Python’s __import__ doesn’t work as expected

From the python docs on __import__:

__import__( name[, globals[, locals[, fromlist[, level]]]])

…

When the name variable is of the form
package.module, normally, the
top-level package (the name up till
the first dot) is returned, not the
module named by name. However, when a
non-empty fromlist argument is given,
the module named by name is returned.
This is done for compatibility with
the bytecode generated for the
different kinds of import statement;
when using “import spam.ham.eggs”, the
top-level package spam must be placed
in the importing namespace, but when
using “from spam.ham import eggs”, the
spam.ham subpackage must be used to
find the eggs variable. As a
workaround for this behavior, use
getattr() to extract the desired
components. For example, you could
define the following helper:

def my_import(name):
    mod = __import__(name)
    components = name.split('.')
    for comp in components[1:]:
        mod = getattr(mod, comp)
    return mod

To paraphrase:

When you ask for somepackage.somemodule, __import__ returns somepackage.__init__.py, which is often empty.

It will return somemodule if you provide fromlist (a list of the variable names inside somemodule you want, which are not actually returned)

You can also, as I did, use the function they suggest.

Note: I asked this question fully intending to answer it myself. There was a big bug in my code, and having misdiagnosed it, it took me a long time to figure it out, so I figured I’d help the SO community out and post the gotcha I ran into here.