Why Do Relative Imports in Python 3 Fail, and How Can I Fix Them?

Relative Imports in Python 3: The Mysterious Errors

In Python 3, relative imports are a convenient way to import modules from within the same directory. However, an unexpected error often arises:

Error Messages:

• ImportError: attempted relative import with no known parent package
• ModuleNotFoundError: No module named 'mymodule'
• SystemError: Parent module '' not loaded, cannot perform relative import

The Root Cause:

To understand these errors, we must delve into Python's module importing mechanism. Relative imports require that the importing module is contained within a package. A package is a directory marked as a package by the presence of an __init__.py file.

Common Layout:

Often, packages have the following structure:

main.py
mypackage/
    __init__.py
    mymodule.py
    myothermodule.py

Example Code:

• mymodule.py

# Exported function
def as_int(a):
    return int(a)

# Test function for module  
def _test():
    assert as_int('1') == 1

if __name__ == '__main__':
    _test()

• myothermodule.py

# Exported function
def add(a, b):
    return as_int(a) + as_int(b)

# Test function for module  
def _test():
    assert add('1', '1') == 2

if __name__ == '__main__':
    _test()

• main.py

from mypackage.myothermodule import add

def main():
    print(add('1', '1'))

if __name__ == '__main__':
    main()

When you run main.py or mypackage/mymodule.py, everything works smoothly. However, running mypackage/myothermodule.py fails due to the relative import:

from .mymodule import as_int

The Solution:

As Guido van Rossum explains, this failure is due to Python treating running scripts from within a package as an antipattern. The recommended solution is to avoid relative imports and use absolute imports instead:

from mypackage.mymodule import as_int

Alternative Approaches:

If you still prefer relative imports, you can use the -m option to specify the module to import, but it can be verbose and inconvenient:

python3 -m mypackage.myothermodule

Another alternative is to manipulate PYTHONPATH to include the parent directory of the package:

import sys
import os

SCRIPT_DIR = os.path.dirname(os.path.abspath(__file__))
sys.path.append(os.path.dirname(SCRIPT_DIR))

from mypackage.mymodule import as_int

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