第1章 概念

1. 错误

• 语法错误：代码不符合解释器或编译器的语法
• 逻辑错误：不完整或不合法的输入或者计算出现问题

2. 异常：代码执行过程中，出现问题导致程序无法执行

• 程序遇到逻辑或算法问题
• 运行过程中计算机错误(内存不够或IO错误)

3. 区别与联系

• 错误：代码运行前的语法或逻辑错误
• 异常：
  异常产生：代码执行中，解释器认为是异常，抛出异常
  异常处理：截获异常并
  处理，否则程序会终止执行
  
  

第2章 常见错误和异常

1. Python标准异常

2. 示例

In [1]: a
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-1-3f786850e387> in <module>()
----> 1 a
NameError: name 'a' is not defined



In [2]: if a
  File "<ipython-input-2-95806c26c887>", line 1
    if a
        ^
SyntaxError: invalid syntax



In [3]: if a:
   ...:     print(a)   
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-3-7856809510fb> in <module>()
----> 1 if a:
      2     print(a)
      3 

NameError: name 'a' is not defined



In [6]: a = int('a10')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-6-5b6f560ee82d> in <module>()
----> 1 a = int('a10')

ValueError: invalid literal for int() with base 10: 'a10'



In [5]: 10/0
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-5-e574edb36883> in <module>()
----> 1 10/0

ZeroDivisionError: division by zero




In [4]: f = open('test.txt')
---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-4-9426bf859622> in <module>()
----> 1 f = open('test.txt')

FileNotFoundError: [Errno 2] No such file or directory: 'test.txt'

第3章 try...except

1. 语法

try:
      try_suite
except Exception as e:
      except_suite

• try和except：语法关键字，是必须的
• try_suite：需要处理的代码
• except_suite：捕获异常后的处理逻辑
• exception as e：设置异常的类，并将异常保存到变量e中

2. 执行过程

• try用来捕获try_suite中的异常，并将异常传给except进行处理；
• except用来处理异常，若捕获的异常与设置的异常一致，使用except_suite进行处理异常；若不一致，则会将捕获的异常传给Python解释器进行处理；
• 需要处理多个异常时，增加多个except语句即可

3. 只能捕获代码运行过程中的错误

• case-1：try...except捕获到一个异常

try:
    a
except BaseException:
    print('Catch an Error')

Catch an Error

• case-2：try...except没有捕获到异常，而是由Python解释器抛出的异常

try:
    if a
except BaseException:
    print('Catch an Error')

  File "<ipython-input-2-a622bb3e387c>", line 2
    if a
        ^
SyntaxError: invalid syntax

• 说明：Python代码执行时，由Python解释器将 .py 文件转换成二进制的字节码文件，转换过程中，若代码存在语法错误，则直接会抛出异常，实际上代码还没有开始运行。

4. 出现的异常与设置的异常不一致时，不会被捕获，但会被Python解释器抛出

• case-1：try...except捕获到一个异常

try:
    a
except NameError as e:
    print('Catch an Error : ' + str(e))

Catch an Error : name 'a' is not defined

• case-2：try...except捕获不到异常，Python解释器抛出异常，并停止代码运行

try:
    a
except IOError as e:
    print('Catch an Error : ' + str(e))

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-6-b89d2b7d14a3> in <module>()
      1 try:
----> 2     a
      3 except IOError as e:
      4     print('Catch an Error : ' + str(e))

NameError: name 'a' is not defined

4. 示例：猜数字游戏

• 开始游戏，产生一个1～100之间的随机数
• 用户输入，游戏根据输入值提示大或者小
• 用户根据提示，继续输入，直到猜对为止
• 如果用户输入错误值，程序可以处理异常
• 使用try...except对输入格式进行保护，确保错误输入时，程序也可以继续进行，增强代码的健壮性

import random
number = random.randint(0, 100) 

while True:
    try:
        guess = int(input('Enter 1~100 :'))    # 输入错误时，提醒用户，而非直接终止程序
    except ValueError as e:
        print('Please enter 1~100 !')
        continue
    if guess > number:
        print('Guess Bigger', guess)
    elif guess < number:
        print('Guess Smaller', guess)
    else:
        print('Guess OK! Game over')
        break
    print('\n')

Enter 1~100 :50
Guess Bigger 50

Enter 1~100 :30
Guess Smaller 30

Enter 1~100 :40d
Please enter 1~100 !

Enter 1~100 :40
Guess Bigger 40

Enter 1~100 :35
Guess Bigger 35

Enter 1~100 :33
Guess OK! Game over

5. try...except...else语句：如果没有异常时，执行else后的代码

try:
    x
except Exception as e:
    print('Catch an Error : ' + str(e))
else:
    print('No Error')

Catch an Error : name 'x' is not defined


x = 0
try:
    x
except Exception as e:
    print('Catch an Error : ' + str(e))
else:
    print('No Error')

No Error

第4章 try...finally

1. 语法

try:
      try_suite
finally:
      do_finally

• try和finally：语法关键字，是必须的
• try_suite：需要处理的代码
• do_finally：清理保护工作(不处理异常)

2. 执行过程

• 如果try没有捕获到异常，继续执行do_finally代码；
• 如果try捕获到异常，仍然继续执行do_finally代码，最后会将捕获的异常传给Python解释器进行处理。

try:
    f = open('test.txt','r')
    number = int(f.read()) 
finally:
    f.close()
    print('File is closed')

File is closed
---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-7-174bacfa803b> in <module>()
      1 try:
----> 2     f = open('test.txt','r')
      3     number = int(f.read())
      4 finally:
      5     f.close()

