359 lines
8.0 KiB
Markdown
359 lines
8.0 KiB
Markdown
# 异常处理
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## 异常和错误
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### 程序中难免出现错误，而错误分成两种
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1. 语法错误（这种错误，根本过不了python解释器的语法检测，必须在程序执行前就改正）
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```python
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#语法错误示范一
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if
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if
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^
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SyntaxError: invalid syntax
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#语法错误示范二
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def test:
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pass
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def test:
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^
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SyntaxError: expected '('
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#语法错误示范三
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print("hello"
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print("hello"
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^
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SyntaxError: '(' was never closed
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```
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### 逻辑错误
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```python
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#用户输入不完整(比如输入为空)或者输入非法(输入不是数字)
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num=input(">>: ")
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res1 = int(num)
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ret = int(num)
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^^^^^^^^
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ValueError: invalid literal for int() with base 10: 'abc'
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#无法完成计算
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res1=1/0
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a = 1/0
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~^~
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ZeroDivisionError: division by zero
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res2=1+'str'
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a = "hello" + 1
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~~~~~~~~^~~
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TypeError: can only concatenate str (not "int") to str
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```
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### 异常
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异常就是程序运行时发生错误的信号
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异常之后的代码就不执行
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### 异常种类
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在python中不同的异常可以用不同的类型（python中统一了类与类型，类型即类）去标识，不同的类对象标识不同的异常，一个异常标识一种错误
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```python
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# 触发IndexError
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l=['eagle','aa']
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l[3]
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# 触发KeyError
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dic={'name':'eagle'}
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dic['age']
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#触发ValueError
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s='hello'
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int(s)
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```
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常见异常
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| 异常 | 描述 |
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| :---------------- | :----------------------------------------------------------- |
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| AttributeError | 试图访问一个对象没有的属性，比如foo.x，但是foo没有属性x |
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| IOError | 输入/输出异常；基本上是无法打开文件 |
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| ImportError | 无法引入模块或包；基本上是路径问题或名称错误 |
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| IndentationError | 语法错误（的子类） ；代码没有正确对齐 |
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| IndexError | 下标索引超出序列边界，比如当x只有三个元素，却试图访问x[5] |
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| KeyError | 试图访问字典里不存在的键 |
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| KeyboardInterrupt | Ctrl+C被按下 |
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| NameError | 使用一个还未被赋予对象的变量 |
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| SyntaxError | Python代码非法，代码不能编译(个人认为这是语法错误，写错了） |
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| TypeError | 传入对象类型与要求的不符合 |
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| UnboundLocalError | 试图访问一个还未被设置的局部变量，基本上是由于另有一个同名的全局变量，导致你以为正在访问它 |
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| ValueError | 传入一个调用者不期望的值，即使值的类型是正确的 |
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其他错误
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```python
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ArithmeticError
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AssertionError
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AttributeError
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BaseException
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BufferError
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BytesWarning
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DeprecationWarning
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EnvironmentError
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EOFError
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Exception
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FloatingPointError
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FutureWarning
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GeneratorExit
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ImportError
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ImportWarning
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IndentationError
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IndexError
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IOError
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KeyboardInterrupt
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KeyError
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LookupError
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MemoryError
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NameError
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NotImplementedError
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OSError
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OverflowError
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PendingDeprecationWarning
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ReferenceError
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RuntimeError
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RuntimeWarning
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StandardError
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StopIteration
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SyntaxError
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SyntaxWarning
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SystemError
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SystemExit
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TabError
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TypeError
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UnboundLocalError
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UnicodeDecodeError
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UnicodeEncodeError
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UnicodeError
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UnicodeTranslateError
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UnicodeWarning
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UserWarning
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ValueError
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Warning
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ZeroDivisionError
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```
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## 异常处理
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- python解释器检测到错误，触发异常（也允许程序员自己触发异常）
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- 程序员编写特定的代码，专门用来捕捉这个异常（这段代码与程序逻辑无关，与异常处理有关）
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- 如果捕捉成功则进入另外一个处理分支，执行你为其定制的逻辑，使程序不会崩溃，这就是异常处理
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**首先须知，异常是由程序的错误引起的，语法上的错误跟异常处理无关，必须在程序运行前就修正**
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```python
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num1=input('>>: ') #输入一个字符串试试
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if num1.isdigit():
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int(num1) #我们的正统程序放到了这里,其余的都属于异常处理范畴
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elif num1.isspace():
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print('输入的是空格,就执行我这里的逻辑')
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elif len(num1) == 0:
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print('输入的是空,就执行我这里的逻辑')
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else:
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print('其他情情况,执行我这里的逻辑')
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'''
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问题一：
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使用if的方式我们只为第一段代码加上了异常处理，但这些if，跟你的代码逻辑并无关系，这样你的代码会因为可读性差而不容易被看懂
