how to properly import a module which need another module to run？

Question

I met an import error called no module named XX. my project file is organized as follows:

-------A.py
          |
          B-----__init__.py        wrote: from .C import C
                        |
                        C.py
                        |​
                        D -----__init__.py           wrote: from .E import E
                                         |
                                         E.py

In A.py, I need to import class C from C.py. but class C needs to use class E(in E.py)to run

In A.py, I wrote import B.C

In C.py, I wrote import DE

when I run the test in A.py, it gives the error: No module named 'D' But if I test C.py, there is no problem at all.

Can anyone tell me why and how to fix it?

Answer 1

When you try to run A.py , it fails because the Python interpreter (the program that runs your Python script) cannot find the D package.

When you run C.py , however, the interpreter does find the D package. This is because the script you are running ( C.py ) is located in the same directory/folder as the D package.

Detailed explanation found in the Python Docs :

When a module named spam is imported, the interpreter first searches for a built-in module with that name. If not found, it then searches for a file named spam.py in a list of directories given by the variable sys.path . sys.path is initialized from these locations:

• The directory containing the input script (or the current directory when no file is specified).

• PYTHONPATH (a list of directory names, with the same syntax as the shell variable PATH).

• The installation-dependent default (by convention including a site-packages directory, handled by the site module).

A quick fix would be to use a relative import in C.py :

from .D import E