Why Do Relative Imports Fail in Python When a Module is Run as `__main__`?

Relative Imports in Python: A Detailed Explanation

When working with a structured directory containing multiple subpackages, importing modules from other subpackages can be a common requirement. However, the standard approach of using relative imports, such as from ..sub2 import mod2, may fail in certain scenarios.

Such was the case for a developer attempting to import mod2 from mod1, within the directory structure provided in the question. The error message "Attempted relative import in non-package" indicates a problem with the relative import.

The root of the issue lies in the fact that the module was being run as __main__ by passing it as an argument to the interpreter. According to PEP 328, relative imports use the module's __name__ attribute to determine its position in the package hierarchy. When the module's name is set to __main__, relative imports are resolved as if the module were a top-level module, regardless of its actual location in the file system.

This behavior can lead to confusion when trying to import modules from other subpackages. To resolve this issue, it's recommended to place the code in a script and import the relevant subpackages explicitly using absolute imports, such as import sub2.mod2.

In Python 2.6 and later, however, the ability to reference modules relative to the main module was introduced. PEP 366 describes this change. To use relative imports effectively, it's essential to reference the main module instead of using __main__. This can be achieved in various ways, such as:

• __init__.py can declare a __package__ attribute to assign a name to the containing package.
• Using the __import__() method to import the subpackage and assign it to a variable.
• Using a placeholder module __init__.py to declare __path__ and __package__ attributes, even if the module is empty.

By adopting these techniques, developers can effectively utilize relative imports for organizing their Python projects and managing dependencies between modules in a structured manner.

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