I recently upgraded versions of pylint, a popular Python style-checker.
It has gone ballistic throughout my code, pointing out places where I import modules in the same package, without specifying the full package path.
The new error message is W0403.
W0403: Relative import %r, should be %r
Used when an import relative to the package directory is detected.
Example
For example, if my packages are structured like this:
/cake
/__init__.py
/icing.py
/sponge.py
/drink
and in the sponge package I write:
import icing
instead of
import cake.icing
I will get this error.
While I understand that not all Pylint messages are of equal importance, and I am not afraid to dismiss them, I don’t understand why such a practice is considered a poor idea.
I was hoping someone could explain the pitfalls, so I could improve my coding style rather than (as I currently plan to do) turning off this apparently spurious warning.
The problem of import icing is that you don’t know whether it’s an absolute import or a relative import. icing could be a module in python’s path, or a package in the current module. This is quite annoying when a local package has the same name as a python standard library package.
You can do from __future__ import absolute_import which turns off implicit relative imports altogether. It is described, including with this justification about ambiguity, in PEP 328. I believe Python 3 has implicit relative imports turned off completely.
You still can do relative imports, but you have to do them explicitly, like this:
from . import icing
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There are a few good reasons:
-
Relative imports break easily, when you move a module around.
Imagine you have a
foo.bar, afoo.bazand abazmodule in your package.foo.barimportsfoo.baz, but using a relative import.Now, if you were to move
foo.bartobar, your module suddenly is importing a differentbaz! -
Relative imports are ambiguous. Even without moving around the
barmodule in the above example, a new developer coming to your project could be forgiven for not realizing thatbazis reallyfoo.bazinstead of the root-levelbazpackage.Absolute imports make it explicit what module is being used. And as
import thispreaches, explicit is better than implicit. -
Python 3 has disabled implicit relative imports altogether; imports are now always interpreted as absolute, meaning that in the above example
import bazwill always import the top-level module. You will have to use the explicit import syntax instead (from . import baz).Porting the example from Python 2 to 3 would thus lead to unexpected problems, using absolute imports now will make your code future-proof.
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