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Python 3 Essential Training
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Operating on parts of a container with the slice operator


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Python 3 Essential Training

with Bill Weinman

Video: Operating on parts of a container with the slice operator

Slices are Python terminology for parts of a container. So as an example, let's go ahead and set up a container here, call it list, and we'll use the mutable list type, and we'll go ahead and assign it a bunch of values here. Now the interesting thing to note about this is that the first element of the list is at index 0.
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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

Operating on parts of a container with the slice operator

Slices are Python terminology for parts of a container. So as an example, let's go ahead and set up a container here, call it list, and we'll use the mutable list type, and we'll go ahead and assign it a bunch of values here. Now the interesting thing to note about this is that the first element of the list is at index 0.

So if I say list sub 0, and you get the number 1. Of I say sub 1, we are going to get the number 2. So it's important to note that the subscripts of container objects in Python are 0-based. That means that the first item is number 0, the second item is number 1, the third item is number 2, and on like that. So we have 10 items here, and they are numbered 0 through 9. So if I look at list sub 9, we have a number 10.

Also it's interesting to note that if I say, if I want to get items 0 through 5 and I use this syntax here, which is called a slice, that is the slice of the first 5 items. And you'll notice that it gives me 0 through 4, because item number 4 is the one with a 5 in it. So when I said list 0 through 5, that actually means list 0 through 5. But it always gives me everything up to, and not including, the last item.

Now many people find this confusing. I find this confusing. The reasons for it are well explained in the Python documentation, and it's still hard to warp your head around. The thing to remember is that ranges in Python are non-inclusive. It's like if I use the range object, and I say I want the range of 0 through 10, and of course it gives it me in that form because it's an iterable. I have to say 4 i in range of 0 through 10, print i. And it's going to give me 0 through 9.

So ranges in Python are non inclusive. They never include the last item. And that second argument in the slice is a range. So if I say list 0 through 10, it's going to give me 0 through 9, which in our case ends with the number 10. Confusing? Yes. Understandable? Yes, it 'sunderstandable. So let's go ahead and make our list a whole lot bigger. I am going to use a shorthand here.

That's going to give us 100 items in our list. And there they are, 0 through 99. And so if I want to look at item 27, it's going to give me item 27, which in this case because we used a range for the entire thing actually has a 27 in it. And that makes it easy for us to deal with our nice, long list. Okay there is a third argument to this slice operator, which is also optional. The first one, of course, is the index. So if I say list sub 27, that's the first one, and that will give me a slice of just one element at index number 27.

If I say list sub 27:42, this will give me a slice that begins at index 27 and ends at index 42 and is non inclusive, so it actually won't give me that last one. So I get 27 through 41. The last one, if I say list sub 27: 42:3, that's going to give me every third element. So it starts at 27, and then it gives me 30 and 33 and 36 and 39.

And it does not give me 42, because our range is non-inclusive, so it didn't include 42. If I had said list sub 27:43:3, then it would have given me that 42 element. So the slice operator actually has three possible arguments. The first one is the index, where the slice begins. And the second one, which is optional, is the index where the slice ends. If it's omitted, then it simply gives me one element at the index that I specified in the first argument.

The third argument is the step. And that indicates how many elements to step over for each iteration, and what this returns, of course, is an iterator. So if I were to say 4 i in list sub 27 :43:3 : print i. We get the elements from our results. Finally, it's important to note that we can actually assign - and that's the reason that I used a mutable object here for our example - we can actually assign to the slice.

I can say list sub 27:43:3 =, and I can put something in there. Let's say 99, 99, 99, 99, 99, 99. And I can put all 99s in there. And now when I look at my list, where I used to have a 27, I now have a 99, and I have a 99 there, and I have a 99 there, and so all of these elements got replaced with 99s, because you can actually assign to a slice as well as just reading from it.

So slices are incredibly useful. More often than not, you are going to use just the first two forms of it. But occasionally, with that third form, you can do some very powerful things, especially for matrix calculations and any thing that has more than one or tow dimensions. So that's the slice operator. It has the three arguments: the start and the stop and the step. And keep in mind that the stop is non- inclusive, just like ranges in Python are non-inclusive.

So when you specify 43 there as the stop, you have to realize that you are not going to get a value from that stop compartment. You are going to get a value from everything up to it but not including it, because ranges are not inclusive in Python. And the step argument tells it how often to step, or how many to step over. So that's the slice operator.

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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