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Exception Handling in python

2016-07-14

1. Catch Exception

1.1 Basic Grammar

Put the code that may cause exception into the block after try, and then catch it with except block, if the exception type match, the execute the code block inside except.

try:
    # write some code
    # that might throw exception
except <ExceptionType>:
    # Exception handler, alert the user

For example, reading a file that does not exist will cause IOError, we can plan ahead for this error.

try:
    f = open('nofile.txt', 'r')
    print f.read()
    f.close()
except IOError:
    print 'file not found'
file not found

The flow is as listed:

  1. First statement between try and except block are executed.
  2. If no exception occurs then code under except clause will be skipped.
  3. If file don’t exists then exception will be raised and the rest of the code in the try block will be skipped
  4. When exceptions occurs, if the exception type matches exception name after except keyword, then the code in that except clause is executed.

1.2 More exception types

try:
    <body>
except <ExceptionType1>:
    <handler1>
except <ExceptionTypeN>:
    <handlerN>
except:
    <handlerExcept>
else:
    <process_else>
finally:
    <process_finally>
  1. except acts like elif,when trythrough an error,match every exception after except keyword. If match, then execute; else execute the one without specified exception type.

  2. else block is executed only when try through no error。

  3. finally will always be executed, with or without exception。

1.3 Example

num1, num2 = 1,0
try:
    result = num1 / num2
    print("Result is", result)

except ZeroDivisionError:
    print("Division by zero is error !!")

except:
    print("Wrong input")

else:
    print("No exceptions")

finally:
    print("This will execute no matter what you input")
Division by zero is error !!
This will execute no matter what you input

2. Throw erroe

Use raise to throw self defined exception。raise ExceptionClass('Your argument')

def enterage(age):
    if age < 0:
        raise ValueError("Only positive integers are allowed")

    if age % 2 == 0:
        print("age is even")

try:
    num = int(input("Enter your age: "))
    enterage(num)
except ValueError:
    print 'Only positive integers are allowd'
except:
    print 'Something went wrong'
Enter your age: -3
Only positive integers are allowd

More information onDocuments
exception

3. Manipulate exception

We can pass the exception to a variable, and it’s pretty easy.

try:
    # this code is expected to throw exception
except ExceptionType as ex:
    # code to handle exception
try:
    number = int(input("Enter a number: "))
    print "The number entered is", number

except NameError as ex:
    print "Exception:", ex
Enter a number: one
Exception: name 'one' is not defined

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