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Python - Exception

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• Python - Exception
  • Try Except
  • Exception Handling
  • Raise an exception
  • Customized Exception Class
  • With Statement

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Try Except

• The try block: test a block of code for errors.
• The except block: handle the error.
• The else block: execute code when there is no error.
• The finally block: execute code, regardless of the result of the try and except blocks.

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Exception Handling

• When an error (or called as exception) occurs, Python will normally stop and generate an error message.

• Since the try block raises an error, the except block will be executed.

  • Without the try block, the program will crash and raise an error.

• define exception blocks

  • if to execute a special block of code for a special kind of error, can define as many exception blocks.

  try:
    print(x)    # x is not defined
  except NameError:
      print("Variable x is not defined")  # execute
  except:
      print("An exception occurred")
  

• Else

  • define a block of code to be executed if no errors were raised

    print("\n--------else--------\n")
    try:
      print("Hello")
    except:
      print("Something went wrong")
    else:
      print("Nothing went wrong")         # executed
    

• Finally

  • finally block, if specified, will be executed regardless if the try block raises an error or not.
  • to close objects and clean up resources: The program can continue, without leaving the file object open.

  print("\n--------finally--------\n")
  
  try:
      print(x)
  except:
      print("Something went wrong")
  finally:
      print("The 'try except' is finished")       # executed
  
  
  print("\n--------finally: close file--------\n")
  try:
      f = open("./demofile.txt")
      try:
          f.write("Lorum Ipsum")
      except:
          print("Something went wrong when writing to the file")
      finally:
          f.close()
  except:
      print("Something went wrong when opening the file")
  

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Raise an exception

• raise to throw (or raise) an exception

print("\n--------raise--------\n")

x = -1
try:
    if x < 0:
        raise Exception("Sorry, no numbers below zero")
except Exception as ex:
    print(ex)           # Sorry, no numbers below zero

x = "hello"
try:
    if not type(x) is int:
        raise TypeError("Only integers are allowed")
except TypeError as ex:
    print("TypeError: {ex}".format(ex=ex))      # TypeError: Only integers are allowed
except Exception as ex:
    print("Exception: {ex}".format(ex=ex))

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Customized Exception Class

• Inheriate from Exception class

class CustomizedException(Exception):
    print("inside")
    pass


try:
    raise CustomizedException()
except CustomizedException:
    print("Raised customized exception.")

# inside
# Raised customized exception.

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With Statement

print("\n--------Example: with statement--------\n")

print("\n--------try except")
filename = "test.txt"


try:
    file = open(filename, 'r')
    try:
        content = file.read()
        print(content)
    except Exception as err:
        print(err)
    finally:
        file.close()
except FileNotFoundError as err:
    print(err)
else:
    print("success")


print("\n--------with statement")
try:
    with open(filename, 'r') as file:
        content = file.read()
        print(content)
except Exception as err:
    print(err)
else:
    print("success")

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