Exceptions and File I/O

Documentation

Docstrings

• Ex:

   def send_message(sender, recipient, message_body, priority=1):
    """Send a message to a recipient
  
    :param str sender: The person sending the message
    :param str recipient: The recipient of the message
    :param str message_body: The body of the message
    :param priority: The priority of the message, can be a number 1-5
    :type priority: integer or None
    :return: the message id
    :rtype: int
    :raises ValueError: if the message_body exceeds 160 characters
    :raises TypeError: if the message_body is not a basestring
    """
    # Function definition
  

• expected to document code on class assignments
• generates web pages and stuff

Exception Handling

Error types

• Syntax errors
  • your code is bad and you should feel bad
  • Python cannot read it
  • Syntax check prior to interpreting code
• Runtime Errors
  • Syntax is fine, logic is bad
  • Python can’t execute it
  • Ex:

    for x in [1,2,3]:
        pront(x)
    

  • Not found until code is interpreted
  • RaiseExceptions

Handling?

• whenever a runtime error occurs, an exception is raised

• helps us track down and fix logical errors

• How they work:
• whenever an exception is raised, it “bubbles up” through the call stack until it is caught or it reaches the top of the call stack
• Ex:

  def print_name(person):
    try:
        print(person["name"])
    except KeyError:
        print("Person has no name.")
  

• All exceptions are instances of classes that derive from BaseException

• Most exceptions you’ll use derive from Exception

• There’s a big list of built-in exceptions

• You can also define your own.

• You can keep the caught exception as a var
• useful for errors & debugging
• Ex:

   try:
    f = open("file.txt")
   except FiltNotFoundError as e:
    print("Caught:",e)
   else:
    #runs if no exceptions raised and handled
   finally:
    #runs no matter what
  

• Raising exceptions
• raise valueError("Frog")

File I/O ——–

 f = open.("file.txt")
 contents = f.read()
 print(contents)
 f.close()

or

 with open("file.txt") as f:
     print(f.read())
#with closes automatically when you outdent