You can start an interactive session on Windows 7 and earlier by clicking your Start button, picking the All Programs option, clicking the Python entry, and selecting the “Python (command line)” menu option. You can also achieve the same effect on Windows and other platforms by typing python as a system command line in your system’s console window (a Command Prompt window on Windows). Another alternative is to launch IDLE, as its main Python shell window is an interactive session. Depending on your platform and Python, if you have not set your system’s PATH variable to find Python, you may need to cd to where Python is installed, or type its full directory path instead of just python (e.g., C:\Python33\python on Windows, unless you’re using the 3.3 launcher).
You type system command lines in whatever your platform provides as a system console: a Command Prompt window on Windows; an xterm or terminal window on Unix, Linux, and Mac OS X; and so on. You type this at the system’s prompt, not at the Python interactive interpreter’s “>>>” prompt—be careful not to confuse these prompts.
Code in a script (really, module) file can be run with system command lines, file icon clicks, imports and reloads, the exec built-in function, and IDE GUI selections such as IDLE’s Run→Run Module menu option. On Unix, they can also be run as executables with the #! trick, and some platforms support more specialized launching techniques (e.g., drag and drop). In addition, some text editors have unique ways to run Python code, some Python programs are provided as standalone “frozen binary” executables, and some systems use Python code in embedded mode, where it is run automatically by an enclosing program written in a language like C, C++, or Java. The latter technique is usually done to provide a user customization layer.
Scripts that print and then exit cause the output file to disappear immediately, before you can view the output (which is why the input trick comes in handy); error messages generated by your script also appear in an output window that closes before you can examine its contents (which is one reason that system command lines and IDEs such as IDLE are better for most development).
Python imports (loads) a module only once per process, by default, so if you’ve changed its source code and want to run the new version without stopping and restarting Python, you’ll have to reload it. You must import a module at least once before you can reload it. Running files of code from a system shell command line, via an icon click, or via an IDE such as IDLE generally makes this a nonissue, as those launch schemes usually run the current version of the source code file each time.
Within the text edit window of the file you wish to run, select the window’s Run→Run Module menu option. This runs the window’s source code as a top-level script file and displays its output back in the interactive Python shell window.
IDLE can still be hung by some types of programs—especially GUI programs that perform multithreading (an advanced technique beyond this book’s scope). Also, IDLE has some usability features that can burn you once you leave the IDLE GUI: a script’s variables are automatically imported to the interactive scope in IDLE and working directories are changed when you run a file, for instance, but Python itself does not take such steps in general.
A namespace is just a package of variables (i.e., names). It takes the form of an object with attributes in Python. Each module file is automatically a namespace—that is, a package of variables reflecting the assignments made at the top level of the file. Namespaces help avoid name collisions in Python programs: because each module file is a self-contained namespace, files must explicitly import other files in order to use their names.