How to import classes from other python files

You don't need to import anything if it's defined globally, e.g.

main.py

class MyAlgo(QCAlgorithm):
    def Initialize(self):
        self.myLib = MyLibrary(123)  # Create new instance from `MyLibrary` class
    def OnData(self, data):
        x = self.myLib.x             # = 123
    
class MyLibrary:
    def __init__(self, x):
        self.x = x

If `MyLibrary` is saved in some other file like `MyLibrary.py`, you can just import the class with `from MyLibrary import MyLibrary`.

Hmmmm my classes aren't defined globally though, they are defined in a different file. For example: 

If this is my project structure, how can I import classes from test.py into main.py?

The library imports are working fine for me, but it's not exactly what I want. Thanks!

Hi Wesley,

Basically just like Adam was writing / like any other import in Python.

Say you have the class MyClass defined in test.py, you write:

from test import MyClass

Fred

Hi Fred, when I try that, I get the following error:

Algorithm.Initialize() Error: During the algorithm initialization, the following exception has occurred: Loader.TryCreatePythonAlgorithm(): Unable to import python module ./cache/algorithm/project/main.pyc. AlgorithmPythonWrapper(): cannot import name 'MyClass'

Any thoughts on why that might be?

Might be easier if you attach a simplified project. Is your project like this?

#main.py

from test import MyClass
class MyAlgo(QCAlgorithm):
    def Initialize(self):
        instanceOfMyClass = MyClass()
    
--

#test.py

class MyClass:
    def __init__(self):
        pass

Hello,

The error “cannot import name 'MyClass'” usually means that you have a circular import dependency to MyClass.

So one file X needs MyClass so it imports it but the file in which MyClass is needs something in X so it imports it. That simple example does not account for indirect circular references but you can also have that and it would also throw an error.

Circular import dependencies are very easy to solve in Python because you can defer the import statement (which in Python is not a static statement for the pre-compiler like in many other languages but an executable statement, just like all other executable statements).

But to help you defer the right statement and at the right place, yes, we would need more info on your project. All we need is the import portion of all your files, which it seems, you only have two, main and test. So your circular dependency must be that main imports something in test but test also imports something in main.

Fred