Relative imports in Python 3

Question

I want to import a function from another file in the same directory.

Sometimes it works for me with from .mymodule import myfunction but sometimes I get a:

SystemError: Parent module '' not loaded, cannot perform relative import 

Sometimes it works with from mymodule import myfunction, but sometimes I also get a:

SystemError: Parent module '' not loaded, cannot perform relative import 

I don't understand the logic here and I couldn't find any explanation. This looks completely random.

Could someone explain to me what's the logic behind all this?

Answer 1

Explanation

From PEP 328

Relative imports use a module's __name__ attribute to determine that module's position in the package hierarchy. If the module's name does not contain any package information (e.g. it is set to '__main__') then relative imports are resolved as if the module were a top level module, regardless of where the module is actually located on the file system.

At some point PEP 338 conflicted with PEP 328:

... relative imports rely on __name__ to determine the current module's position in the package hierarchy. In a main module, the value of __name__ is always '__main__', so explicit relative imports will always fail (as they only work for a module inside a package)

and to address the issue, PEP 366 introduced the top level variable __package__:

By adding a new module level attribute, this PEP allows relative imports to work automatically if the module is executed using the -m switch. A small amount of boilerplate in the module itself will allow the relative imports to work when the file is executed by name. [...] When it [the attribute] is present, relative imports will be based on this attribute rather than the module __name__ attribute. [...] When the main module is specified by its filename, then the __package__ attribute will be set to None. [...] When the import system encounters an explicit relative import in a module without __package__ set (or with it set to None), it will calculate and store the correct value (__name__.rpartition('.')[0] for normal modules and __name__ for package initialisation modules)

(emphasis mine)

If the __name__ is '__main__', __name__.rpartition('.')[0] returns empty string. This is why there's empty string literal in the error description:

SystemError: Parent module '' not loaded, cannot perform relative import 

The relevant part of the CPython's PyImport_ImportModuleLevelObject function:

if (PyDict_GetItem(interp->modules, package) == NULL) {     PyErr_Format(PyExc_SystemError,             "Parent module %R not loaded, cannot perform relative "             "import", package);     goto error; } 

CPython raises this exception if it was unable to find package (the name of the package) in interp->modules (accessible as sys.modules). Since sys.modules is "a dictionary that maps module names to modules which have already been loaded", it's now clear that the parent module must be explicitly absolute-imported before performing relative import.

Note: The patch from the issue 18018 has added another if block, which will be executed before the code above:

if (PyUnicode_CompareWithASCIIString(package, "") == 0) {     PyErr_SetString(PyExc_ImportError,             "attempted relative import with no known parent package");     goto error; } /* else if (PyDict_GetItem(interp->modules, package) == NULL) {     ... */ 

If package (same as above) is empty string, the error message will be

ImportError: attempted relative import with no known parent package 

However, you will only see this in Python 3.6 or newer.

Solution #1: Run your script using -m

Consider a directory (which is a Python package):

. ├── package │   ├── __init__.py │   ├── module.py │   └── standalone.py 

All of the files in package begin with the same 2 lines of code:

from pathlib import Path print('Running' if __name__ == '__main__' else 'Importing', Path(__file__).resolve()) 

I'm including these two lines only to make the order of operations obvious. We can ignore them completely, since they don't affect the execution.

__init__.py and module.py contain only those two lines (i.e., they are effectively empty).

standalone.py additionally attempts to import module.py via relative import:

from . import module  # explicit relative import 

We're well aware that /path/to/python/interpreter package/standalone.py will fail. However, we can run the module with the -m command line option that will "search sys.path for the named module and execute its contents as the __main__ module":

vaultah@base:~$ python3 -i -m package.standalone Importing /home/vaultah/package/__init__.py Running /home/vaultah/package/standalone.py Importing /home/vaultah/package/module.py >>> __file__ '/home/vaultah/package/standalone.py' >>> __package__ 'package' >>> # The __package__ has been correctly set and module.py has been imported. ... # What's inside sys.modules? ... import sys >>> sys.modules['__main__'] <module 'package.standalone' from '/home/vaultah/package/standalone.py'> >>> sys.modules['package.module'] <module 'package.module' from '/home/vaultah/package/module.py'> >>> sys.modules['package'] <module 'package' from '/home/vaultah/package/__init__.py'> 

-m does all the importing stuff for you and automatically sets __package__, but you can do that yourself in the

Solution #2: Set __package__ manually

Please treat it as a proof of concept rather than an actual solution. It isn't well-suited for use in real-world code.

