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Python 3 Essential Training
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Creating a sequence with a generator function


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Python 3 Essential Training

with Bill Weinman

Video: Creating a sequence with a generator function

A generator function is a function that returns an iterator object. So this is how you create functionality that can be used in a for loop or any place an iterator is allowable in Python. So let's go ahead and create a generator object. We will make a working copy of generator.py, and we'll go ahead and open that up. And here we have an example of using the range object.
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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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Python 3 Essential Training
6h 36m Beginner Jul 29, 2010

Viewers: in countries Watching now:

Due to its power, simplicity, and complete object model, Python has become the scripting language of choice for many large organizations, including Google, Yahoo, and IBM. In Python 3 Essential Training, Bill Weinman demonstrates how to use Python 3 to create well-designed scripts and maintain existing projects. This course covers the basics of the language syntax and usage, as well as advanced features such as objects, generators, and exceptions. Example projects include a normalized database interface and a complete working CRUD application. Exercise files accompany the course.

Topics include:
  • A Python 3 quick start for experienced developers
  • Creating functions and objects
  • Using Python’s built-in objects and classes
  • Repeating code with loops and iterators
  • Understanding and using conditional expressions
  • Creating sequences with generators
  • Reusing code with objects and libraries
  • Handling errors with exceptions
Subjects:
Developer Web Programming Languages Web Development
Software:
Python
Author:
Bill Weinman

Creating a sequence with a generator function

A generator function is a function that returns an iterator object. So this is how you create functionality that can be used in a for loop or any place an iterator is allowable in Python. So let's go ahead and create a generator object. We will make a working copy of generator.py, and we'll go ahead and open that up. And here we have an example of using the range object.

And if we run this, you'll notice that we get our range of numbers up to and not including the 25, because the range object is naturally noninclusive. So a lot of times you might want an inclusive range object. So let's go ahead and create one. We'll call this inclusive_range, and we'll define a function. Call it inclusive_range. And the range object takes three arguments: start, stop, and step.

And we'll go ahead and say i=start, while i<=stop, and that will make it inclusive. i+=step. And then just before we increment, we're going to return, although we're not going to use return. We're going to yield i. What that does is it returns I, but because we're using yield instead of return, the next time the function is called execution will continue right after the yield statement.

So the next thing that will happen is i will get incremented by step and then the loop will be tested again. And then assuming that the loop condition is still true, yield will be called again and it will yield another iteration in the sequence. So again, what makes the yield different than return is that as the function gets called over and over again, each time execution begins right after the yield and continues as if the function were running continually.

And yield returns each time the next item in the sequence. So let's go ahead and run this. In order to run it, we are going to need to give it a start and a step. So we'll start at 0, and we'll step by 1, and we should get the same sequence, but including a 25 at the end. Save it and run it, and there we are, got the same sequence, and there is our 25 at the end. So that is how yield works and that is how you create a generator.

Let's go ahead and exercise all the techniques we've been learning in this function chapter and make this function work exactly like range does. Range can be called with one, two, or three arguments. If it's called with just one argument, stop is the value that's used and start is given a default value of 0 and step is given a default value of 1. But Python doesn't really have a way to do that. If I give this one here a default value of 0, I say start=0, and I give step a default value of 1, and I call this with just the 25 by itself.

Save that and run it. We get this SyntaxError. Non-default argument follows default argument. So this is not allowable in Python. The only way we can really do this is with a tuple. So let's go ahead and process our arguments. Let's start by taking a number of arguments, because we are going to different things with different numbers of arguments. numargs = the length of tuple, and if numargs<1, we're going to raise an error.

Requires at least one argument. And then elif numargs == 1, and we'll just set these up, elif numargs == 2, elif numargs == 3, or else, and we'll raise another error. TypeError, inclusive_range expected at most 3 arguments, got some number, format (numargs)).

Now I've got this little x here because it's expecting something to be in that suite. So if there's just one argument, then start is going to be 0 and step is going to be 1, and we'll go up to the top of the list, and we'll use our argument. We'll say stop=args0. All right. And if we have two arguments, then our arguments are start and stop, and we can assign those like this and give step the default value.

If we have three arguments, and we can assign them all like this. start, stop, and step=args. All right. Now, this should work. Let's save it and see how we are calling it up here? With just the 25 by itself. So we'll run it, and there we have all 25 numbers, just like we expected. It works just like range, except it's inclusive and it gives us that last number. If we have a 5 at the beginning of it, it should give us 5 through 25, there we go.

And if we step by three, let's say, save that and run it. That's exactly what we expect. Let's say we have too many arguments. Let's give it some other number. Now we're going to expect to get this one here. It's not less than 1. It's not 1. It's not 2. It's not 3. It's this something else. So save that and run it. And we get inclusive_range expected at most 3 arguments, and got 4.

And let's trigger this error here and give it no arguments at all. Save that and run it. And requires at least one argument is our error message. So there we have it. We've duplicated the functionality of the range object. We've got a function that returns an iterator object and it does exactly what range does, except that it's inclusive. And we've accomplished this using the yield statement, which turns our function into a generator, and the way the yield works is each time yield is run, it returns the value.

And the next time the function is called, execution picks up from right after the yield. And this turns the function into a generator and what it generates is an iterator object, which can be used exactly like any iterator object in Python.

Find answers to the most frequently asked questions about Python 3 Essential Training.


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Q: The installation process for the PyDev Eclipse plug-in doesn’t work as described in the videos. What should I do?
A: The version of Eclipse used in the recording of Python 3 Essential Training was Eclipse 3.5.2 (Galileo SR2) and the version of PyDev was 1.5.7. Soon after recording, the Eclipse project was updated to version 3.6, called Helios. As of this writing, the current version of Eclipse is 3.6.1 (Helios SR1).
Around the same time as these updates were being released, the PyDev project was updated as well. As of this writing, the current version of PyDev is 1.6.2. If you are using these most recent versions, the procedure for installing Eclipse itself has not changed, but the process documented in the videos for installing PyDev may not work.

We will be releasing new versions of the videos soon, but the author has posted a document describing the new installation procedure at:
<a href="http://python.bw.org/pydev-installation/" target="blank>http://python.bw.org/pydev-installation/</a>
Q: How do you install the pydev-interpreter in Eclipse Indigo on Mac OS X Lion? It's significantly different than what's shown in the video "Installing Python 3 and Eclipse for Mac."
A: Since this course was publishing, there have been upgrades to Mac OS X and Eclipse. In this scenario, after installing Python and Eclipse and the Pydev interpreter, there is a different directory to go to when modifying the preferences. As instructed in the movie (at around 6:40) restart Eclipse and then go to Eclipse > Preferences and drill down to Pydev > Interpreter - Python in the sidebar. Click New and in the Select interpreter dialog that opens, click Browse.

The Open dialog box will open, but does not appear to display your hard drive. You must press and hold the Command+Shift+Period keys. This will display all hidden files on your system. Navigate to the new path ~/Library/Frameworks/Python.framework/versions/3.2/bin/python 3.2 and click Open.

You should be able to proceed normally from there.
Q: When I try to install PyDev, it's not showing up in the Available Software window.
A:  This can happen if the pydev.org/updates site is down.
 
    You can manually download PyDev from the pydev.org web site, or from my site here:
 
        http://python.bw.org/
 
    Installation is simple. I've included instructions on my web site above.
 
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