1 try-except

比如我們要打開(kāi)一個(gè)不存在的文件，它會(huì)報(bào)錯(cuò)

f=open('no.txt','r') f.read()''' --------------------------------------------------------------------------- FileNotFoundError Traceback (most recent call last) <ipython-input-54-2709fabe0da7> in <module> ----> 1 f=open('no.txt','r')2 f.read()FileNotFoundError: [Errno 2] No such file or directory: 'no.txt' '''

我們可以用try-except 實(shí)現(xiàn)

try:f=open('no.txt','r')print(f.read()) except FileNotFoundError as e:print(e)f=open('no.txt','w')f.write('new file')f.close()''' [Errno 2] No such file or directory: 'no.txt' '''

?但是如果我們except里面使用了不匹配的error 還是會(huì)報(bào)錯(cuò)

try:f=open('no1.txt','r')print(f.read()) except KeyError as e:print(e)f=open('no1.txt','w')f.write('new file')f.close()''' --------------------------------------------------------------------------- FileNotFoundError Traceback (most recent call last) <ipython-input-56-5e0e21b6da44> in <module>1 try: ----> 2 f=open('no1.txt','r')3 print(f.read())4 except KeyError as e:5 print(e)FileNotFoundError: [Errno 2] No such file or directory: 'no1.txt' '''

解決方法是使用基類(lèi)Error

try:f=open('no1.txt','r')print(f.read()) except BaseException as e:print(e)f=open('no1.txt','w')f.write('new file')f.close()

2 多except異常處理

d = {"name": "f1", "age": 2} l = [1,2,3]try:v = d["gender"]l[3] = 4 except KeyError as e:print("key error for:", e)d["gender"] = "x" except IndexError as e:print("index error for:", e)l.append(4) print(d) print(l)''' key error for: 'gender' {'name': 'f1', 'age': 2, 'gender': 'x'} [1, 2, 3] '''

????????但是此時(shí)有一點(diǎn)需要注意：它不會(huì)同時(shí)處理字典的?KeyError?和列表的?IndexError

????????因?yàn)樵诔绦蝽樞驁?zhí)行的時(shí)候，只要是報(bào)錯(cuò)了， 那么就會(huì)終止錯(cuò)誤之后的代碼，進(jìn)入錯(cuò)誤?回收?環(huán)節(jié)。這個(gè)回收環(huán)節(jié)在上面的案例中也就是?except?的錯(cuò)誤處理環(huán)節(jié)。?

? ? ? ?換句話(huà)說(shuō)，except語(yǔ)句只能執(zhí)行一個(gè)。

? ? ? ? 所以上述例子中，列表l并沒(méi)有被append

3 try-except-else 語(yǔ)句

try-except和前面的一樣，在?else?中處理沒(méi)有報(bào)錯(cuò)的情況。

l = [1,2,3] try:l[3] = 4 except IndexError as e:print(e) else:print("no error, now in else")print(l) ''' list assignment index out of range ''' #此時(shí)是有異常的，所以執(zhí)行的是except里面的語(yǔ)句l = [1,2,3,-4] try:l[3] = 4 except IndexError as e:print(e) else:print("no error, now in else")print(l) ''' no error, now in else [1, 2, 3, 4] ''' #此時(shí)沒(méi)有異常，所以執(zhí)行的是try和else里面的語(yǔ)句

4 try-except-finally 語(yǔ)句

try-except和前面的一樣，finally中的語(yǔ)句，不管有沒(méi)有異常，都會(huì)執(zhí)行

在有些情況下，加不加finally，好像沒(méi)啥區(qū)別

沒(méi)有異常：

l = [1,2,3,-1] try:l[3] = 4 except IndexError as e:print(e) finally:print("reach finally") ''' reach finally ''' l = [1,2,3,-1] try:l[3] = 4 except IndexError as e:print(e)print("reach finally") ''' reach finally '''

有異常：

l = [1,2,3] try:l[3] = 4 except IndexError as e:print(e)print("reach finally") ''' list assignment index out of range reach finally ''' l = [1,2,3] try:l[3] = 4 except IndexError as e:print(e) finally:print("reach finally") ''' list assignment index out of range reach finally '''

所以，finally語(yǔ)句什么使用需要用到呢？

我們看下面這個(gè)例子：

l = [1,2,3] try:l[3] = 4print("reach finally") '''File "<ipython-input-71-5a32d8788477>", line 6print("reach finally")^ SyntaxError: invalid syntax '''l = [1,2,3] try:l[3] = 4 finally:print("reach finally") ''' reach finally --------------------------------------------------------------------------- IndexError Traceback (most recent call last) <ipython-input-75-26ba6fe359fd> in <module>1 l = [1,2,3]2 try: ----> 3 l[3] = 44 finally:5 print("reach finally")IndexError: list assignment index out of range '''

