Python 3 Essential Training
Illustration by John Hersey

Using strings


Python 3 Essential Training

with Bill Weinman

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Video: Using strings

Python has some very powerful facilities for working with strings. And in fact, in Python 3, they become refined, and in some cases even more powerful. Let's get right into strings, and we'll start by making a working copy of And we can call this, go ahead and open that working copy, and we'll start by defining a String. I'll just call it s, and I'll say 'This is a string!' and we'll go ahead, and we'll print(s) and save that and run it.
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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

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Watch the Online Video Course 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
Developer Web
Bill Weinman

Using strings

Python has some very powerful facilities for working with strings. And in fact, in Python 3, they become refined, and in some cases even more powerful. Let's get right into strings, and we'll start by making a working copy of And we can call this, go ahead and open that working copy, and we'll start by defining a String. I'll just call it s, and I'll say 'This is a string!' and we'll go ahead, and we'll print(s) and save that and run it.

And there we have the string. Strings in Python are immutable objects, and they're created with either single quotes or double quotes. And so if we change those single quotes to double quotes, we'll see - if we save and run, we'll see that we get exactly the same result. You can use Escape characters in a string, as you can in many languages. So if I put in here a \n, that'll introduce a new line in the middle of the string.

And if I save that and run it, we see that there is a new line now between the a and the s of string. On the other hand, if I want this \n to actually be a part of the string, rather than getting replaced with a new line, I can put the letter r before the definition of the string like that, and if I save that and run it, I get the \n as part of the string. This is called a raw string, and the place where this gets used the most is when creating regular expressions.

And we'll get into regular expressions in some detail in another chapter. In addition to these sorts of Escapes with the backslashes, you can also do some formatting and replacing of variables in the string. For example, if I create a number and call this, say n, and give it a value of 42, I could put that 42 right in the middle of the string here. And I'm going to show you the Python 3 way to do this.

And we'll look briefly at the Python 2 way to do it as well, because you'll see that a lot. For all your new code, you want to use the Python 3 way, which is with the format method of the string object, because the Python 2 way is going to go away. It's considered obsolescent, and it will be dropped in the next version of Python. So I'm going to save this and run. And you'll see what this does. This actually inserts the value of n in the middle of the string.

We have these curly braces here, and those get replaced with the format. So format is a method of the string object. And so this literal string is actually an object. Remember, everything in Python is an object. We can use this objecty-referencing operator, this period, to access a method of that object, and do this variable replacement. So this is very powerful, and this is very common.

And you'll see it done this way in new Python 3 code. In Python 2 code, you'll see it's done this other way. I'm going to put %s here, and over here a % sign and the letter n. And so we'll save that and we'll run it. And you see we get the same result. This is the way that it was done in Python 2. And this is a bit of a hack. It's perfectly valid, and you'll see it a lot.

You'll even see it some in Python 3 code that's written by people who are used to Python 2, and that's fine. The reason you don't want to use this construct is because it is considered obsolescent, and it will be dropped in the next version of Python. So you'll see this, you want to know what it does, but the right way to do it in Python 3 is to use the new format method of the string object.

And that will look like this. I'll save that and run it. And that's how that works. There's one more way I want to show you for defining a string, and this is using the triple quotes. And you can triple either the single quotes or the double quotes. So I can either do it this way, or instead of these triple quotes, I can do it this way with the double quotes, like this. And you'll see it's done both ways. I tend to use the singles tripled instead of the doubles tripled.

Now what this does is it allows you to have a string that spans several lines. And so, what I'll often do is I'll start with a backslash and a new line and go all the way back to the end of the line like that, and this will allow me to just have line after line of text and more text, and to have it actually started at the beginning of it.

I'll explain what this does in a moment. Let's save this and run. And then you can see what this does. This is useful if you have lines and lines and lines of text. And so the way you do this is with three quotes, either single quotes or double quotes, at the beginning and the end. And the result will be a string that has the new lines, and it's all formatted exactly like you type it in. What this here does, this backslash and a new line, is that it escapes the new line, so that it doesn't actually show up in the string.

If I didn't have this, and I save this and run it - that's scrolled out of the way - there you see that we get a black line at the beginning, because this blank line here is actually inserted in the string. The way to get that not to happen is to use the backslash before it. The backslash has to be the very last character on the line, so that it's escaping the new line and not a space or something. So I save that and I run it, and you can see that we don't have a scrollbar over here. This is actually at the top, and there is no new line at the beginning of the string.

So that's the triple quote way of defining strings. And that's often used in docstrings in function, which we'll get to later on in the chapter on functions.

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:
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 site is down.
    You can manually download PyDev from the web site, or from my site here:

    Installation is simple. I've included instructions on my web site above.
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