What are good rules of thumb for Python imports?

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In production code in our company, we try to follow the following rules.

We place imports at the beginning of the file, right after the main file’s docstring, e.g.:

"""
Registry related functionality.
"""
import wx
# ...

Now, if we import a class that is one of few in the imported module, we import the name directly, so that in the code we only have to use the last part, e.g.:

from RegistryController import RegistryController
from ui.windows.lists import ListCtrl, DynamicListCtrl

There are modules, however, that contain dozens of classes, e.g. list of all possible exceptions. Then we import the module itself and reference to it in the code:

from main.core import Exceptions
# ...
raise Exceptions.FileNotFound()

We use the import X as Y as rarely as possible, because it makes searching for usage of a particular module or class difficult. Sometimes, however, you have to use it if you wish to import two classes that have the same name, but exist in different modules, e.g.:

from Queue import Queue
from main.core.MessageQueue import Queue as MessageQueue

As a general rule, we don’t do imports inside methods — they simply make code slower and less readable. Some may find this a good way to easily resolve cyclic imports problem, but a better solution is code reorganization.

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