Python異常處理

異常

異常產生，檢測到錯誤且直譯器認為是異常，丟擲異常；異常處理，截獲異常，忽略或終止程式處理異常。

首先給出兩個實驗txt檔案，內容如下：

try…except…

捕獲指定型別的異常

try:
    a
except NameError as e:   #捕獲NameError類的異常
    print('catch Error:',e)

print('exec over')

b=0
try:
    b
except NameError as e:
    print('catch Error:',e)

print('exec over')

捕獲所有型別的異常

try:
    undefvar   #之前沒定義過的變數
except:   #捕獲所有異常
    print('catch error')
#except Exception as e:
#Exception可以將所有的異常包括在內；將異常賦予變數e

指定捕獲異常型別與實際觸發異常型別不同

try:
    undefvar   #之前沒定義過的變數
except FileNotFoundError as e:   #捕獲FileNotFoundError異常
    print('catch error',e)

try:
    open('test1.txt')   #開啟之前不存在檔案
except FileNotFoundError as e:   #捕獲FileNotFoundError異常
    print('catch error',e)

捕獲多種異常型別

try:
    f=open('one.txt')
    line=f.read(2)   #讀取檔案one.txt裡的前兩個字元
    numd=int(line)
    print('read num=%d' % numd)
except FileNotFoundError as e:
    print('catct FileNotFoundError',e)
except ValueError as e:
    print('ValueError',e)

try…except…else…

try語句捕獲到異常執行except語句
try語句沒有捕獲到異常執行else語句

try:
    f=open('changeone.txt')
    line=f.read(2)   #讀取檔案changeone.txt裡的前兩個字元
    numd=int(line)
    print('read num=%d' % numd)
except FileNotFoundError as e:
    print('catct FileNotFoundError',e)
except ValueError as e:
    print('ValueError',e)
else:
    print('No Error')

try:
    f=open('changeone1.txt')
    line=f.read(2)   #讀取檔案changeone.txt裡的前兩個字元
    numd=int(line)
    print('read num=%d' % numd)
except FileNotFoundError as e:
    print('catct FileNotFoundError',e)
except ValueError as e:
    print('ValueError',e)
else:
    print('No Error')

try…finally…

如果try沒有捕獲到異常，先執行finally後的程式碼塊；
如果try捕獲到異常，先執行finally後的程式碼塊，由python直譯器處理捕獲到的異常

try-finally語句：無論是否檢測到異常，都會執行finally程式碼；不是為解釋異常而設計，主要是做一些清理工作，關閉檔案或釋放系統資源

try:
    f=open('one.txt')
    print(int(f.read()))
finally:
    print('close file')
    f.close()

組合

try-except-finally

若try沒有捕獲到異常，執行完try程式碼塊後，執行finally
若try捕獲到異常，首先執行except處理錯誤，然後執行finally

try:
    ft=open('changeone.txt')
    line=ft.read(2)   #讀取檔案changeone.txt裡的前兩個字元
    numd=int(line)
    print('read num=%d' % numd)
except FileNotFoundError as e:
    print('catct FileNotFoundError',e)
except ValueError as e:
    print('ValueError',e)
finally:
    print('close file')
    ft.close()

try:
    ff=open('changeone1.txt')   #open時發生錯誤，f變數定義失敗
    line=ff.read(2)   
    numd=int(line)
    print('read num=%d' % numd)
except FileNotFoundError as e:
    print('catct FileNotFoundError',e)
except ValueError as e:
    print('ValueError',e)
finally:
    print('close file')
    ff.close()

try:
    ff=open('changeone1.txt')   #open時發生錯誤，f變數定義失敗
    line=ff.read(2)   
    numd=int(line)
    print('read num=%d' % numd)
except FileNotFoundError as e:
    print('catct FileNotFoundError',e)
except ValueError as e:
    print('ValueError',e)
finally:
    try:
        print('close file')
        ff.close()
    except NameError as e:   #捕獲變數ff的錯誤
        print('NameError:,',e)

try-except-else-finally

若try沒有捕獲到異常，執行完try程式碼段後，執行else程式碼段，最後執行finally
若try捕獲到異常，首先執行except處理錯誤，然後執行finally

try:
    feef=open('changeone.txt')   #open時正常讀入，沒發生錯誤
    line=feef.read(2)   
    numd=int(line)
    print('read num=%d' % numd)
except FileNotFoundError as e:
    print('catct FileNotFoundError',e)
except ValueError as e:
    print('ValueError',e)
else:
    print('No Error')
finally:
    try:
        print('close file')
        feef.close()
    except NameError as e:
        print('NameError:,',e)

with語句

with語句，作用等同於try-except-finally語句的

import os
try:
    with open('one.txt') as fw:
        print('in with fw.read:',fw.read())
        fw.seek(-5,os.SEEK_SET)   #出現異常
except ValueError as e:
    print('in with catch ValueError:',e)
    print('with:',fw.closed)