Python Tutorial – How to Run Python Script in Windows Command Line Prompt

May 29, 2012, by admin

Python Tutorial For Beginners

This is not essentially a straightforward question. If you are already known with running programs from the Windows command line then everything will seem understandable; otherwise, you might need a little more guidance. There are also differentiations between Windows 95, 98, NT, ME, 2000 and XP which can add to the confusion.

Unless you use some sort of integrated development setting, you will end up typing Windows commands into what is variously referred to as a “DOS window” or “Command prompt window”. generally you can create such a window from your Start menu; under Windows 2000 the menu selection is Start ‣ Programs ‣ Accessories ‣ Command Prompt. You should be able to distinguish when you have started such a window because you will see a Windows “command prompt”, which usually looks like this:

The letter may be different, and there might be other things after it, so you might just as easily see something like:

Depending on how your computer has been set up and what else you have recently done with it. Once you have started such a window, you are well on the way to running Python programs.

You have to understand that your Python scripts have to be processed by another program called the Python interpreter. The interpreter reads your script, compiles it into byte codes, and then implements the byte codes to run your program. So, how do you position for the interpreter to handle your Python?

First, you need to make sure that your command window identifies the word “python” as an instruction to start the interpreter. If you have opened a command window, you should try entering the command python and hitting return. You should then see something like:

You have started the interpreter in “interactive mode”. That means you can enter Python statements or expressions interactively and have them implemented or evaluated while you wait. This is one of Python’s strongest aspects. Check it by entering a few expressions of your choice and seeing the results:

Many people use the interactive mode as a expedient yet highly programmable calculator. When you want to end your interactive Python session, hold the Ctrl key down while you enter a Z, then hit the “Enter” key to get back to your Windows command prompt.

You may also find that you have a Start-menu entry such as Start ‣ Programs ‣ Python 2.2 ‣ Python (command line) that results in you seeing the >>> prompt in a new window. If so, the window will disappear after you enter the Ctrl-Z character; Windows is running a single “python” command in the window, and closes it when you terminate the interpreter.

If the python command, instead of displaying the interpreter prompt >>>, gives you a message like:

or

then you have to make sure that your computer knows where to find the Python interpreter. To do this you will have to change a setting called PATH, which is a list of directories where Windows will look for programs.

You should position for Python’s installation directory to be added to the PATH of every command window as it starts. If you installed Python fairly recently then the command

will probably tell you where it is installed; the normal location is something like C:Python23. Otherwise you will be cut to a search of your whole disk … use Tools ‣ Find or hit the Search button and look for “python.exe”. Supposing you discover that Python is installed in the C:Python23 directory (the default at the time of writing), you should make sure that entering the command

starts up the interpreter as above (and don’t forget you’ll need a “CTRL-Z” and an “Enter” to get out of it). Once you have verified the directory, you need to add it to the start-up routines your computer goes through. For older versions of Windows the easiest way to do this is to edit the C:AUTOEXEC.BAT file. You would want to add a line like the following to AUTOEXEC.BAT:

For Windows NT, 2000 and (I assume) XP, you will need to add a string such as

to the current setting for the PATH environment variable, which you will find in the properties window of “My Computer” under the “Advanced” tab. Note that if you have enough privilege you might get a choice of installing the settings either for the Current User or for System. The latter is preferred if you want everybody to be able to run Python on the machine.

If you aren’t sure doing any of these manipulations yourself, ask for help! At this stage you may want to reboot your system to make absolutely sure the new setting has taken effect. You probably won’t need to reboot for Windows NT, XP or 2000. You can also avoid it in earlier versions by editing the file C:WINDOWSCOMMANDCMDINIT.BAT instead of AUTOEXEC.BAT.

You should now be able to start a new command window, enter python at the C:> (or whatever) prompt, and see the >>> prompt that indicates the Python interpreter is reading interactive commands.

Let’s presume you have a program called pytest.py in directory C:SteveProjectsPython. A session to run that program might look like this:

Because you added a file name to the command to start the interpreter, when it starts up it reads the Python script in the named file, compiles it, executes it, and terminates, so you see another C:> prompt. You might also have entered

if you hadn’t wanted to change your current directory.

Under NT, 2000 and XP you may well find that the installation process has also arranged that the command pytest.py (or, if the file isn’t in the current directory, C:SteveProjectsPythonpytest.py) will automatically distinguish the ”.py” extension and run the Python interpreter on the named file. Using this aspect is fine, but some versions of Windows have bugs which mean that this form isn’t precisely comparable to using the interpreter unambiguously, so be careful.

The vital things to remember are:

Start Python from the Start Menu, or make sure the PATH is set correctly so Windows can find the Python interpreter.

should give you a ‘>>>’ prompt from the Python interpreter. Don’t forget the CTRL-Z and ENTER to finish the interpreter (and, if you started the window from the Start Menu, make the window disappear).

Once this works, you run programs with commands:

When you know the commands to use you can build Windows shortcuts to run the Python interpreter on any of your scripts, naming particular working directories, and adding them to your menus. Take a look at

if your needs are complex.

Interactive mode (where you see the >>> prompt) is best used for checking that individual statements and expressions do what you think they will, and for developing code by experiment.