变量类型-python笔记

变量赋值

变量的本质是，开辟的一块内存，用来存放值。给变量赋值很容易：

[python]
view plaincopy

#!/usr/local/bin/python2.7
count1=100
count2=1000.9
count3="What the fuck."
print count1,'\n',count2,'\n',count3

连续赋同一个值：a=b=c=1，那么a b c共享同一块存储区，存储的值为1.
连续分别赋值：a,b,c = 2, 4, "hey"那就是三个不同变量。

基本数据类型

数字Numbers

赋值：val=20 val2=3.242等。不管是什么数值类型，直接赋值即可。
删除赋值：del val,val2
python支持的四种数值类型：int long float complex。例子如下表

字符串String

引号之间的字符，构成字符串。
可以用+将两个字符串连起来，用*使得字符串重复自身，变长。





另外，字符串的子串，可以用[:]提取出来。比如：

[python]
view plaincopy

#!/usr/bin/python

str = 'Hello World!'

print str # 打印全字符串
print str[0] # 打印第一个字符H
print str[2:5] # 打印第三到第五个字符
print str[2:] # 打印从第三个字符到末尾的所有字符
print str * 2 # Prints string two times
print str + "TEST" # Prints concatenated string

列表List

列表用[ ]标识。是python最通用的复合数据类型。看这段代码就明白。

[python]
view plaincopy

#!/usr/bin/python

list = [ 'abcd', 786 , 2.23, 'john', 70.2 ]
tinylist = [123, 'john']

print list # Prints complete list
print list[0] # Prints first element of the list
print list[1:3] # Prints elements starting from 2nd till 3rd
print list[2:] # Prints elements starting from 3rd element
print tinylist * 2 # Prints list two times
print list + tinylist # Prints concatenated lists

3.向 list 中增加元素

>>> li
['a', 'b', 'mpilgrim', 'z', 'example']
>>> li.append("new")
>>> li
['a', 'b', 'mpilgrim', 'z', 'example', 'new']
>>> li.insert(2, "new")
>>> li
['a', 'b', 'new', 'mpilgrim', 'z', 'example', 'new']
>>> li.extend(["two", "elements"])
>>> li
['a', 'b', 'new', 'mpilgrim', 'z', 'example', 'new', 'two', 'elements']

4.搜索 list

>>> li
['a', 'b', 'new', 'mpilgrim', 'z', 'example', 'new', 'two', 'elements']
>>> li.index("example")
5
>>> li.index("new")
2
>>> li.index("c")
Traceback (innermost last):
File "<interactive input>", line 1, in ?
ValueError: list.index(x): x not in list
>>> "c" in li
False

列表的元素用逗号隔开，是可以对某元素多次赋值的。

5.从 list 中删除元素

>>> li

['a', 'b', 'new', 'mpilgrim', 'z', 'example', 'new', 'two', 'elements']

>>> li.remove("z")

>>> li

['a', 'b', 'new', 'mpilgrim', 'example', 'new', 'two', 'elements']

>>> li.remove("new")

>>> li

['a', 'b', 'mpilgrim', 'example', 'new', 'two', 'elements']

>>> li.remove("c")

Traceback (innermost last):

File "<interactive input>", line 1, in ?

ValueError: list.remove(x): x not in list

>>> li.pop()

'elements'

>>> li

['a', 'b', 'mpilgrim', 'example', 'new', 'two']

6.list 运算符

>>> li = ['a', 'b', 'mpilgrim']

>>> li = li + ['example', 'new']

>>> li

['a', 'b', 'mpilgrim', 'example', 'new']

>>> li += ['two']

>>> li

['a', 'b', 'mpilgrim', 'example', 'new', 'two']

>>> li = [1, 2] * 3

>>> li

[1, 2, 1, 2, 1, 2]

元组Tuple

元组用()标识。内部元素用逗号隔开。但是元素不能二次赋值，相当于只读列表。

[python]
view plaincopy

#!/usr/bin/python

tuple = ( 'abcd', 786 , 2.23, 'john', 70.2 )
tinytuple = (123, 'john')

print tuple # Prints complete list
print tuple[0] # Prints first element of the list
print tuple[1:3] # Prints elements starting from 2nd till 3rd
print tuple[2:] # Prints elements starting from 3rd element
print tinytuple * 2 # Prints list two times
print tuple + tinytuple # Prints concatenated lists

字典Dictionary

字典用{ }标识。字典由索引(key)和它对应的值value组成。

[python]
view plaincopy

#!/usr/bin/python

dict = {}
dict['one'] = "This is one"
dict[2] = "This is two"

tinydict = {'name': 'john','code':6734, 'dept': 'sales'}

print dict['one'] # Prints value for 'one' key
print dict[2] # Prints value for 2 key
print tinydict # Prints complete dictionary
print tinydict.keys() # Prints all the keys
print tinydict.values() # Prints all the values

对字典dict，one 、2就是索引。而"This is one"和"This is two"就是相对应的值。
字典的元素是没有排序的。
dict.keys()返回所有key，dict.values()返回所有value值。

变量类型转换

int(x [,base])

Converts x to an integer. base specifies the base if x is a string.

long(x [,base] )

Converts x to a long integer. base specifies the base if x is a string.

float(x)

Converts x to a floating-point number.

complex(real [,imag])

Creates a complex number.

str(x)

Converts object x to a string representation.

repr(x)

Converts object x to an expression string.

eval(str)

Evaluates a string and returns an object.

tuple(s)

Converts s to a tuple.

list(s)

Converts s to a list.

set(s)

Converts s to a set.

dict(d)

Creates a dictionary. d must be a sequence of (key,value) tuples.

frozenset(s)

Converts s to a frozen set.

chr(x)

Converts an integer to a character.

unichr(x)

Converts an integer to a Unicode character.

ord(x)

Converts a single character to its integer value.

hex(x)

Converts an integer to a hexadecimal string.

oct(x)

Converts an integer to an octal string.