python-practices

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name: python-practices description: Apply whenever writing or reviewing Python code — prescriptive best practices for Python 3.14: which idioms to prefer, which anti-patterns to avoid, and which modern language features to reach for first.

Language Standard

Python 3.14 is the target. Always prefer the most modern idiomatic form available in 3.14. When unsure about current best practice for a topic, web-search before writing.

Living Document

When you discuss, teach, or implement a Python pattern that reveals a best practice not yet listed here, add it to the appropriate section before the conversation ends.

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1. Types & Truthiness

• Use isinstance() over type() for type checks — respects inheritance and subclasses
• Use is None / is not None, never == None
• Prefer truthiness checks: if items: not if len(items) > 0:; if not items: not if len(items) == 0:
• Use x or default for falsy-fallback values (understand short-circuit: returns first truthy or last value)
• Use is only for singletons: None, True, False

2. Modern Syntax

• Walrus operator := (3.8+) — assign inside expressions to avoid double evaluation: if (n := len(data)) > 10: or while chunk := f.read(8192):
• Match/case structural pattern matching (3.10+) — prefer over long if/elif chains for multi-way dispatch on type or structure; use:
  • Guard clauses: case x if x > 0:
  • Sequence patterns: case [first, *rest]:
  • Mapping patterns: case {"type": "click", "pos": pos}:
  • Class patterns: case Point(x=0, y=0):
  • as patterns: case [_, *_] as full_list:
  • Dotted-name value patterns (not captures): case Color.RED:
  • Catch-all: case _:
• zip(..., strict=True) (3.10+) — always use when zipping sequences that must be equal length
• dict | other / |= merging (3.9+) — prefer over {**a, **b} unless interleaving inline keys
• ExceptionGroup + except* (3.11+) — use for concurrent/async error aggregation where multiple independent exceptions occur
• Chained comparisons — 1 < x < 10 instead of x > 1 and x < 10; x is evaluated only once

3. Data Structures

• collections.Counter — for counting hashable items; missing keys return 0 (not KeyError); use .most_common(n), and arithmetic operators (+ adds, - subtracts dropping negatives, & keeps minimum, | keeps maximum)
• collections.defaultdict — for auto-initializing missing keys (defaultdict(list), defaultdict(int)); prefer over manual setdefault loops
• typing.NamedTuple class syntax — prefer over collections.namedtuple; supports defaults and type annotations; use _replace() for modified copies, _asdict() for dict conversion
• dict.get(key, default) — prefer over if key in d: ... d[key]
• dict.pop(key, None) — prefer over check-then-delete
• dict.setdefault(key, val) — inserts only if key absent; never overwrites
• Tuple as dict key — for composite keys; lists are not hashable
• Lists as stacks — append()/pop() is O(1); use collections.deque for queues (pop(0) is O(n))
• Avoid [[0]*n]*m — all rows share the same list object; use [[0]*n for _ in range(m)]
• Dict views (d.keys(), d.values(), d.items()) are live — they reflect later mutations

4. Iteration

• enumerate(iterable, start=N) — always prefer over manual index tracking
• zip() — for parallel iteration; zip(*pairs) for unzipping / transposing
• any() / all() — short-circuit boolean queries; any([]) == False, all([]) == True (vacuous truth)
• next(iterator, default) — use the default to avoid StopIteration
• Generator expressions over map()/filter() — equally lazy, more readable; use map/filter only when directly passing to higher-order functions

5. Comprehensions

• Prefer comprehensions over explicit loops for transformations
• Filter form: [x for x in items if cond] — shorter list
• Ternary form: [x if cond else y for x in items] — same length, transformed values (these are different, don't confuse them)
• Flatten with nested comprehension: [cell for row in matrix for cell in row]
• Invert a mapping: {v: k for k, v in d.items()}
• Comprehension variables do not leak into enclosing scope (Python 3); walrus := is the exception — it leaks

6. Functions

• Mutable default arguments — always use None sentinel, construct inside: def f(items=None): items = [] if items is None else items; default objects are shared across all calls
• Keyword-only arguments (*) — use def f(a, *, b): to force callers to name b; good for Boolean flags and optional config
• Positional-only arguments (/) — use def f(a, /): to prevent callers from using keyword form; good for library APIs
• functools.singledispatch — for type-based dispatch; register implementations per type, decouples type-specific logic from the base function
• functools.partial() — for specializing callables without writing wrapper functions
• operator.itemgetter() / attrgetter() / methodcaller() — prefer over lambdas as key/sort functions
• Avoid assigning lambdas to variables — use def instead; lambdas show <lambda> in tracebacks
• for/else — else runs when loop completes without break; idiomatic "search found nothing" pattern, no flag variable needed
• functools.reduce() — for cumulative operations with an initial value; supply initial argument to handle empty sequences

7. Error Handling

• Always catch specific exception types; never bare except: (catches SystemExit, KeyboardInterrupt, etc.)
• except (ValueError, TypeError): — multiple types in one clause
• try/except/else/finally — put "only-if-no-exception" code in else, not after the try block
• Bare raise inside except re-raises preserving the original traceback; raise e adds an extra frame
• raise NewError(...) from original — explicit exception chaining; sets __cause__; never silently drop the original with raise NewError() from None unless intentional
• Inherit custom exceptions from Exception, not BaseException; add custom attributes for structured error context
• contextlib.suppress(ExcType) — prefer over try: ... except ExcType: pass
• ExceptionGroup / except* (3.11+) — for multiple simultaneous errors (e.g., asyncio.TaskGroup)

8. Strings

• f-strings for all string formatting (3.6+); avoid legacy % formatting and .format()
• str.join() for concatenating iterables — ", ".join(words); never s += word in a loop
• Raw strings r"" for regex patterns and Windows paths
• .split() without arguments splits on any whitespace, handles multiple spaces/tabs/newlines
• .strip() / .lstrip() / .rstrip() for whitespace trimming

9. Context Managers

• with for all resource management (files, locks, connections) — guarantees cleanup even on exception
• @contextlib.contextmanager for simple generator-based CMs: code before yield is setup; always wrap yield in try/finally for guaranteed teardown
• contextlib.ExitStack for composing a dynamic number of context managers without nested with statements
• __exit__ returning True suppresses the exception; returning None/False propagates it