Chapter 7: Error and Exception Handling in Python¶

Writing robust Python programs necessitates an understanding of how to handle errors and exceptions.
Python provides a powerful set of tools for controlling the flow of a program when an error occurs, preventing the program from terminating unexpectedly.
In this chapter, we will explore Python's error and exception handling mechanisms, including built-in exceptions, the try-except block, and how to define your own exceptions.

1. Basic Try-Except Block¶

he expression of try...except as following

try:
    # the main body
except(Error1,Error2,...) as e:
    alert message
goto retry

If Python gets an exception but not be cached by an except module, the entire program will crash

Here is a simple go game example

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board_size = 5
board = []
def initBoard():
    for i in range(board_size):
        row  = ['+'] * board_size
        board.append(row)

def printBoard():
    for i in range(board_size):
        for j in range(board_size):
            print(board[i][j], end = '')
        # equal to print('\n')
        print()
initBoard()
printBoard()

inputStr = input('Please input the coordinates with "x,y" style: \n')
while inputStr!= 'Done':
    try:
        x_str,y_str = inputStr.split(sep=',')
        if board[int(y_str)-1][int(x_str) - 1] != '+':
            inputStr = input('There already have a piece, please re-input')
            continue
        board[int(y_str)-1][int(x_str)-1] = '●'
        printBoard()
    except Exception:
        inputStr = input('Your input is illegal, please re-input')
        continue
board_size = 5
board = []
def initBoard():
    for i in range(board_size):
        row  = ['+'] * board_size
        board.append(row)

def printBoard():
    for i in range(board_size):
        for j in range(board_size):
            print(board[i][j], end = '')
        # equal to print('\n')
        print()
initBoard()
printBoard()

inputStr = input('Please input the coordinates with "x,y" style: \n')
while inputStr!= 'Done':
    try:
        x_str,y_str = inputStr.split(sep=',')
        if board[int(y_str)-1][int(x_str) - 1] != '+':
            inputStr = input('There already have a piece, please re-input')
            continue
        board[int(y_str)-1][int(x_str)-1] = '●'
        printBoard()
    except Exception:
        inputStr = input('Your input is illegal, please re-input')
        continue

+++++
+++++
+++++
+++++
+++++
●++++
+++++
+++++
+++++
+++++
●++++
+++++
+●+++
+++++
+++++

---------------------------------------------------------------------------
KeyboardInterrupt                         Traceback (most recent call last)
Cell In[1], line 27
     25     printBoard()
     26 except Exception:
---> 27     inputStr = input('Your input is illegal, please re-input')
     28     continue

File d:\Anaconda3\envs\mybase\Lib\site-packages\ipykernel\kernelbase.py:1191, in Kernel.raw_input(self, prompt)
   1189     msg = "raw_input was called, but this frontend does not support input requests."
   1190     raise StdinNotImplementedError(msg)
-> 1191 return self._input_request(
   1192     str(prompt),
   1193     self._parent_ident["shell"],
   1194     self.get_parent("shell"),
   1195     password=False,
   1196 )

File d:\Anaconda3\envs\mybase\Lib\site-packages\ipykernel\kernelbase.py:1234, in Kernel._input_request(self, prompt, ident, parent, password)
   1231 except KeyboardInterrupt:
   1232     # re-raise KeyboardInterrupt, to truncate traceback
   1233     msg = "Interrupted by user"
-> 1234     raise KeyboardInterrupt(msg) from None
   1235 except Exception:
   1236     self.log.warning("Invalid Message:", exc_info=True)

KeyboardInterrupt: Interrupted by user

2. `Exception` in Python¶

Python exceptions are organized in a hierarchy, allowing you to catch specific errors or groups of errors.

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import sys
try:
    a = int(sys.argv[1])
    b = int(sys.argv[2])
    c = a / b
    print('The quotient is :',c)
except IndexError:
    print('index error: not enough input parameters')
except ValueError:
    print('Value Error: could only input integers')
except ArithmeticError:
    print('The denominator is 0')
except Exception:
    print('Unkown error')
import sys
try:
    a = int(sys.argv[1])
    b = int(sys.argv[2])
    c = a / b
    print('The quotient is :',c)
except IndexError:
    print('index error: not enough input parameters')
except ValueError:
    print('Value Error: could only input integers')
except ArithmeticError:
    print('The denominator is 0')
except Exception:
    print('Unkown error')

Value Error: could only input integers

3. Handling multiple errors¶

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try:
    a = int(sys.argv[1])
    b = int(sys.argv[2])
    c = a / b
    print('The quotient is :',c)
except(IndexError,ValueError,ArithmeticError):
    print('one of IndexError, ValueError and ArithmeticError happened')
except:
    print('unknown error')
try:
    a = int(sys.argv[1])
    b = int(sys.argv[2])
    c = a / b
    print('The quotient is :',c)
except(IndexError,ValueError,ArithmeticError):
    print('one of IndexError, ValueError and ArithmeticError happened')
except:
    print('unknown error')

one of IndexError, ValueError and ArithmeticError happened

4. Access error messages¶

Every instance of Exception contains the following properties:

args: The index of the exception and it's descriptions
errno: The index of the exception
strerror: The description string of the exception
with_traceback(): See the trajectory of the exception

