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

User · Answer

I think the best solution is to create a package for your module: Here is more info on how to do it.

Once you have a package you don't need to worry about relative import, you can just do absolute imports.

User · Answer

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- gt 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- gt 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- gt 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  gt  gt  gt    file     home vaultah package standalone py   gt  gt  gt    package    package   gt  gt  gt    The   package   has been correctly set and module py has been imported        What s inside sys modules      import sys  gt  gt  gt  sys modules    main      lt module  package standalone  from   home vaultah package standalone py  gt   gt  gt  gt  sys modules  package module    lt module  package module  from   home vaultah package module py  gt   gt  gt  gt  sys modules  package    lt module  package  from   home vaultah package   init   py  gt    -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    Add the parent directory of the Nth predecessor of the current module to sys path Remove the current file s directory from sys path Import the parent module of the current module using its fully-qualified name Set   package   to the fully-qualified name from 2 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 -   Replace explicit relative imports with equivalent absolute imports 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  gt  gt  gt  module  lt module  package module  from   home vaultah  local lib python3 6 site-packages your package name-0 0 0-py3 6 egg package module py  gt   gt  gt  gt  import sys  gt  gt  gt  sys modules  package    lt module  package  from   home vaultah  local lib python3 6 site-packages your package name-0 0 0-py3 6 egg package   init   py  gt   gt  gt  gt  sys modules  package module    lt module  package module  from   home vaultah  local lib python3 6 site-packages your package name-0 0 0-py3 6 egg package module py  gt    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    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  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  gt  gt  gt  module  lt module  package module  from   home vaultah package module py  gt   gt  gt  gt  import sys  gt  gt  gt  sys modules  package    lt module  package  from   home vaultah package   init   py  gt   gt  gt  gt  sys modules  package module    lt module  package module  from   home vaultah package module py  gt    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

User · Answer

I ran into this issue   A hack workaround is importing via an if else block like follows      usr bin env python3  myothermodule  if   name         main         from mymodule import as int else      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

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Hopefully  this will be of value to someone out there - I went through half a dozen stackoverflow posts trying to figure out relative imports similar to whats posted above here  I set up everything as suggested but I was still hitting ModuleNotFoundError  No module named  my module name   Since I was just developing locally and playing around  I hadn t created run a setup py file  I also hadn t apparently set my PYTHONPATH    I realized that when I ran my code as I had been when the tests were in the same directory as the module  I couldn t find my module     python3 test my module module test py                                                                                                               2 4 0 Traceback  most recent call last     File  test my module module test py   line 6  in  lt module gt      from my module module import   ModuleNotFoundError  No module named  my module    However  when I explicitly specified the path things started to work     PYTHONPATH   python3 test my module module test py                                                                                                  2 4 0             ---------------------------------------------------------------------- Ran 11 tests in 0 001s  OK   So  in the event that anyone has tried a few suggestions  believes their code is structured correctly and still finds themselves in a similar situation as myself try either of the following if you don t export the current directory to your PYTHONPATH    Run your code and explicitly include the path like so    PYTHONPATH   python3 test my module module test py  To avoid calling PYTHONPATH    create a setup py file with contents like the following and run python setup py development to add packages to the path       setup py from setuptools import setup  find packages  setup      name  sample       packages find packages

User · Answer

I had a similar problem  I needed a Linux service and cgi plugin which use common constants to cooperate  The  natural  way to do this is to place them in the init py of the package  but I cannot start the cgi plugin with the -m parameter   My final solution was similar to Solution  2 above   import sys import pathlib as p import importlib  pp   p Path sys argv 0   pack   pp resolve   parent  pkg   importlib import module    init     package str pack     The disadvantage is that you must prefix the constants  or common functions  with pkg   print pkg Glob

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TLDR  Append Script path to the System Path by adding following in the entry point of your python script  import os path import sys PACKAGE PARENT        SCRIPT DIR   os path dirname os path realpath os path join os getcwd    os path expanduser   file       sys path append os path normpath os path join SCRIPT DIR  PACKAGE PARENT     Thats it now you can run your project in PyCharma as well as from Terminal

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if both packages are in your import path  sys path   and the module class you want is in example example py  then to access the class without relative import try   from example example import fkt

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My boilerplate to make a module with relative imports in a package runnable standalone  package module py    Standalone boilerplate before relative imports if   package   is None                        DIR   Path   file    resolve   parent     sys path insert 0  str DIR parent         package     DIR name  from   import variable in  init  py from   import other module in package      Now you can use your module in any fashion   Run module as usual  python -m package module Use it as a module  python -c  from package import module  Run it standalone  python package module py or with shebang     bin env python  just  package module py  NB  Using sys path append instead of sys path insert will give you a hard to trace error if your module has the same name as your package  E g  my script my script py Of course if you have relative imports from higher levels in your package hierarchy  than this is not enough  but for most cases  it s just okay

User · Answer

I had a similar problem and solved it by creating a symbolic link to the package in the working directory  ln -s          my package my package and then import it as usual  import my package I know this is more like a  quot Linux quot  solution rather than a  quot Python quot  solution  but it s a valid approach nonetheless

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Moving the file from which you are importing to an outside directory helps  This is extra useful when your main file makes any other files in its own directory  Ex  Before  Project  ---dir1  -------main py  -------module1 py After  Project  ---module1 py  ---dir1  -------main py

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I tried all of the above to no avail  only to realize I mistakenly had a - in my package name  In short  don t have - in the directory where   init   py is  I ve never felt elated after finding out such inanity

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For PyCharm users  I also was getting ImportError  attempted relative import with no known parent package because I was adding the   notation to silence a PyCharm parsing error  PyCharm innaccurately reports not being able to find  lib thing import function If you change it to   lib thing import function it silences the error but then you get the aforementioned ImportError  attempted relative import with no known parent package  Just ignore PyCharm s parser  It s wrong and the code runs fine despite what it says

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Put this inside your package s   init   py file     For relative imports to work in Python 3 6 import os  sys  sys path append os path dirname os path realpath   file        Assuming your package is like this    -- project       -- package            --   init   py            -- module1 py            -- module2 py       -- setup py   Now use regular imports in you package  like     in module2 py from module1 import class1   This works in both python 2 and 3

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If none of the above worked for you  you can specify the module explicitly  Directory   -- Project         -- Dir               --   init   py               -- module py               -- standalone py  Solution   in standalone py from Project Dir module import       module - the module to be imported

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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  PACKAGE PARENT        SCRIPT DIR   os path dirname os path realpath os path join os getcwd    os path expanduser   file       sys path append os path normpath os path join SCRIPT DIR  PACKAGE PARENT     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

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I needed to run python3 from the main project directory to make it work   For example  if the project has the following structure   project demo   -- main py  -- some package        --   init   py       -- project configs py  -- test       -- test project configs py   Solution  I would run python3 inside folder project demo  and then perform a  from some package import project configs

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To obviate this problem  I devised a solution with the repackage package  which has worked for me for some time  It adds the upper directory to the lib path   import repackage repackage up   from mypackage mymodule import myfunction   Repackage can make relative imports that work in a wide range of cases  using an intelligent strategy  inspecting the call stack

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I was getting this ImportError  attempted relative import with no known parent package  In my program I was using the file from current path for importing its function   from  filename import function   Then I modified the current path  Dot  with package name  Which resolved my issue   from package name filename import function   I hope the above answer helps you

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