Python 3 教程一:入门

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Python已经是3.1版本了,与时俱进更新教程.

本文适合有Java编程经验的程序员快速熟悉Python

本文程序在windows xp+python3.1a1 测试通过.

本文提到的idle指python shell,即安装python后你在菜单看到的IDLE(python gui)

在idle里ctrl+n可以打开一个新窗口,输入源码后ctrl+s可以保存,f5运行程序.

凡打开新窗口即指ctrl+n的操作.

1 你好

#
打开新窗口,输入:

#
! /usr/bin/python

#
-*- coding: utf8 -*-

s1
=
input(
"
Input your name:
"
)

print
(
"
你好,%s
"

%
s1)

'''

知识点:

* input("某字符串")函数:显示"某字符串",并等待用户输入.

* print()函数:如何打印.

* 如何应用中文

* 如何用多行注释

'''

2 字符串和数字

但有趣的是,在javascript里我们会理想当然的将字符串和数字连接,因为是动态语言嘛.但在Python里有点诡异,如下:

#
! /usr/bin/python

a
=
2

b
=
"
test
"

c
=
a
+
b

运行这行程序会出错,提示你字符串和数字不能连接,于是只好用内置函数进行转换

#
! /usr/bin/python

#
运行这行程序会出错,提示你字符串和数字不能连接,于是只好用内置函数进行转换

a
=
2

b
=
"
test
"

c
=
str(a)
+
b

d
=
"
1111
"

e
=
a
+
int(d)

#
How to print multiply values

print
(
"
c is %s,e is %i
"

%
(c,e))

'''

知识点:

* 用int和str函数将字符串和数字进行转换

* 打印以#开头,而不是习惯的//

* 打印多个参数的方式

'''

3 列表

#
! /usr/bin/python

#
-*- coding: utf8 -*-

#
列表类似Javascript的数组,方便易用

#
定义元组

word
=
[
'
a
'
,
'
b
'
,
'
c
'
,
'
d
'
,
'
e
'
,
'
f
'
,
'
g
'
]

#
如何通过索引访问元组里的元素

a
=
word[
2
]

print
(
"
a is:
"
+
a)

b
=
word[
1
:
3
]

print
(
"
b is:
"
)

print
(b)
#
index 1 and 2 elements of word.

c
=
word[:
2
]

print
(
"
c is:
"
)

print
(c)
#
index 0 and 1 elements of word.

d
=
word[0:]

print
(
"
d is:
"
)

print
(d)
#
All elements of word.

#
元组可以合并

e
=
word[:
2
]
+
word[
2
:]

print
(
"
e is:
"
)

print
(e)
#
All elements of word.

f
=
word[
-
1
]

print
(
"
f is:
"
)

print
(f)
#
The last elements of word.

g
=
word[
-
4
:
-
2
]

print
(
"
g is:
"
)

print
(g)
#
index 3 and 4 elements of word.

h
=
word[
-
2
:]

print
(
"
h is:
"
)

print
(h)
#
The last two elements.

i
=
word[:
-
2
]

print
(
"
i is:
"
)

print
(i)
#
Everything except the last two characters

l
=
len(word)

print
(
"
Length of word is:
"
+
str(l))

print
(
"
Adds new element
"
)

word.append(
'
h
'
)

print
(word)

#
删除元素

del
word[0]

print
(word)

del
word[
1
:
3
]

print
(word)

'''

知识点:

* 列表长度是动态的,可任意添加删除元素.

* 用索引可以很方便访问元素,甚至返回一个子列表

* 更多方法请参考Python的文档

'''

4 字典

#
! /usr/bin/python

x
=
{
'
a
'
:
'
aaa
'
,
'
b
'
:
'
bbb
'
,
'
c
'
:
12
}

print
(x[
'
a
'
])

print
(x[
'
b
'
])

print
(x[
'
c
'
])

for
key
in
x:

print
(
"
Key is %s and value is %s
"

%
(key,x[key]))

'''

知识点:

* 将他当Java的Map来用即可.

'''

5 字符串

比起C/C++,Python处理字符串的方式实在太让人感动了.把字符串当列表来用吧.

#
! /usr/bin/python

word
=
"
abcdefg
"

a
=
word[
2
]

print
(
"
a is:
"
+
a)

b
=
word[
1
:
3
]

print
(
"
b is:
"
+
b)
#
index 1 and 2 elements of word.

c
=
word[:
2
]

print
(
"
c is:
"
+
c)
#
index 0 and 1 elements of word.

d
=
word[0:]

print
(
"
d is:
"
+
d)
#
All elements of word.

e
=
word[:
2
]
+
word[
2
:]

print
(
"
e is:
"
+
e)
#
All elements of word.

f
=
word[
-
1
]

print
(
"
f is:
"
+
f)
#
The last elements of word.

g
=
word[
-
4
:
-
2
]

print
(
"
g is:
"
+
g)
#
index 3 and 4 elements of word.

h
=
word[
-
2
:]

print
(
"
h is:
"
+
h)
#
The last two elements.

i
=
word[:
-
2
]

print
(
"
i is:
"
+
i)
#
Everything except the last two characters

l
=
len(word)

print
(
"
Length of word is:
"
+
str(l))

中文和英文的字符串长度是否一样?