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def foo():
    try:
        fis = open('a.text')
    except Exception as e:
        # Get the index of the exception and it's descriptions
        print(e.args)
        # print(e.with_traceback())
        print(e.__repr__())
        print(e.errno)
foo()
def foo():
    try:
        fis = open('a.text')
    except Exception as e:
        # Get the index of the exception and it's descriptions
        print(e.args)
        # print(e.with_traceback())
        print(e.__repr__())
        print(e.errno)
foo()

(2, 'No such file or directory')
FileNotFoundError(2, 'No such file or directory')
2

5. `else` module¶

If there is no error within the try section, the program would go to the else part

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s = input('Please input the denominator: ')
try:
    result = 20/ int(s)
    print('the quotient between 20 and %s is %g' %(s,result))
except ValueError:
    print('input error')
except ArithmeticError:
    print('Could not input 0')
else:
    print('No exception')
s = input('Please input the denominator: ')
try:
    result = 20/ int(s)
    print('the quotient between 20 and %s is %g' %(s,result))
except ValueError:
    print('input error')
except ArithmeticError:
    print('Could not input 0')
else:
    print('No exception')

the quotient between 20 and 5 is 4
No exception

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def else_test():
    s = input('Please input the denominator: ')
    result = 20 / int(s)
    print('the quotient between 20 and %s is %g' %(s,result))
def right_main():
    try:
        print('try section, no exception')
    except:
        print('Error!')
    else:
        else_test()
def wrong_main():
    try:
        print('try section, no exception')
        else_test()
    except:
        print('Error!')
wrong_main()
print('--------------------------------')
right_main()
def else_test():
    s = input('Please input the denominator: ')
    result = 20 / int(s)
    print('the quotient between 20 and %s is %g' %(s,result))
def right_main():
    try:
        print('try section, no exception')
    except:
        print('Error!')
    else:
        else_test()
def wrong_main():
    try:
        print('try section, no exception')
        else_test()
    except:
        print('Error!')
wrong_main()
print('--------------------------------')
right_main()

try section, no exception
Error!
--------------------------------
try section, no exception

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
Cell In[13], line 20
     18 wrong_main()
     19 print('--------------------------------')
---> 20 right_main()

Cell In[13], line 11, in right_main()
      9     print('Error!')
     10 else:
---> 11     else_test()

Cell In[13], line 3, in else_test()
      1 def else_test():
      2     s = input('Please input the denominator: ')
----> 3     result = 20 / int(s)
      4     print('the quotient between 20 and %s is %g' %(s,result))

ZeroDivisionError: division by zero

6. `finally` module¶

No matter there is error within try section or not, the code in final module would be executed.
Because of that, do not use return or raise in finally part

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import os
def test():
    fis = None
    try:
        fis = open('a.text')
    except OSError as e:
        print(e.strerror)
        return
    finally:
        print('Perform finally part!')
        if fis is not None:
            try:
                fis.close()
            except OSError as ios:
                print(ios.strerror)
test()
import os
def test():
    fis = None
    try:
        fis = open('a.text')
    except OSError as e:
        print(e.strerror)
        return
    finally:
        print('Perform finally part!')
        if fis is not None:
            try:
                fis.close()
            except OSError as ios:
                print(ios.strerror)
test()

No such file or directory
Perform finally part!

7. Raise an exception¶

raise: would raise a RuntimeError or get the error in the except part
raise xxxError

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def main():
    try:
        mtd(3)
    except Exception as e:
        print('Get an error')
        print(e)
    
def mtd(a):
    if a > 0:
        raise ValueError('a > 0!')
main()
def main():
    try:
        mtd(3)
    except Exception as e:
        print('Get an error')
        print(e)
    
def mtd(a):
    if a > 0:
        raise ValueError('a > 0!')
main()

Get an error
a > 0!

8. Self Defined Exception, raise and except¶

8.1 Self defined exception¶

Every error objects should be the subclass of Exception

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class AuctionException(Exception): pass
raise AuctionException
class AuctionException(Exception): pass
raise AuctionException

---------------------------------------------------------------------------
AuctionException                          Traceback (most recent call last)
Cell In[20], line 2
      1 class AuctionException(Exception): pass
----> 2 raise AuctionException

AuctionException:

8.2 `except` and `raise`¶

Transfer system-defined error to self defined error

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class AuctionTest:
    def __init__(self,init_price):
        self.init_price = init_price
    def bid(self,bid_price):
        d = 0.0
        try:
            d = float(bid_price)
        except Exception as e:
            print('Find a error, ',e)
            raise AuctionException('The bid price must be a number!')
        if self.init_price > d:
            raise AuctionException('The bid price is too low!')
        init_price = d

        
def main():
    at = AuctionTest(200)
    try:
        at.bid('df')
    except AuctionException as e:
        print('The error in the main body,',e)

main()
class AuctionTest:
    def __init__(self,init_price):
        self.init_price = init_price
    def bid(self,bid_price):
        d = 0.0
        try:
            d = float(bid_price)
        except Exception as e:
            print('Find a error, ',e)
            raise AuctionException('The bid price must be a number!')
        if self.init_price > d:
            raise AuctionException('The bid price is too low!')
        init_price = d

        
def main():
    at = AuctionTest(200)
    try:
        at.bid('df')
    except AuctionException as e:
        print('The error in the main body,',e)

main()

Find a error,  could not convert string to float: 'df'
The error in the main body, The bid price must be a number!