§5.3塊

：表示縮進

§5.4條件判斷

代表False的有：False None

0 "" () [] {}，其他都為True.

>>>

bool('I think, therefore I am')

True

python會自動進行這種類型轉換。

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'

比較操作：

x

== y x equals y.

x < y x is less than y.

x > y x is greater than y.

x >= y x is greater than or equal to y.

x <= y x is less than or equal to y.

x != y x is not equal to y.

x is y x and y are the same object.

x is not y x and y are different objects.

x in y x is a member of the container

(e.g., sequence) y.

x not in y x is not a member of the

container (e.g., sequence) y.

COMPARING INCOMPATIBLE

比較也可以嵌套：0 < age < 100

>>>

x = y = [1, 2, 3]

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

>>> x == y

True

>>> x == z

True

>>> x is y

True

>>> x is z

False

不要在基本的，不可改變的類型，比如numbers and strings中使用is，這些類型在python內部處理。

三元運算：a if b else c

斷言：

>>>

age = -1

>>> assert 0 < age < 100,

'The age must be realistic'

Traceback (most recent call last):

File "", line 1, in

?

AssertionError: The age must be realistic

§5.5循環

name = ''

while not name:

name = raw_input('Please enter your name:

')

print 'Hello, %s!' % name

for number in range(1,101):

print number

xrange一次只產生一個數，在大量循環的時候可以提高效率，一般情況沒有明顯效果。

與字典配合使用：

d

= {'x': 1, 'y': 2, 'z': 3}

for key in d:

print key, 'corresponds to', d[key]

不過排序的為您提要自己處理。

names

= ['anne', 'beth', 'george', 'damon']

ages = [12, 45, 32, 102]

for i in range(len(names)):

print names[i], 'is', ages[i], 'years old'

>>>

zip(names, ages)

[('anne', 12), ('beth', 45), ('george',

32), ('damon', 102)]

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

print name, 'is', age, 'years old'

>>> zip(range(5),

xrange(100000000))

[(0, 0), (1, 1), (2, 2), (3, 3), (4, 4)]

for index, string in enumerate(strings):

if 'xxx' in string:

strings[index] = '[censored]'

from math import sqrt

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

root = sqrt(n)

if root == int(root):

print n

break

from math import sqrt

for n in range(99, 81, -1):

root = sqrt(n)

if root == int(root):

print n

break

else:

print "Didn't find it!"

可見for語句也可以有else。

§5.6List Comprehension

[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']

這里類似數據庫里面的連接。一種更有效的方法：

girls

= ['alice', 'bernice', 'clarice']

boys = ['chris', 'arnold', 'bob']

letterGirls = {}

for girl in girls:

letterGirls.setdefault(girl[0],

[]).append(girl)

print [b+'+'+g for b in boys for g in

letterGirls[b[0]]]

§5.7pass, del,和exec

pass：什么都不做

del：刪除變量

>>> from math import sqrt

>>> scope = {}

>>> exec 'sqrt = 1' in scope

>>> sqrt(4)

2.0

>>> scope['sqrt']

1

>>> len(scope)

2

>>> scope.keys()

['sqrt', '__builtins__']

eval是內置的，和exec類似。exec針對陳述，eval針對表達式。Python 3.0中，raw_input被命名為input.此處尚未完全理解。

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