如何导入和使用从其他文件中导入类的python模块？

Question

I am an experienced C# developer but have recently had to write some python code. I really like the language but I'm struggling with splitting some of my code into modules. For simplicity, I have essentially 2 modules with the first looking like this:

file1.py

from file2 import addOperation, volumeOperation

class utils:

  def add(self, num1, num2):
    op = addOperation(num1, num2)
    return op.calculate()

  def volume(self, length, width, height):
    op = volumeOperation(length, width, height)
    return op.calculate()

def main():
  util = utils()
  print(f"Adding 2 and 3 produces { util.add(2, 3) }")
  print(f"Volume of a 2x2x2 cube is { util.volume(2,2,2) }")

if __name__ == '__main__':
  main()

file2.py

class addOperation:
    def __init__(self, num1, num2):
        self.num1 = num1
        self.num2 = num2

    def calculate(self):
        return self.num1 + self.num2

class volumeOperation:
    def __init__(self, length, width, height):
        self.width = width
        self.height = height
        self.length = length

    def calculate(self):
        return self.width * self.height * self.length

_init__.py

import file1, file2

Those 3 files sit in a folder called "mylibrary" and when I run "python .\\file1.py" everything works as expected. The module then gets built into a wheel using this setup.py:

import setuptools

print(setuptools.find_packages())

setuptools.setup(
    name="mylibrary",
    version="1.0.0",
    author="me",
    author_email="me@me.com",
    packages=setuptools.find_packages(),
    classifiers=[
        "Programming Language :: Python :: 3",
        "Operating System :: OS Independent"
    ],
)

The resulting wheel then gets installed using "python -m pip install --user mylibrary-1.0.0-py3-none-any.whl" which appears to work just fine.

My second module is the following file "consumer.py":

import mylibrary.file1

if __name__ == '__main__':
    util = mylibrary.file1.utils()
    print(f"Adding 2 and 3 produces { util.add(2, 3) }")
    print(f"Volume of a 2x2x2 cube is { util.volume(2,2,2) }")

When I try to run "python .\\consumer.py" (I'm on windows using python 3.7, btw) I get "ModuleNotFoundError: No module named 'file2'".

I have tried different ways of importing, I've tried using an empty __init__.py in the mylibrary module, and I have read a few other stackoverflow questions that at least have similar problems but I haven't been able to get it to work.

What should I modify for this to work?

Answer 1

In Python 3, implicit relative imports got removed :

The only acceptable syntax for relative imports is from .[module] import name. All import forms not starting with . are interpreted as absolute imports. ( PEP 0328 )

Your

from file2 import ...

in file1.py is such an implicit relative import (ie, it would have worked as a relative import in Python 2, but is now treated as an absolute import in Python 3).

---

So, you'll need to change it to either an absolute import:

from mylibrary.file2 import ...

or an explicit relative import (using a leading dot):

from .file2 import ...

If you really do need them, you'll also need to convert the imports in your __init__.py to

from . import file1
from . import file2

Other than that, your setuptools distribution seems to be layed out properly, and works for me (nice minimal example BTW!).