#
! /usr/bin/python

#
-*- coding: utf8 -*-

s
=
input(
"
输入你的中文名,按回车继续
"
);

print
(
"
你的名字是  :
"

+
s)

l
=
len(s)

print
(
"
你中文名字的长度是:
"
+
str(l))

知识点:

类似Java,在python3里所有字符串都是unicode,所以长度一致.

6 条件和循环语句

#
! /usr/bin/python

#
条件和循环语句

x
=
int(input(
"
Please enter an integer:
"
))

if
x
<
0:

x
=
0

print
(
"
Negative changed to zero
"
)

elif
x
==
0:

print
(
"
Zero
"
)

else
:

print
(
"
More
"
)

#
Loops List

a
=
[
'
cat
'
,
'
window
'
,
'
defenestrate
'
]

for
x
in
a:

print
(x, len(x))

#
知识点:

#
* 条件和循环语句

#
* 如何得到控制台输入

7 函数

#
! /usr/bin/python

#
-*- coding: utf8 -*-

def
sum(a,b):

return
a
+
b

func
=
sum

r
=
func(
5
,
6
)

print
(r)

#
提供默认值

def
add(a,b
=
2
):

return
a
+
b

r
=
add(
1
)

print
(r)

r
=
add(
1
,
5
)

print
(r)

一个好用的函数

#
! /usr/bin/python

#
-*- coding: utf8 -*-

#
The range() function

a
=
range (
1
,
10
)

for
i
in
a:

print
(i)

a
=
range(
-
2
,
-
11
,
-
3
)
#
The 3rd parameter stands for step

for
i
in
a:

print
(i)

知识点:

Python 不用{}来控制程序结构,他强迫你用缩进来写程序,使代码清晰.

定义函数方便简单

方便好用的range函数

8 异常处理

#
! /usr/bin/python

s
=
input(
"
Input your age:
"
)

if
s
==
""
:

raise
Exception(
"
Input must no be empty.
"
)

try
:

i
=
int(s)

except
Exception as err:

print
(err)

finally
:
#
Clean up action

print
(
"
Goodbye!
"
)

9 文件处理

对比Java,python的文本处理再次让人感动

#
! /usr/bin/python

spath
=
"
D:/download/baa.txt
"

f
=
open(spath,
"
w
"
)
#
Opens file for writing.Creates this file doesn't exist.

f.write(
"
First line 1.\n
"
)

f.writelines(
"
First line 2.
"
)

f.close()

f
=
open(spath,
"
r
"
)
#
Opens file for reading

for
line
in
f:

print
(
"
每一行的数据是:%s
"
%
line)

f.close()

知识点:

open的参数:r表示读,w写数据,在写之前先清空文件内容,a打开并附加内容.

打开文件之后记得关闭

10 类和继承

class
Base:

def

__init__
(self):

self.data
=
[]

def
add(self, x):

self.data.append(x)

def
addtwice(self, x):

self.add(x)

self.add(x)

#
Child extends Base

class
Child(Base):

def
plus(self,a,b):

return
a
+
b

oChild
=
Child()

oChild.add(
"
str1
"
)

print
(oChild.data)

print
(oChild.plus(
2
,
3
))

'''

知识点:

* self:类似Java的this参数

'''

11 包机制

每一个.py文件称为一个module,module之间可以互相导入.请参看以下例子:

#
a.py

def
add_func(a,b):

return
a
+
b

#
b.py

from
a
import
add_func
#
Also can be : import a

print
(
"
Import add_func from module a
"
)

print
(
"
Result of 1 plus 2 is:
"
)

print
(add_func(
1
,
2
))
#
If using "import a" , then here should be "a.add_func"

module可以定义在包里面.Python定义包的方式稍微有点古怪,假设我们有一个parent文件夹,该文件夹有一个child子文件夹.child中有一个module a.py . 如何让Python知道这个文件层次结构?很简单,每个目录都放一个名为_init
_.py 的文件.该文件内容可以为空.这个层次结构如下所示:

parent

--
__init_.py

--
child

--
__init_.py

--
a.py

b.py

那么Python如何找到我们定义的module?在标准包sys中,path属性记录了Python的包路径.你可以将之打印出来:

import sys

print(sys.path)

通常我们可以将module的包路径放到环境变量PYTHONPATH中,该环境变量会自动添加到sys.path属性.另一种方便的方法是编程中直接指定我们的module路径到sys.path 中:

import
sys

import
os

sys.path.append(os.getcwd()
+
'
\\parent\\child
'
)

print
(sys.path)

from
a
import
add_func

print
(sys.path)

print
(
"
Import add_func from module a
"
)

print
(
"
Result of 1 plus 2 is:
"
)

print
(add_func(
1
,
2
))

知识点:

如何定义模块和包

如何将模块路径添加到系统路径,以便python找到它们

如何得到当前路径

12 内建帮助手册

对比C++,Java的突出进步是内建Javadoc机制,程序员可以通过阅读Javadoc了解函数用法.Python也内建了一些方便函数以便程序员参考.

dir函数: 查看某个类/对象的方法. 如果有某个方法想不起来,请敲dir. 在idle里,试试 dir(list)

help函数: 详细的类/对象介绍. 在idle里, 试试 help(list)