雖然都是報(bào)錯(cuò)，但是加了finally之后，就是先執(zhí)行完finally中的語(yǔ)句，再報(bào)錯(cuò)

5 raise

手動(dòng)觸發(fā)異常

def no_negative(num):if num < 0:raise ValueError("I said no negative")return numprint(no_negative(-1))''' Traceback (most recent call last):File "/lib/python3.8/asyncio/futures.py", line 178, in resultraise self._exceptionFile "/lib/python3.8/asyncio/tasks.py", line 280, in __stepresult = coro.send(None)File "/lib/python3.8/site-packages/pyodide/_base.py", line 419, in eval_code_asyncreturn await CodeRunner(File "/lib/python3.8/site-packages/pyodide/_base.py", line 276, in run_asyncres = eval(last_expr, self.globals, self.locals)File "<exec>", line 6, in <module>File "<exec>", line 3, in no_negative ValueError: I said no negative '''

?注：這里raise 后面的error類(lèi)型，必須是python 中定義的異常錯(cuò)誤類(lèi)型名稱(chēng)（見(jiàn)第6小節(jié)）

6 python 異常錯(cuò)誤名稱(chēng)表

異常名稱(chēng)描述

BaseException	所有異常的基類(lèi)
SystemExit	解釋器請(qǐng)求退出
KeyboardInterrupt	用戶(hù)中斷執(zhí)行(通常是輸入^C)
Exception	常規(guī)錯(cuò)誤的基類(lèi)
StopIteration	迭代器沒(méi)有更多的值
GeneratorExit	生成器(generator)發(fā)生異常來(lái)通知退出
StandardError	所有的內(nèi)建標(biāo)準(zhǔn)異常的基類(lèi)
ArithmeticError	所有數(shù)值計(jì)算錯(cuò)誤的基類(lèi)
FloatingPointError	浮點(diǎn)計(jì)算錯(cuò)誤
OverflowError	數(shù)值運(yùn)算超出最大限制
ZeroDivisionError	除(或取模)零 (所有數(shù)據(jù)類(lèi)型)
AssertionError	斷言語(yǔ)句失敗
AttributeError	對(duì)象沒(méi)有這個(gè)屬性
EOFError	沒(méi)有內(nèi)建輸入,到達(dá)EOF 標(biāo)記
EnvironmentError	操作系統(tǒng)錯(cuò)誤的基類(lèi)
IOError	輸入/輸出操作失敗
OSError	操作系統(tǒng)錯(cuò)誤
WindowsError	系統(tǒng)調(diào)用失敗
ImportError	導(dǎo)入模塊/對(duì)象失敗
LookupError	無(wú)效數(shù)據(jù)查詢(xún)的基類(lèi)
IndexError	序列中沒(méi)有此索引(index)
KeyError	映射中沒(méi)有這個(gè)鍵
MemoryError	內(nèi)存溢出錯(cuò)誤(對(duì)于Python 解釋器不是致命的)
NameError	未聲明/初始化對(duì)象 (沒(méi)有屬性)
UnboundLocalError	訪問(wèn)未初始化的本地變量
ReferenceError	弱引用(Weak reference)試圖訪問(wèn)已經(jīng)垃圾回收了的對(duì)象
RuntimeError	一般的運(yùn)行時(shí)錯(cuò)誤
NotImplementedError	尚未實(shí)現(xiàn)的方法
SyntaxError	Python 語(yǔ)法錯(cuò)誤
IndentationError	縮進(jìn)錯(cuò)誤
TabError	Tab 和空格混用
SystemError	一般的解釋器系統(tǒng)錯(cuò)誤
TypeError	對(duì)類(lèi)型無(wú)效的操作
ValueError	傳入無(wú)效的參數(shù)
UnicodeError	Unicode 相關(guān)的錯(cuò)誤
UnicodeDecodeError	Unicode 解碼時(shí)的錯(cuò)誤
UnicodeEncodeError	Unicode 編碼時(shí)錯(cuò)誤
UnicodeTranslateError	Unicode 轉(zhuǎn)換時(shí)錯(cuò)誤
Warning	警告的基類(lèi)
DeprecationWarning	關(guān)于被棄用的特征的警告
FutureWarning	關(guān)于構(gòu)造將來(lái)語(yǔ)義會(huì)有改變的警告
OverflowWarning	舊的關(guān)于自動(dòng)提升為長(zhǎng)整型(long)的警告
PendingDeprecationWarning	關(guān)于特性將會(huì)被廢棄的警告
RuntimeWarning	可疑的運(yùn)行時(shí)行為(runtime behavior)的警告
SyntaxWarning	可疑的語(yǔ)法的警告
UserWarning	用戶(hù)代碼生成的警告

?參考內(nèi)容：如何控制異常 try-except - 交互式學(xué)Python | 莫煩Python (mofanpy.com)

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