Learning Python - Mark Lutz [38]
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>>>
Alternatively, you can run a change-directory command to go to Python’s install directory before typing “python”—try the cd c:\python30 command on Windows, for example:
C:\misc> cd C:\Python30
C:\Python30> python
Python 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) [MSC v.1500 32 bit (Intel)] ...
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>>>
On Windows, besides typing python in a shell window, you can also begin similar interactive sessions by starting IDLE’s main window (discussed later) or by selecting the “Python (command line)” menu option from the Start button menu for Python, as shown in Figure 2-1 back in Chapter 2. Both spawn a Python interactive prompt with equivalent functionality; typing a shell command isn’t necessary.
Running Code Interactively
However it’s started, the Python interactive session begins by printing two lines of informational text (which I’ll omit from most of this book’s examples to save space), then 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 after you press the Enter key.
For instance, here are the results of two Python print statements (print is really a function call in Python 3.0, but not in 2.6, so the parentheses here are required in 3.0 only):
% python
>>> print('Hello world!')
Hello world!
>>> print(2 ** 8)
256
Again, you don’t need to 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 (2 ** 8 means 2 raised to the power 8 in Python).
When coding interactively like this, you can type as many Python commands as you like; each is run immediately after it’s entered. Moreover, because the interactive session automatically prints the results of expressions you type, you don’t usually need to say “print” explicitly at this prompt:
>>> lumberjack = 'okay'
>>> lumberjack
'okay'
>>> 2 ** 8
256
>>> <== Use Ctrl-D (on Unix) or Ctrl-Z (on Windows) to exit
%
Here, the first line saves a value by assigning it to a variable, and the last two lines typed are expressions (lumberjack and 2 ** 8)—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 didn’t do much in this session’s code—just typed some Python print and assignment statements, along with a few expressions, which we’ll study in detail later. The main thing to notice is that the interpreter executes the code entered on each line immediately, when the Enter key is pressed.
For example, when we typed the first print statement at the >>> prompt, the output (a Python string) was echoed back right away. There was no need to create a source-code file, and 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; such a statement runs immediately after you’ve entered all of its lines and pressed Enter twice to add a blank line.
Why the Interactive Prompt?
The interactive prompt runs code and echoes results as you go, but it doesn’t save your code in a file. Although this means you won’t do the bulk of your coding in interactive sessions, the interactive prompt turns out to be a great place to both experiment with the language and test program files on the fly.
Experimenting
Because code is executed immediately, the interactive prompt is a perfect place to experiment with the language and will be used often in this book to demonstrate smaller