Decision Checklists Reference

Read when: Final review before committing Python code, need quick lookup of requirements

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Before Writing try/except

• Is this at an error boundary? (CLI/API level)
• Can I check the condition proactively with a cheap, precise test?
• If not, is a small try/except around the authoritative operation clearer?
• Am I adding meaningful context, or just hiding?
• Is a third-party API forcing me to use exceptions because no precise pre-check exists?
• Have I encapsulated the violation?
• Am I catching specific exceptions, not broad?
• If catching at error boundary, am I logging/warning? (Never silently swallow)

Default: Let exceptions bubble up

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Before Path Operations

• Am I checking .exists() because filesystem presence matters for this operation?
• If missing paths should fail during .resolve(), did I pass strict=True?
• Am I treating .is_relative_to() as a boolean check instead of wrapping it for ValueError?
• Am I using pathlib.Path, not os.path?
• Did I specify encoding="utf-8"?

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Before Using typing.cast()

• Have I added a runtime assertion to verify the cast?
• Is the assertion cost trivial (O(1))? If yes, always add it.
• If skipping, is it because I just performed an isinstance check (redundant)?
• If skipping for performance, have I documented the measured overhead?

Default: Always add runtime assertion before cast when cost is trivial

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Before Defining an Interface (ABC or Protocol)

• Do I own all implementations? -> Prefer ABC
• Am I wrapping a third-party library? -> Prefer Protocol
• Do I need runtime isinstance() validation? -> Use ABC
• Is this a minimal interface (1-2 methods)? -> Protocol may be simpler
• Do I need shared method implementations? -> Use ABC

Default for internal application code you own: ABC. Default for external library facades: Protocol.

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Before Preserving Backwards Compatibility

• Did the user explicitly request it?
• Is this a public API with external consumers?
• Have I documented why it's needed?
• Is migration cost prohibitively high?

Default: Break the API and migrate callsites immediately

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Before Inline Imports

• Is this to break a circular dependency?
• Is this for TYPE_CHECKING?
• Is this for conditional features?
• If for startup time: Have I MEASURED the import cost?
• If for startup time: Is the cost significant (>100ms)?
• If for startup time: Have I documented the measured cost in a comment?
• Have I documented why the inline import is needed?

Default: Module-level imports

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Before Importing/Re-Exporting Symbols

• Is there already a canonical location for this symbol?
• Am I creating a second import path for the same symbol?
• If this is a shim module, am I importing only what's needed for this module's purpose?
• Have I avoided __all__ exports?

Default: Import from canonical location, never re-export

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Before Declaring a Local Variable

• Is this variable used more than once?
• Is this variable used close to where it's declared?
• Would inlining the computation hurt readability?
• Am I extracting object fields into locals that are only used once?

Default: Inline single-use computations at the call site; access object attributes directly

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Before Adding a Default Parameter Value

• Do 95%+ of callers actually want this default?
• Would forgetting to pass this parameter cause a subtle bug?
• Is there a safer design that makes the choice explicit?
• If the default is never overridden anywhere, should this parameter exist at all?

Default: Require explicit values; eliminate unused defaults

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Before Adding a Function with 5+ Parameters

• Have I added * after the first (or ctx) parameter?
• Is only self/ctx positional?
• Is this an ABC/Protocol method? (exempt from rule)
• If using ThreadPoolExecutor.submit(), am I using a lambda wrapper?

Default: All parameters after the first should be keyword-only

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Before Writing Module-Level Code

• Does this involve any computation (even Path() construction)?
• Does this involve I/O (file, network, environment)?
• Could this fail or raise exceptions?
• Would tests need to mock this value?

If any answer is "yes", wrap in a @cache-decorated function instead.