3.1 Interactive Coding

Perhaps the simplest way to run Python programs is to type them at Python's interactive command line. There are a variety of ways to start this command line—in an IDE, from a system console, and so on. Assuming the interpreter is installed as an executable program on your system, the most platform-neutral way to start an interactive interpreter session is usually to type just "python" at your operating system's prompt, without any arguments. For example:

% python
Python 2.2 (#28, Dec 21 2001, 12:21:22) [MSC 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>

Here the word "python" is typed at your system shell prompt, to begin an interactive Python session (the "%" character stands for your system's prompt, not your input). The notion of a system shell prompt is generic, but varies per platform:

• On Windows, you can type python in a DOS console window (a.k.a. Command Prompt), or the Start/Run... dialog box.

• On Unix and Linux, you might type this in a shell window (e.g., in an xterm or console, running a shell such as ksh or csh).

• Other systems may use similar or platform-specific devices. On PalmPilots, for example, click the Python home icon to launch an interactive session; on a Zaurus PDA, open a Terminal window.

If you have not set your shell's PATH environment variable to include Python, you may need to replace the word "python" with the full path to the Python executable on your machine. For instance, on Windows, try typing C:\Python22\python (or C:\Python23\python for Version 2.3); on Unix and Linux, /usr/local/bin/python (or /usr/bin/python) will often suffice.

Once the Python interactive session starts, it begins by printing two lines of informational text (which we normally omit in our examples to save space), and prompts for input with >>> when it's waiting for you to type a new Python statement or expression. When working interactively, the results of your code are displayed after the >>> lines—here are the results of two Python print statements:

% python
>>> print 'Hello world!'
Hello world!
>>> print 2 ** 8
256

Again, don't worry about the details of the print statements shown here yet (we'll start digging into syntax in the next chapter). In short, they print a Python string and an integer, as shown by the output lines that appear after each >>> input line.

When working interactively like this, we can type as many Python commands as we like; each is run immediately after entered. Moreover, because the interactive session automatically prints the results of expressions typed, we don't usually need to say "print" explicitly at this prompt:

>>> lumberjack = 'okay'   
>>> lumberjack
'okay'
>>> 2 ** 8
256
>>>                      use Ctrl-D or Ctrl-Z to exit
%

Here, the last two lines typed are expressions (lumberjack and 2 ** 8), and their results are displayed automatically. To exit an interactive session like this one and return to your system shell prompt, type Ctrl-D on Unix-like machines; on MS-DOS and Windows systems, type Ctrl-Z to exit. In the IDLE GUI discussed later, either type Ctrl-D, or simply close the window.

Now, we're not doing much in this session's code: we type Python print and assignment statements, and a few expressions, which we'll study in detail later. The main thing to notice is that the code entered is executed immediately by the interpreter, when the Enter key is pressed at the end of the line.

For instance, after typing the first print statement at the >>> prompt, the output (a Python string) is echoed back right away. There's no need to run the code through a compiler and linker first, as you'd normally do when using a language such as C or C++. As you'll see in later chapters, you can also run multiline statements at the interactive prompt; the statement runs immediately after you've entered all its lines.

Besides typing python in a shell window, you can also begin similar interactive sessions by starting IDLE's main window, or on Windows via the Start button menus for Python and select the Python (command-line) menu option as shown in Figure 2-1. Both spawn a >>> prompt with equivalent functionality—code is run as it is typed.

Testing Code at the Interactive Prompt Because code is executed immediately, the interactive prompt turns out to be a great place to experiment with the language. It will be used often in this book to demonstrate smaller examples. In fact, this is the first rule of thumb to remember: if you're ever in doubt about how a piece of Python code works, fire up the interactive command line and try it out to see what happens. Chances are good that you won't break anything. (You need to know more about system interfaces before you will become dangerous.) Although you won't do the bulk of your coding in interactive sessions (because the code you type there is not saved), the interactive interpreter is a great place to test code you've put in files. You can import your module files interactively, and run tests on the tools they define by typing calls at the interactive prompt. More generally, the interactive prompt is a place to test program components, regardless of their source—you can type calls to linked-in C functions, exercise Java classes under Jython, and more. Partly because of this interactive nature, Python supports an experimental and exploratory programming style you'll find convenient when starting out.

3.1.1 Using the Interactive Prompt

Although simple to use, there are a few ways that the interactive prompt seems to trip up beginners:

• Type Python commands only. First of all, remember that you can only type Python code at the Python prompt, not system commands. There are ways to run system commands from within Python code (e.g., os.system), but they are not as direct as simply typing the command itself.

• Print statements are required only in files. Because the interactive interpreter automatically prints the results of expressions, you do not need to type complete print statements interactively. This is a nice feature, but tends to confuse users when they move on to writing code in files: within a code file, you really must use print statements to see your output, because expression results are not automatically echoed. You must say print in files, but not interactively.

• Don't indent at the interactive prompt (yet). When typing Python programs, either interactively or into a text file, be sure to start all your unnested statements in column 1 (that is, all the way to the left). If you don't, Python may print a "SyntaxError" message. Until Chapter 9, all statements will be unnested, so this includes everything for now. This seems to be a recurring confusion in introductory Python classes. A leading space generates an error message.

• Prompts and compound statements. We won't meet compound (multiline) statements until Chapter 9 , but as a preview, you should know that when typing lines two and beyond of a compound statement interactively, the prompt may change. In the simple shell window interface, the interactive prompt changes to ... instead of >>> for lines 2 and beyond; in the IDLE interface, lines after the first are automatically indented. In either case, a blank line (hitting the Enter key at the start of a line) is needed to tell interactive Python that you're done typing the multiline statement; by contrast, blank lines are ignored in files.

  You'll see why this matters in Chapter 9. For now, if you happen to come across a ... prompt or blank line when entering your code, it probably means that you've somehow confused interactive Python into thinking you're typing a multiline statement. Try hitting the Enter key, or a Ctrl-C combination to get back to the main prompt. The >>> and ... prompts can also be changed (they are available in built-in module sys), but we'll assume they have not been in our examples.