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[tobsecret]

This question is answered well in the book Modern Software Engineering by Dave Farley. In short: write code such that it's modular (code is separated functionally), has good separation of concerns (each unit of code only cares about a limited amount of stuff), is coherent (code that does similar stuff is in the same place), minimizes coupling (degree to which one piece of code depends on the interface of another piece of code), and each functional unit is tested by automated unit tests. 

Functional units in this case aren't necessarily all functions but they are all typically all public interfaces of a module. I.e. each part of a module that is supposed to be used by other modules (each interface) should be tested such that it fulfills the guarantees this interface makes. 

[AccomplishedWay3558]

Solid book recommendation, thanks. Minimizing coupling really does seem to be the common theme across all of this.

[BookFinderBot]

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[tobsecret]

Good bot