Pythonic Idioms

Pythonic idioms

Idioms earn their place by making intent legible, not by proving fluency. A comprehension, an unpack, or a context manager is right when it removes noise; a plain loop is right when the compact form buries what happens.

Method

1. Use a comprehension when the whole point is to build one collection. [transform(x) for x in xs if keep(x)] beats an append loop because the result type and shape are stated up front. The moment the body needs a side effect, a try, or two statements, switch back to a for loop; a comprehension run for effect is a misuse.
2. Unpack instead of indexing. first, *rest = seq, for key, value in d.items(), a, b = b, a. Named targets document what each slot means; row[0] and row[3] force the reader to reconstruct it. Use _ for values you deliberately drop.
3. Pair every acquire with a context manager. with open(...), locks, database sessions, tempfile, and any resource you close. If the library gives no context manager, wrap it in contextlib.contextmanager or closing. Manual try/finally is a smell that a with is missing.
4. Prefer EAFP when the happy path dominates. Attempt the operation and catch the specific failure (try: d[k] except KeyError) rather than pre-checking (if k in d). EAFP avoids the time-of-check/time-of-use race and reads as one intention. Use LBYL only when the check is cheap and the failure is expensive or unrecoverable.
5. Reach for the standard idiom before hand-rolling. enumerate not a manual counter, zip not parallel indexing, dict.get/setdefault and collections.defaultdict not existence dances, any/all not flag loops, f-strings not % or +.
6. Stop when nesting or density hurts. One if filter and one loop in a comprehension is fine; a second for or a ternary inside a nested comprehension is a loop trying to escape. Optimize for the reader who arrives at 3 a.m., not for line count.

Boundaries

• Idioms are about clarity, not speed. A comprehension is not automatically faster than a loop; profile before rewriting for performance.
• Do not chain generator expressions and comprehensions so deeply that a debugger cannot step into them; extract a named function instead.
• EAFP needs a narrow except. Catching broad Exception to avoid a check hides bugs and belongs to python-error-handling, not to idiom.