[Python-Dev] question/comment about documentation of relative imports (original) (raw)

Darren Dale dsdale24 at gmail.com
Tue Oct 5 16:56:11 CEST 2010

I have a couple questions/comments about the use of PEP 328-style relative imports. For example, the faq at http://docs.python.org/py3k/faq/programming.html#what-are-the-best-practices-for-using-import-in-a-module reads:

"Never use relative package imports. If you’re writing code that’s in the package.sub.m1 module and want to import package.sub.m2, do not just write from . import m2, even though it’s legal. Write from package.sub import m2 instead. See PEP 328 for details."

There is no explanation to support the claim that relative imports should "never" be used. It seems to me that someone read the following in PEP 328::

from .moduleY import spam from .moduleY import spam as ham from . import moduleY from ..subpackage1 import moduleY from ..subpackage2.moduleZ import eggs from ..moduleA import foo from ...package import bar from ...sys import path

Note that while that last case is legal, it is certainly discouraged ("insane" was the word Guido used).

... and interpreted it to mean that relative imports are in general discouraged. I interpreted it to mean that relative imports should not be used to import from python's standard library.

There are cases where it is necessary to use relative imports, like a package that is included as a subpackage of more than one other project (when it is not acceptable to add an external dependency, for example due to version/compatibility issues). There is some additional context on relative imports in the programming faq for python-2.7 at http://docs.python.org/faq/programming.html#what-are-the-best-practices-for-using-import-in-a-module : "Never use relative package imports. If you’re writing code that’s in the package.sub.m1 module and want to import package.sub.m2, do not just write import m2, even though it’s legal. Write from package.sub import m2 instead. Relative imports can lead to a module being initialized twice, leading to confusing bugs. See PEP 328 for details."

Is there some documentation explaining why the module may be initialized twice? I don't see it in PEP 328. Is this also the case for python-3, or does it only apply to the old-style (pre-PEP 328) relative imports in python-2? If relative imports are truly so strongly discouraged, then perhaps warnings should also be included in places like http://docs.python.org/library/functions.html#_import_ , and especially http://docs.python.org/tutorial/modules.html#intra-package-references and http://www.python.org/dev/peps/pep-0328/ (which, if I have misinterpreted, is ambiguously written. Though I doubt this is the case).

There is also this warning against relative imports in PEP 8:

- Relative imports for intra-package imports are highly discouraged.
  Always use the absolute package path for all imports.
  Even now that PEP 328 [7] is fully implemented in Python 2.5,
  its style of explicit relative imports is actively discouraged;
  absolute imports are more portable and usually more readable.

... but one could argue, as I just have, that relative imports are more portable, not less. In a sense, the statement "explicit relative imports is actively discouraged" is objectively false. They are passively discouraged. If they were actively discouraged, perhaps performing a relative import would raise a warning, or maybe distutils would raise a warning at install time, or maybe an additional import would be required to enable them. Up until now, I was not aware that use of PEP 328 relative imports might be discouraged. I'm still unclear as to why they might be discouraged. I recently helped convert a popular package to use PEP 328 relative imports. Would the python devs consider this a mistake?

Thanks, Darren

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