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The example application is a service that performs mathematical operations. It's a pointless service, or rather its only point is to provide an excuse for me to talk about structuring the project. The shared code is located in the layer/shared folder, while the lambda handlers live in the functions folder. Tests have been separated from the application code in the tests folder - since we don't want them to be included with the deployed code.
This structure is essentially a variant of what's known as the src package layout - with the src directory renamed as layer. For this project I'm using setuptools as the packaging tool, and I'm configuring the package declaratively using a setup.cfg file.
The first step is to turn the internal package into a wheel (a *.whl file). We can use the build tool for this purpose. After installing build with pip we can run it as follows:
There is a closed issue in the mypy repo with a discussion about this problem. The problem can be boiled down to this: mypy only understands Python packages and relationships between them, while our functions folder holds multiple discrete, parallel entry-points into the codebase that don't make sense when interpreted as a package. Contents of the functions directory, in other words, is a bit like a monorepo with multiple distinct projects located in separate directories, and mypy doesn't understand monorepos.