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Raising exceptions

From: Python 3 Essential Training

Video: Raising exceptions

Python uses exceptions as its primary method of handling errors. We can raise our own exceptions in the functions and modules that we write. Let's take a look at how we do that. Make a working copy of exceptions.py, call it exceptions-working.py, and we'll go ahead and open our working copy, and here we have our simple print the lines of text from this file. Go ahead and run that. Now, let's say that we want it to write a function that will open a file and return its lines of text.

Raising exceptions

Python uses exceptions as its primary method of handling errors. We can raise our own exceptions in the functions and modules that we write. Let's take a look at how we do that. Make a working copy of exceptions.py, call it exceptions-working.py, and we'll go ahead and open our working copy, and here we have our simple print the lines of text from this file. Go ahead and run that. Now, let's say that we want it to write a function that will open a file and return its lines of text.

It's pretty easy little function, call it readfile, and we'll pass it a filename and open(filename), like that, and we'll return the readlines method from the filehandle and that's actually an Iterator and it allow us to do this. We can now get rid off that and here we can say readfile lines.text. Save that and run it and there it works exactly like we expect.

Now, let's say that we misspell lines.text here. We'll get our same error message that we got before, IOError, and of course we can handle that in a try block like this. And there we have our error handling, our exception catching, and there we get our error message like that, but let's say that we have another condition that we want to raise a different kind of an error for. We could check the filename for example, using the endswith method of the string object to see if it ends with .TXT. And in that case, we'll just go ahead and do what we normally do, an else. We're going to raise an exception.

We're going to use the raise statement, we're going to raise a ValueError and we're going to give it a little message, "Filename must end with .TXT," all right and then give it a different kind of a name here. We save this and run it. Now, we have an unhandled exception. We get exactly the same kind of error reporting that Python does when it raises its own exceptions and we see it gives us the trace back.

It's just an unhandled exception. That's what Python does with an unhandled exception. And so, we can handle that exception with a separate except, except ValueError as e and we can print bad filename and the message. So now when we save it and run it, we get this nice little error message here. So that's how we raise an exception in Python. You'll notice it's very simple. We use the raise keyword and we give it the exception.

Python has predefined a number of exceptions in its library, pretty much anything you are going to need, and on your screen is the URL where you can find that list with a lot of descriptions of them. In this way you don't need to define your own and your programs will work more consistently with the rest of the Python universe, by using the exception names that are already defined.

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This video is part of

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Python 3 Essential Training

87 video lessons · 38676 viewers

Bill Weinman
Author

 
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  1. 5m 14s
    1. Welcome
      1m 32s
    2. Understanding prerequisites for Python
      2m 4s
    3. Using the exercise files
      1m 38s
  2. 33m 29s
    1. Getting started with "Hello World"
      4m 43s
    2. Selecting code with conditionals
      4m 45s
    3. Repeating code with a loop
      4m 13s
    4. Reusing code with a function
      2m 43s
    5. Creating sequences with generator functions
      2m 46s
    6. Reusing code and data with a class
      4m 39s
    7. Greater reusability with inheritance and polymorphism
      7m 17s
    8. Handling errors with exceptions
      2m 23s
  3. 22m 32s
    1. Installing Python 3 and Eclipse for Windows
      11m 24s
    2. Installing Python 3 and Eclipse for Mac
      11m 8s
  4. 28m 0s
    1. Creating a main script
      3m 27s
    2. Understanding whitespace in Python
      4m 8s
    3. Commenting code
      3m 28s
    4. Assigning values
      3m 37s
    5. Selecting code and values with conditionals
      4m 46s
    6. Creating and using functions
      3m 54s
    7. Creating and using objects
      4m 40s
  5. 31m 23s
    1. Understanding variables and objects in Python
      2m 46s
    2. Distinguishing mutable and immutable objects
      2m 41s
    3. Using numbers
      3m 34s
    4. Using strings
      6m 38s
    5. Aggregating values with lists and tuples
      4m 55s
    6. Creating associative lists with dictionaries
      4m 24s
    7. Finding the type and identity of a variable
      4m 45s
    8. Specifying logical values with True and False
      1m 40s
  6. 9m 42s
    1. Selecting code with if and else conditional statements
      2m 22s
    2. Setting multiple choices with elif
      2m 14s
    3. Understanding other strategies for multiple choices
      2m 38s
    4. Using the conditional expression
      2m 28s
  7. 11m 26s
    1. Creating loops with while
      1m 27s
    2. Iterating with for
      3m 54s
    3. Enumerating iterators
      3m 22s
    4. Controlling loop flow with break, continue, and else
      2m 43s
  8. 23m 28s
    1. Performing simple arithmetic
      2m 14s
    2. Operating on bitwise values
      3m 30s
    3. Comparing values
      3m 32s
    4. Operating on Boolean values
      2m 59s
    5. Operating on parts of a container with the slice operator
      6m 52s
    6. Understanding operator precedence
      4m 21s
  9. 11m 34s
    1. Using the re module
      1m 4s
    2. Searching with regular expressions
      3m 12s
    3. Replacing with regular expressions
      3m 29s
    4. Reusing regular expressions with re.compile
      3m 49s
  10. 9m 10s
    1. Learning how exceptions work
      1m 18s
    2. Handling exceptions
      4m 15s
    3. Raising exceptions
      3m 37s
  11. 23m 1s
    1. Defining functions
      6m 23s
    2. Using lists of arguments
      2m 26s
    3. Using named function arguments
      4m 32s
    4. Returning values from functions
      1m 55s
    5. Creating a sequence with a generator function
      7m 45s
  12. 47m 29s
    1. Understanding classes and objects
      5m 12s
    2. Using methods
      6m 12s
    3. Using object data
      10m 4s
    4. Understanding inheritance
      5m 11s
    5. Applying polymorphism to classes
      7m 13s
    6. Using generators
      9m 48s
    7. Using decorators
      3m 49s
  13. 18m 54s
    1. Understanding strings as objects
      3m 25s
    2. Working with common string methods
      5m 24s
    3. Formatting strings with str.format
      5m 31s
    4. Splitting and joining strings
      2m 49s
    5. Finding and using standard string methods
      1m 45s
  14. 25m 27s
    1. Creating sequences with tuples and lists
      4m 6s
    2. Operating on sequences with built-in methods
      5m 50s
    3. Organizing data with dictionaries
      4m 56s
    4. Operating on character data with bytes and byte arrays
      10m 35s
  15. 11m 46s
    1. Opening files
      2m 4s
    2. Reading and writing text files
      4m 33s
    3. Reading and writing binary files
      5m 9s
  16. 21m 27s
    1. Creating a database with SQLite 3
      6m 56s
    2. Creating, retrieving, updating, and deleting records
      7m 31s
    3. Creating a database object
      7m 0s
  17. 18m 27s
    1. Using standard library modules
      8m 0s
    2. Finding third-party modules
      5m 47s
    3. Creating a module
      4m 40s
  18. 23m 11s
    1. Dealing with syntax errors
      8m 19s
    2. Dealing with runtime errors
      4m 0s
    3. Dealing with logical errors
      4m 22s
    4. Using unit tests
      6m 30s
  19. 19m 56s
    1. Normalizing a database interface
      6m 39s
    2. Deconstructing a database application
      8m 9s
    3. Displaying random entries from a database
      5m 8s
  20. 29s
    1. Goodbye
      29s

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