FileNotFoundError: [Errno 2] No such file or directory: 'test.txt'

3. 作用

• 无论try...finally语句是否捕捉到异常，都会执行finally代码；
• 作用：为异常处理事件提供清理机制，用来关闭文件或释放系统资源。
  
  

第5章 try...except...else...finally

1. try...except...finally

• 如果try语句没有捕获异常，执行完try代码后，执行finally代码；
• 如果try语句捕获到异常，首先执行except语句处理错误，再执行finally代码；
• case-1：try语句捕获到ValueError错误、文件被关闭

try:
    f = open('test.txt')
    number = int(f.read())
except ValueError as e:
    print('Catch an Error : ' + str(e))
except IOError as e:
    print('Catch an Error : ' + str(e))
finally:
    print('File will be closed')
    f.close()

Catch an Error : invalid literal for int() with base 10: 'x123'
File will be closed

• case-2：try语句捕获到错误，执行finally时，被解释器抛出另一个错误，是因为test---1文件不存在，即f没有并定义。

try:
    f = open('test---1.txt')
    number = int(f.read())
except ValueError as e:
    print('Catch an Error : ' + str(e))
except IOError as e:
    print('Catch an Error : ' + str(e))
finally:
    print('File will be closed')
    f.close() 

Catch an Error : [Errno 2] No such file or directory: 'test1.txt'
File will be closed
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-1-215addff53c9> in <module>()
      8 finally:
      9     print('File will be closed')
---> 10     f.close()

NameError: name 'f' is not defined

• case-3：为finally增加try...except异常处理，保护f.close()，避免发生异常

try:
    f = open('test1.txt')
    number = int(f.read())
except ValueError as e:
    print('Catch an Error : ' + str(e))
except IOError as e:
    print('Catch an Error : ' + str(e))
finally:
    try:
        print('File will be closed')
        f.close()
    except NameError as e:
        print('Catch another Error : ' + str(e))

Catch an Error : [Errno 2] No such file or directory: 'test1.txt'
File will be closed
Catch another Error : name 'f' is not defined

2. try...except...else...finally

• 如果try语句没有捕获到异常，执行完try代码后，执行else代码，最后执行finally代码
• 如果try语句捕获到异常，首先执行except语句处理异常，然后再执行finally代码
• try语句捕获到异常，except语句处理后，没有执行else语句，直接执行了finally语句

try:
    f = open('test.txt')
    number = int(f.read())
except ValueError as e:
    print('Catch an Error : ' + str(e))
except IOError as e:
    print('Catch an Error : ' + str(e))
else:
    print('No Error')
finally:
    print('File will be closed')
    f.close()

Catch an Error : invalid literal for int() with base 10: 'x123'
File will be closed

• try语句没有捕获到异常，会依次执行else语句和finally语句

try:
    f = open('test.txt')
    number = int(f.read())
except ValueError as e:
    print('Catch an Error : ' + str(e))
except IOError as e:
    print('Catch an Error : ' + str(e))
else:
    print('No Error')
finally:
    print('File will be closed')
    f.close()

No Error
File will be closed

第6章 标准和自定义异常

1. raise语句

• 作用：用于主动抛出异常
• 语法格式：raise [exception [(args)]]
• exception：异常的类
• args：描述异常信息的字符串元组
• 示例

In [1]: raise ValueError ('test is not defined')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-9-3e52ef4eb7b2> in <module>()
----> 1 raise ValueError ('test name is not defined')

ValueError: test name is not defined

2. assert语句

• 作用：是断言语句，用于检查表达式是否为真，如果为假会引发AssertionError错误，并抛出
• 语法格式：assert expression [, args]
• expression：表达式内容
• args：判断条件的字符串描述信息
• 应用场景：函数或类中的方法，对参数格式有严格的定义时，可以使用assert对传入的参数进行断言判断
• 示例

In [2]: assert 0, 'test 0 is False'
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-14-0c60f121c0f6> in <module>()
----> 1 assert 0, 'test 0 is False'

AssertionError: test 0 is False

3. 标准异常
   标准异常：Python内建的异常，程序执行前就已经存在

   
   
4. 自定义异常

• Python允许自定义异常，用于描述Python中没有涉及的异常情况
• 自定义异常必须继承Exception类
• 自定义异常，只能主动触发
• 简单demo

In [13]: class FileError(IOError):
    ...:     pass
    ...: 

In [14]: raise FileError ('test customize error')
---------------------------------------------------------------------------
FileError                                 Traceback (most recent call last)
<ipython-input-17-b91b6d033ece> in <module>()
----> 1 raise FileError ('test customize error')

• 示例

# 自定义错误异常
In [15]: class CustomizeError(Exception):    # 自定义的错误类继承于Exception基类
    ...:     def __init__(self, customize_error_info):    # 初始化实例。传入了一个参数：用户自定义错误的信息
    ...:         Exception.__init__(self)    # 初始化了Exception类，
    ...:         self.system_error_info = customize_error_info    # 把用户自定义的错误信息传入
    ...:         
    ...:     def __str__(self):    # 支撑print打印语句的方法
    ...:         return "Customize_error %s" % self.system_error_info    # 打印自定义错误的信息
    ...:     

# 调用自定义的错误异常，并使用try...except进行异常处理
In [16]: try:
    ...:     raise CustomizeError ('test_customeize_error')
    ...: except CustomizeError as e:
    ...:     print("Error_info : %s" % str(e))
    ...:     
Error_info : Customize_error test_customeize_error