PEP 366 has a workaround to this problem, however, it's incomplete, because setting __package__ alone is not enough. You're going to need to import at least N preceding packages in the module hierarchy, where N is the number of parent directories (relative to the directory of the script) that will be searched for the module being imported.

Thus,

1. Add the parent directory of the Nth predecessor of the current module to sys.path

2. Remove the current file's directory from sys.path

3. Import the parent module of the current module using its fully-qualified name

4. Set __package__ to the fully-qualified name from 2

5. Perform the relative import

I'll borrow files from the Solution #1 and add some more subpackages:

package ├── __init__.py ├── module.py └── subpackage     ├── __init__.py     └── subsubpackage         ├── __init__.py         └── standalone.py 

This time standalone.py will import module.py from the package package using the following relative import

from ... import module  # N = 3 

We'll need to precede that line with the boilerplate code, to make it work.

import sys from pathlib import Path  if __name__ == '__main__' and __package__ is None:     file = Path(__file__).resolve()     parent, top = file.parent, file.parents[3]      sys.path.append(str(top))     try:         sys.path.remove(str(parent))     except ValueError: # Already removed         pass      import package.subpackage.subsubpackage     __package__ = 'package.subpackage.subsubpackage'  from ... import module # N = 3 

It allows us to execute standalone.py by filename:

vaultah@base:~$ python3 package/subpackage/subsubpackage/standalone.py Running /home/vaultah/package/subpackage/subsubpackage/standalone.py Importing /home/vaultah/package/__init__.py Importing /home/vaultah/package/subpackage/__init__.py Importing /home/vaultah/package/subpackage/subsubpackage/__init__.py Importing /home/vaultah/package/module.py 

A more general solution wrapped in a function can be found here. Example usage:

if __name__ == '__main__' and __package__ is None:     import_parents(level=3) # N = 3  from ... import module from ...module.submodule import thing 

Solution #3: Use absolute imports and setuptools

The steps are -

1. Replace explicit relative imports with equivalent absolute imports

2. Install package to make it importable

For instance, the directory structure may be as follows

. ├── project │   ├── package │   │   ├── __init__.py │   │   ├── module.py │   │   └── standalone.py │   └── setup.py 

where setup.py is

from setuptools import setup, find_packages setup(     name = 'your_package_name',     packages = find_packages(), ) 

The rest of the files were borrowed from the Solution #1.

Installation will allow you to import the package regardless of your working directory (assuming there'll be no naming issues).

We can modify standalone.py to use this advantage (step 1):

from package import module  # absolute import 

Change your working directory to project and run /path/to/python/interpreter setup.py install --user (--user installs the package in your site-packages directory) (step 2):

vaultah@base:~$ cd project vaultah@base:~/project$ python3 setup.py install --user 

Let's verify that it's now possible to run standalone.py as a script:

vaultah@base:~/project$ python3 -i package/standalone.py Running /home/vaultah/project/package/standalone.py Importing /home/vaultah/.local/lib/python3.6/site-packages/your_package_name-0.0.0-py3.6.egg/package/__init__.py Importing /home/vaultah/.local/lib/python3.6/site-packages/your_package_name-0.0.0-py3.6.egg/package/module.py >>> module <module 'package.module' from '/home/vaultah/.local/lib/python3.6/site-packages/your_package_name-0.0.0-py3.6.egg/package/module.py'> >>> import sys >>> sys.modules['package'] <module 'package' from '/home/vaultah/.local/lib/python3.6/site-packages/your_package_name-0.0.0-py3.6.egg/package/__init__.py'> >>> sys.modules['package.module'] <module 'package.module' from '/home/vaultah/.local/lib/python3.6/site-packages/your_package_name-0.0.0-py3.6.egg/package/module.py'> 

Note: If you decide to go down this route, you'd be better off using virtual environments to install packages in isolation.

