I have this code repo

I created manual UML which look like this:

I am trying to auto generate the UML via pyreverse:

pyreverse -o png -p ShoppingCart ./mainService.py

Format png is not supported natively. Pyreverse will try to generate it using Graphviz...

Unfortunately, it gives me blank diagram. What can I do to get the project's classes in the diagram?

This is the file structure:

.
├── Entity
│   ├── Apple.py
│   ├── Buy1Get1FreeApple.py
│   ├── Buy3OnPriceOf2Orange.py
│   ├── Offer.py
│   ├── Orange.py
│   ├── Product.py
│   └── ShoppingCart.py
├── Enum
│   └── ProductType.py
└── mainService.py

In short

Assuming the installation of pyreverse and graphviz is correct, all you need to do is to package your project adding some emplty __init__py files in each folder. Alternatively, you'd hjhave to add all the modules manually in the command line.

More details - step by step

About the error message

Assuming everything is installed correctly, your command line should give you the warning message, which is absolutely normal:

Format png is not supported natively. Pyreverse will try to generate it using Graphviz...

What's wrong?

The diagram will stay empty because you tell pyreverse to analyse a single file and there is no class defined in that file. If you'd add manually the different modules to be analysed:

pyreverse -o png -p ShoppingCart mainService.py Entity\Apple.py Enum\ProductType.py Entity\Orange.py Entity\ShoppingCart.py

you would then very well obtain a rudimentary diagram:

If you would add the options -AS upfront you'd get all ancestors in the project and all associated classes recursively:

How to package your project?

This is cumbersome. Fortunately, there's little missing to package your project. For a full reference, you may look here. But in short, it is sufficient to add an empty __init__.py file in your project folder, and each subfolder where you store the modules:

.
├── Entity
│   ├── __init__.py                   <<===== add this empty file
│   ├── Apple.py
│   ├── Buy1Get1FreeApple.py
│   ├── Buy3OnPriceOf2Orange.py
│   ├── Offer.py
│   ├── Orange.py
│   ├── Product.py
│   └── ShoppingCart.py
├── Enum
│   ├── __init__.py                   <<===== add this empty file
│   └── ProductType.py
├── __init__.py                       <<===== add this empty file
└── mainService.py

You will then be able to run the simpler command line:

pyreverse -AS -o png -p ShoppingCart .

and obtain this magnificient diagram:

The packaging helps python and pyreverse to understand that these are not files to be analysed in isolation, but in the context of a package made of subpackages, etc.