DI, composition root, the clock

Dependency injection without a framework

DI means dependencies arrive from outside instead of being constructed inside. Python needs no framework for this — an __init__ is a container:

class ReportService:
    def __init__(self, repo, clock, mailer):     # all seams visible
        self._repo, self._clock, self._mailer = repo, clock, mailer

• Constructor injection is the default: required collaborators are arguments. Everything the object needs is visible in the signature.
• Functional core, imperative shell: keep decisions in pure functions (data) -> data, push I/O to the edges. The shell is thin and boring; the core is trivially testable.
• Composition root: one place (often main() or a tiny factory) builds real objects and wires them. Tests build a different graph with fakes.
• The clock, the RNG, the UUID factory are dependencies. Time-dependent logic is untestable only because datetime.now() is hidden inside. Inject clock and pass a fixed fake in tests.
• When construction graphs grow large, a tiny factory function beats a DI framework: explicit, greppable, debuggable.