Absolute import module in same package

问题:

I've simplified my import problems down to this simple base case. Say I have a Python package:

mypkg/
   __init__.py
   a.py
   b.py

a.py contains:

def echo(msg):
    return msg

b.py contains:

from mypkg import a       # possibility 1, doesn't work
#import a                 # possibility 2, works
#from mypkg.a import echo  # import also fails

print(a.echo())

Running python b.py produces ImportError: No module named mypkg on both Python 2.7.6 and Python 3.3.5. I have also tried adding from __future__ import absolute_import in both cases, same issue.

Expected:

I expect possibility 1 to work just fine.

Why do I want to do this:

Possibility 2 is less desirable. Hypothetically, the standard library could introduce a package called a (unlikely in this case, but you get the idea). While Python 2 searches the current package first, Python 3+ includes absolute import changes so that the standard library is checked first. No matter what my reason, possibility 1 is supposed to work, no? I could swear I've done it thousands of times before.

Note: If you write a script external to mypkg, from mypkg import a works without issue.

My question is similar to python - absolute import for module in the same directory, but the author implies that what I have should be working.

回答1:

from mypkg import a is the correct form. Don't run scripts from inside the Python package directory, it makes the same module available using multiple names that may lead to bugs. Run python -m mypkg.b from the directory that contains mypkg instead.

To be able to run from any directory, mypkg should be in pythonpath.

回答2:

Try this:

import sys
import os
this_dir = os.path.dirname(os.path.realpath(__file__))
sys.path.append(os.path.dirname(this_dir))

from mypkg import a
print(a.echo())

回答3:

I think the problem comes from the fact that you don't have a reference to mypkg inside the mypkg folder. Look at what Python is doing when I try to run your example (using the verbose option):

# trying /vagrant/mypkg/mypkg.py

That's why it can find the module, because it doesn't exist. One hack you could do is to have a file called mypkg.py with the line

import a

but that's just your 2nd possibility above in another jacket. Without knowing what you want to accomplish, I would choose the first example on the Intra-package Reference text. I would write b.py as such:

from a import echo

print(echo('message'))

回答4:

Yes it will not work, because at the moment you call print(mypkg.a.echo()), mypkg is still loading (mypkg.__init__ -> mypkg.b). This is because Python loads parent modules first. https://docs.python.org/3/reference/import.html#searching

What you can do is wrap print(mypkg.a.echo()) into a function:

def echo():
   mypkg.a.echo()

And then:

import mypkg.b
mypkg.b.echo()

Or even:

print(sys.modules['mypkg.a'].echo())

Also you can help Python to find your module:

import importlib
mypkg.a = importlib.import_module('mypkg.a')
mypkg.a.echo()