This article aims to solve the common problem of submodules inside Python packages referencing root directory modules. We will explain the key role of the __init__.py file in the Python package structure, and explain in detail how to use relative imports (such as from ..module import Object) to ensure correct references between modules, thereby building a clear and maintainable Python project.

Understand Python package structure

In Python, a directory is considered a package if it contains a file named __init__.py. The existence of this file tells the Python interpreter that this directory and the modules within it can be imported. The __init__.py file can be empty or contain package-level initialization code. It is the foundation for building modular and reusable code bases.

When you create a Python project and want to publish it as an installable package (e.g. via pip install), it is crucial to ensure that each level of the package is correctly defined as a package.

Typical scenarios and import dilemmas

Suppose you are building a Python package called my_package with the following file structure:

 my_package
├── model
│ └── do_stuff.py
└── request_models.py

Among them, request_models.py is located in the root directory of the package and defines some request models, such as:

 # my_package/request_models.py
classMyModel:
    def __init__(self, data):
        self.data = data

Do_stuff.py is located in the model subdirectory and needs to import and use MyModel. If you try to use the following import statement directly in do_stuff.py:

 # my_package/model/do_stuff.py
from request_models import MyModel
#...

You will encounter an error like ModuleNotFoundError: No module named 'request_models'. This is because the Python interpreter cannot find a top-level module named request_models in do_stuff.py's current module search path.

Solution One: Ensure Correct Package Structure

The first step to fix this problem is to make sure all your directories are recognized as packages by Python. This means that the __init__.py file needs to be added to both the my_package directory and its subdirectory model.

The correct package file structure should look like this:

 my_package
├── __init__.py # Make my_package a package ├── model
│ ├── __init__.py # Make model a subpackage of my_package │ └── do_stuff.py
└── request_models.py

After adding these __init__.py files, the Python interpreter can correctly understand that my_package is a whole including the model subpackage and the request_models module.

Solution 2: Use relative imports

Once the package structure is set up correctly, we can use relative imports in do_stuff.py to reference the content in request_models.py. Relative imports allow you to specify which modules to import based on the current module's location in the package.

• . indicates the current package.
• .. represents the parent package of the current package.
• ... represents the grandfather package of the current package, and so on.

In my_package/model/do_stuff.py, to import the request_models module located in its parent package (i.e. my_package), you should use two dots (..) to refer to the parent package:

 # my_package/model/do_stuff.py
from ..request_models import MyModel

def process_data(data):
    model_instance = MyModel(data)
    print(f"Processing data with MyModel: {model_instance.data}")

if __name__ == "__main__":
    process_data({"key": "value"})

This code tells the Python interpreter: "From the parent package (my_package) of the package (model) where the current module (do_stuff.py) is located, import the request_models module and get MyModel from it."

How relative import works

When Python executes a module, it knows which package the module belongs to. Relative imports use this information to resolve import paths. For example, when do_stuff.py is executed, Python knows that it is located in the my_package.model package. Therefore, .. will be parsed as my_package, and from ..request_models import MyModel is equivalent to from my_package.request_models import MyModel.

Things to note and best practices

• When to use relative imports? Relative imports are often used for inter-module references inside a package, especially when you want to avoid hardcoding the absolute path to the package. It helps keep your code flexible, especially when package names may change or packages are nested in deeper structures.
• When to use absolute import? Absolute imports should be used when you need to import modules from different top-level packages or third-party libraries. For example, import os or from django.db import models. Absolute imports are also preferred in the top-level __init__.py file of a package, or when the script is run directly.
• Avoid circular references: Whether using relative imports or absolute imports, care should be taken to avoid circular references between modules, which may cause the program to crash or behave abnormally.
• Clear package hierarchy: Keeping the package structure clear and logical helps to understand and maintain the code. Unnecessarily deep nesting can complicate relative imports.

Summarize

Properly organizing Python packages and using the appropriate import mechanism are keys to building robust, maintainable projects. By ensuring that each directory is marked as a package by the __init__.py file, and leveraging relative imports (such as from ..module import Object) to handle module references inside the package, you can effectively solve the problem of submodules referencing root directory modules, thus improving the modularity and readability of your code.

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