Solution #4: Use absolute imports and some boilerplate code

Frankly, the installation is not necessary - you could add some boilerplate code to your script to make absolute imports work.

I'm going to borrow files from Solution #1 and change standalone.py:

1. Add the parent directory of package to sys.path before attempting to import anything from package using absolute imports:

   import sys from pathlib import Path # if you haven't already done so file = Path(__file__).resolve() parent, root = file.parent, file.parents[1] sys.path.append(str(root))  # Additionally remove the current file's directory from sys.path try:     sys.path.remove(str(parent)) except ValueError: # Already removed     pass 

2. Replace the relative import by the absolute import:

   from package import module  # absolute import 

standalone.py runs without problems:

vaultah@base:~$ python3 -i package/standalone.py Running /home/vaultah/package/standalone.py Importing /home/vaultah/package/__init__.py Importing /home/vaultah/package/module.py >>> module <module 'package.module' from '/home/vaultah/package/module.py'> >>> import sys >>> sys.modules['package'] <module 'package' from '/home/vaultah/package/__init__.py'> >>> sys.modules['package.module'] <module 'package.module' from '/home/vaultah/package/module.py'> 

I feel that I should warn you: try not to do this, especially if your project has a complex structure.

---

As a side note, PEP 8 recommends the use of absolute imports, but states that in some scenarios explicit relative imports are acceptable:

Absolute imports are recommended, as they are usually more readable and tend to be better behaved (or at least give better error messages). [...] However, explicit relative imports are an acceptable alternative to absolute imports, especially when dealing with complex package layouts where using absolute imports would be unnecessarily verbose.

Answer 2

unfortunately, this module needs to be inside the package, and it also needs to be runnable as a script, sometimes. Any idea how I could achieve that?

It's quite common to have a layout like this...

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

...with a mymodule.py like this...

#!/usr/bin/env python3  # Exported function def as_int(a):     return int(a)  # Test function for module   def _test():     assert as_int('1') == 1  if __name__ == '__main__':     _test() 

...a myothermodule.py like this...

#!/usr/bin/env python3  from .mymodule import as_int  # 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() 

...and a main.py like this...

#!/usr/bin/env python3  from mypackage.myothermodule import add  def main():     print(add('1', '1'))  if __name__ == '__main__':     main() 

...which works fine when you run main.py or mypackage/mymodule.py, but fails with mypackage/myothermodule.py, due to the relative import...

from .mymodule import as_int 

The way you're supposed to run it is...

python3 -m mypackage.myothermodule 

...but it's somewhat verbose, and doesn't mix well with a shebang line like #!/usr/bin/env python3.

The simplest fix for this case, assuming the name mymodule is globally unique, would be to avoid using relative imports, and just use...

from mymodule import as_int 

...although, if it's not unique, or your package structure is more complex, you'll need to include the directory containing your package directory in PYTHONPATH, and do it like this...

from mypackage.mymodule import as_int 

...or if you want it to work "out of the box", you can frob the PYTHONPATH in code first with this...

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 

It's kind of a pain, but there's a clue as to why in an email written by a certain Guido van Rossum...

I'm -1 on this and on any other proposed twiddlings of the __main__ machinery. The only use case seems to be running scripts that happen to be living inside a module's directory, which I've always seen as an antipattern. To make me change my mind you'd have to convince me that it isn't.

Whether running scripts inside a package is an antipattern or not is subjective, but personally I find it really useful in a package I have which contains some custom wxPython widgets, so I can run the script for any of the source files to display a wx.Frame containing only that widget for testing purposes.