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Handling exceptions

From: Python 3 Essential Training

Video: Handling exceptions

Exceptions are a primary method of handling errors in Python. So let's take a look at how we handle exceptions. We will make a working copy of exceptions.py, call it exceptions-working.py. We will open our working copy and here we have a very simple little script that reads lines of text from a file and there is the text file there and it prints them on the screen. Go ahead and we will run this and there is our lines of text. You'll notice, we did this in different way this time.

Handling exceptions

Exceptions are a primary method of handling errors in Python. So let's take a look at how we handle exceptions. We will make a working copy of exceptions.py, call it exceptions-working.py. We will open our working copy and here we have a very simple little script that reads lines of text from a file and there is the text file there and it prints them on the screen. Go ahead and we will run this and there is our lines of text. You'll notice, we did this in different way this time.

Instead of using end = a blank string, ctually using the strip method in the string itself and that just strips any trailing new lines from the end of the string. It's another way to do it, using the string method rather than using the print function to resolve that discrepancy. Now if I had typed the name wrong or if I am referring to a file that doesn't exist and I save this and I run it, you'll notice that we get one of Python's lovely little error messages and it has the Traceback (most recent call last).

It's doing what it can to help you to find the error in your code and then down at the bottom here, it says IOError and it has a message. This IOError before the colon is actually the name of the exception. That's the exception that's being raised by the Python interpreter and then after the colon is its error message. Now I can trap that exception. And then I could do something different with it in my code. So here is how I do that. I use try and I am just going to put that line there in the try and then I use except like this, and I can print a message.

I can say, "could not open the file." And then I don't want this to run, so I can put that in the else. Put the else there and so now what I have, I save this and run it. I will just get this little message, "could not open the file, come back tomorrow" and it doesn't try to read the lines in it. And if the typo is not there, I save that and run it, now I get the actual lines of text.

Now this will actually catch any error at all, because I just said except like that. If I want to just kept that particular error, I say except IOError, and if I save that, well let's go ahead and just type this again. Save that, run it, then I get that error. So it's sometimes useful to go ahead and let Python give you that error message, because that's the easy way to find out which error is going to raise. I can also get the error message itself. I can say as e: like this and then instead of come back tomorrow, I can give it that error message like this and then save and run and it says, "Could not open the file: Error 2 No such file or directory:".

And there we have something it's actually useful for the user. Now we can actually if we want to, we can put all of this code up here in the try and it doesn't hurt anything to do it that way and then we don't need an else clause. Save that and run it. We are still getting our error message. Let's go ahead and cut that out and we see that it works the same way. So in the event that you have more than one line of code in your try clause, execution will stop after the error is raised.

So the error gets raised here and then it will go right to handling the error in the except clause. So this line won't get run when the exception happens. So if we have this misspelling in here, you save it and run it, we get the error message and it doesn't try to use the file handle that it never got, because as soon as that exception is raised, which happens in this open function, then the exception clause is run right away. So you'll see this pattern commonly where a number of lines of code will be put in a try block and then in fact sometimes there will be several different except clauses as well, listing out a number of different errors that could be raised.

So this is how exceptions work in Python and this is how you handle exceptions in your code using try and except.

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This video is part of

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Python 3 Essential Training

87 video lessons · 43036 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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