Using Command Lines and Files

Running program files from system command lines is also a fairly straightforward launch option, especially if you are familiar with command lines in general from prior work. For newcomers, though, here are a few pointers about common beginner traps that might help you avoid some frustration:

 

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  • Beware of automatic extensions on Windows. If you use the Notepad program to code program files on Windows, be careful to pick the type All Files when it comes time to save your file, and give the file a .py suffix explicitly. Otherwise, Notepad will save your file with a .txt extension (e.g., as script1.py.txt), making it difficult to run in some launching schemes.
    Worse, Windows hides file extensions by default, so unless you have changed your view options you may not even notice that you’ve coded a text file and not a Python file. The file’s icon may give this away—if it doesn’t have a snake on it, you may have trouble. Uncolored code in IDLE and files that open to edit instead of run when clicked are other symptoms of this problem.
    Microsoft Word similarly adds a .doc extension by default; much worse, it adds formatting characters that are not legal Python syntax. As a rule of thumb, always pick All Files when saving under Windows, or use a more programmer-friendly text editor such as IDLE. IDLE does not even add a .py suffix automatically—a feature programmers tend to like, but users do not.
  • Use file extensions and directory paths at system prompts, but not for imports. Don’t forget to type the full name of your file in system command lines—that is, use python script1.py rather than python script1. By contrast, Python’s import statements, which we’ll meet later in this chapter, omit both the .py file suffix and the directory path (e.g., import script1). This may seem trivial, but confusing these two is a common mistake.
    At the system prompt, you are in a system shell, not Python, so Python’s module file search rules do not apply. Because of that, you must include both the .py extension and, if necessary, the full directory path leading to the file you wish to run. For instance, to run a file that resides in a different directory from the one in which you are working, you would typically list its full path (e.g., python d:\tests\spam.py). Within Python code, however, you can just say import spam and rely on the Python module search path to locate your file, as described later.
  • Use print statements in files. Yes, we’ve already been over this, but it is such a common mistake that it’s worth repeating at least once here. Unlike in interactive coding, you generally must use print statements to see output from program files. If you don’t see any output, make sure you’ve said “print” in your file. Again, though, print statements are not required in an interactive session, since Python automatically echoes expression results; prints don’t hurt here, but are superfluous extra typing.