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问题二：
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这只是我们代码中的一个小逻辑，如果类似的逻辑多，那么每一次都需要判断这些内容，就会倒置我们的代码特别冗长。
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'''
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```
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总结：
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1. if判断式的异常处理只能针对某一段代码，对于不同的代码段的相同类型的错误你需要写重复的if来进行处理。
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2. 在你的程序中频繁的写与程序本身无关，与异常处理有关的if，会使得你的代码可读性极其的差
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3. if是可以解决异常的，只是存在1,2的问题，所以，千万不要妄下定论if不能用来异常处理。
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```python
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def test():
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print('test.runing')
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choice_dic = {
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'1':test
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}
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while True:
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choice = (input('>>: ').strip())
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if not choice or choice not in choice_dic:continue
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choice_dic[choice]()
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```
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**python：为每一种异常定制了一个类型，然后提供了一种特定的语法结构用来进行异常处理**
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### 基本语法
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```python
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try:
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被检测的代码块
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except 异常类型：
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try中一旦检测到异常，就执行这个位置的逻辑
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```
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将文件的每一行变成一个迭代器，然后读出来
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```python
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f = open('a.txt')
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g = (line.strip() for line in f)
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for line in g:
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print(line)
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else:
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f.close()
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```
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但是如果超出了迭代器的范围就会出现`StopIteration`错误
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使用异常处理
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```python
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try:
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f = open('a.txt')
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g = (line.strip() for line in f)
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print(next(g))
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print(next(g))
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print(next(g))
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print(next(g))
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print(next(g))
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print(next(g))
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print(next(g))
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print(next(g))
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print(next(g))
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print(next(g))
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except StopIteration:
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f.close()
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print('读取出错')
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```
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### 异常类只能用来处理指定的异常情况
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```python
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s1 = 'hello'
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try:
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int(s1)
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except IndexError as e:
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print e
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```
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### 多分支
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主要是用来针对不同的错误情况进行错误处理
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```python
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s1 = 'hello'
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try:
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int(s1)
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except IndexError as e:
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print(e)
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except KeyError as e:
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print(e)
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except ValueError as e:
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print(e)
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```
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### 万能异常:Exception
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```python
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s1 = 'hello'
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try:
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int(s1)
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except Exception as e:
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print(e)
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```
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多分支加万能异常
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```python
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s1 = 'hello'
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try:
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int(s1)
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except IndexError as e:
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print(e)
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except KeyError as e:
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print(e)
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except ValueError as e:
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print(e)
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except Exception as e:
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print(e)
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```
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## 其他异常情况
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```python
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s1 = '10'
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try:
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int(s1)
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except IndexError as e:
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print(e)
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except KeyError as e:
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print(e)
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except ValueError as e:
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print(e)
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except Exception as e:
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print(e)
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else:
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print('try内代码块没有异常则执行我')
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finally:
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print('无论异常与否,都会执行该模块,通常是进行清理工作')
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```
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### 主动触发异常
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```python
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try:
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raise TypeError('类型错误')
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except Exception as e:
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print(e)
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```
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### 自定义异常
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```python
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class EvaException(BaseException):
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def __init__(self,msg):
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self.msg=msg
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def __str__(self):
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return self.msg
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try:
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raise EvaException('类型错误')
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except EvaException as e:
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print(e)
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```
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### 断言
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表达式为True时，程序继续运行，表达式为False时程序终止运行，并报AssertionError错误
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```python
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assert 1 == 1
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assert 1 == 2
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```
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### try..except的方式比较if的方式的好处
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1. 把错误处理和真正的工作分开来
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2. 代码更易组织，更清晰，复杂的工作任务更容易实现
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3. 毫无疑问，更安全了，不至于由于一些小的疏忽而使程序意外崩溃了
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