Python Study Notes (10): exception

Some exceptions may occur during program execution, which may exist. For example, when you read a file, the file does not exist. In this case, we can handle exceptions.

1. Error

Let's do a simple test. We intentionally write the print method into the print method.

>>> Print ("Hello World ")
Traceback (most recent call last ):
File "<pyshell #0>", line 1, in <module>
Print ("Hello World ")
Nameerror: Name 'print' is not defined
>>>

The system will throw a nameerror exception.

2. try... try t

We can use try... try t to handle exceptions.

try:    f = open("test.txt")    f.close()except(IOError):    print("The file is not exist.")except:    print("Some error occurred.")print("Done")

Running result:

The file is not exist.
Done

3. Exception

You can use the raise statement to raise an exception. First, we need to customize a shortinputerror exception and inherit the exception class.

class ShortInputError(Exception):    '''A user-defined exception class.'''    def __init__(self, length, atleast):        Exception.__init__(self)        self.length = length        self.atleast = atleasttry:    s = input("Enter something -->")    if len(s) < 3:        raise(ShortInputError(len(s), 3))    #Other work can continue as usual hereexcept(EOFError):    print("Why did you do an EOF on me?")except ShortInputError as e:    print("ShortInputError: The input was of length %d, \was expecting at least %d" % (e.length, e.atleast))else:    print("No exception was raised.")

Enter two characters to run the result:

>>>
Enter something --> tr
Shortinputerror: the input was of length 2, was expecting at least 3
>>>

Enter three or more characters to run the result:

>>>
Enter something --> Test
No exception was raised.
>>>

4. Try... finally

What should you do if you want to close the file regardless of whether an exception occurs? This can be done using finally blocks. Note: In a try block, you can use the limit t clause and the Finally block. If you want to use them at the same time, you need to embed one into another.

import timetry:    f = open("poem.txt")    while True: # our usual file-reading idiom        line = f.readline()        if len(line) == 0:            break        time.sleep(2)        print(line),finally:    f.close()    print("Cleaning up...closed the file")

Running result:

>>>
Programming is fun

When the work is done

If you wanna make your work also fun:

Use python!

Cleaning up... closed the file
>>>

def divide(x, y):    try:        result = x/ y    except ZeroDivisionError:        print("Division by zero!")    else:        print("result is", result)    finally:        print("executing finally clause")

Test results:

>>> Divide (2, 1)
Result is 2.0
Executing finally Clause
>>> Divide (2, 0)
Division by zero!
Executing finally Clause
>>> Divide ("2", "1 ")
Executing finally Clause
Traceback (most recent call last ):
File "<pyshell #13>", line 1, in <module>
Divide ("2", "1 ")
File "<pyshell #10>", line 3, in divide
Result = x/y
Typeerror: Unsupported operand type (s) for/: 'str' and 'str'
>>>