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reload, from, and Interactive Testing

In fact, the prior gotcha is even more subtle than it appears. Chapter 3 warned that it’s usually better not to launch programs with imports and reloads because of the complexities involved. Things get even worse when from is brought into the mix. Python beginners often stumble onto its issues in scenarios like the one outlined next. Say that after opening a module file in a text edit window, you launch an interactive session to load and test your module with from:

from module import function
function(1, 2, 3)

Finding a bug, you jump back to the edit window, make a change, and try to reload the module this way:

from imp import reload
reload(module)

This doesn’t work, because the from statement assigned the name function, not module. To refer to the module in a reload, you have to first load it with an import statement at least once:

from imp import reload
import module
reload(module)
function(1, 2, 3)

However, this doesn’t quite work either—reload updates the module object, but as discussed in the preceding section, names like function that were copied out of the module in the past still refer to the old objects (in this instance, the original version of the function). To really get the new function, you must refer to it as module.function after the reload, or rerun the from:

from imp import reload
import module
reload(module)
from module import function        # Or give up and use module.function()
function(1, 2, 3)

Now, the new version of the function will finally run.

As you can see, there are problems inherent in using reload with from: not only do you have to remember to reload after imports, but you also have to remember to rerun your from statements after reloads. This is complex enough to trip up even an expert once in a while. (In fact, the situation has gotten even worse in Python 3.0, because you must also remember to import reload itself!)

The short story is that you should not expect reload and from to play together nicely. The best policy is not to combine them at all—use reload with import, or launch your programs other ways, as suggested in Chapter 3: using the Run→Run Module menu option in IDLE, file icon clicks, system command lines, or the exec built-in function.