5.9 Exercises

1. Basics, import. With your favorite text editor, write a Python module called mymod.py, which exports three top-level names:

   • A countLines(name) function that reads an input file and counts the number of lines in it (hint: file.readlines() does most of the work for you)

   • A countChars(name) function that reads an input file and counts the number of characters in it (hint: file.read() returns a single string)

   • A test(name) function that calls both counting functions with a given input filename

   A filename string should be passed into all three mymod functions. Now, test your module interactively, using import and name qualification to fetch your exports. Does your PYTHONPATH include the directory where you created mymod.py ? Try running your module on itself: e.g., test("mymod.py"). Note that test opens the file twice; if you're feeling ambitious, you might be able to improve this by passing an open file object into the two count functions.

2. from/from. Test your mymod module from Exercise 1 interactively, by using from to load the exports directly, first by name, then using the from* variant to fetch everything.

3. __main__. Now, add a line in your mymod module that calls the test function automatically only when the module is run as a script. Try running your module from the system command line; then import the module and test its functions interactively. Does it still work in both modes?

4. Nested imports. Finally, write a second module, myclient.py, which imports mymod and tests its functions; run myclient from the system command line. If myclient uses from to fetch from mymod, will mymod's functions be accessible from the top level of myclient? What if it imports with import instead? Try coding both variations in myclient and test interactively, by importing myclient and inspecting its __ dict__.

5. Reload. Experiment with module reloads: perform the tests in the changer.py example, changing the called function's message and/or behavior repeatedly, without stopping the Python interpreter. Depending on your system, you might be able to edit changer in another window, or suspend the Python interpreter and edit in the same window (on Unix, a Ctrl-Z key combination usually suspends the current process, and a fg command later resumes it).

6. Circular imports (and other acts of cruelty).^[10] In the section on recursive import gotchas, importing recur1 raised an error. But if we restart Python and import recur2 interactively, the error doesn't occur: test and see this for yourself. Why do you think it works to import recur2, but not recur1? (Hint: Python stores new modules in the built-in sys.modules table (a dictionary) before running their code; later imports fetch the module from this table first, whether the module is "complete" yet or not.) Now try running recur1 as a script: % python recur1.py. Do you get the same error that occurs when recur1 is imported interactively? Why? (Hint: when modules are run as programs they aren't imported, so this case has the same effect as importing recur2 interactively; recur2 is the first module imported.) What happens when you run recur2 as a script?

   ^[10] We should note that circular imports are extremely rare in practice. In fact, we have never coded or come across a circular import in six years of Python coding—except on the Internet (where such things receive an inordinate amount of attention), and when writing books like this. On the other hand, if you can understand why it's a potential problem, you know a lot about Python's import semantics.