Toolchains Python Core

Python Core Patterns

Modern Python development patterns targeting 3.13+ for minimal, efficient, reusable, and performant code.

Quick Start

New code: Use @dataclass(slots=True) for data classes, PEP 695 generics (def first[T](...)), Protocol for structural typing, TaskGroup for async concurrency.

Performance: Profile first with py-spy or scalene. Use generators for single-pass iteration, set for membership tests, connection pooling for I/O.

Minimalism Patterns

• Use walrus operator (:=) in while-loops and comprehension filters to eliminate duplicate calls — avoid in deeply nested expressions
• Prefer match/case over if-elif chains for structural/type dispatch; powerful with dataclass destructuring
• Use @dataclass(frozen=True) for value objects, @dataclass(slots=True) for memory savings (~200 bytes/instance)
• Use NamedTuple for lightweight immutable records; dataclass when you need defaults, mutability, or methods
• Always use field(default_factory=list) for mutable defaults — never def f(items=[])
• Prefer dict.get(key, default) over if key in dict for simple lookups
• Use contextlib.suppress(Exception) instead of empty try/except/pass

Efficiency Patterns

• Generator expressions for single-pass iteration; list comprehensions for building lists (20-30% faster than loops)
• __slots__ or @dataclass(slots=True) reduces per-instance memory by 200+ bytes
• set for membership testing (O(1) vs O(n) for lists), frozenset for hashable sets
• str.join() over += for string building; f-strings over .format() (fastest string formatting)
• itertools for composable lazy iteration: chain, islice, groupby, batched (3.12+)
• memoryview for zero-copy binary slicing on large buffers
• @functools.lru_cache / @functools.cache (3.9+) for memoization of pure functions
• collections.deque for O(1) append/pop from both ends; defaultdict to avoid key existence checks
• Batch database operations — use executemany() or bulk inserts, never N+1 loops

Code Reuse

• Protocol over ABC when you want structural typing without inheritance — duck typing with type safety
• ABC when you need shared implementation or runtime isinstance() enforcement
• Composition over inheritance; functools.partial for specialization without subclassing
• functools.cached_property for one-time expensive computed attributes (thread-safe in 3.12+)
• Small, single-responsibility Protocols composed together beat large interfaces
• Use typing.overload for functions with type-dependent return signatures
• Extract shared logic into standalone functions, not base classes — flat is better than nested

Modern Python (3.13+)

• PEP 695 generics (3.12+): def first[T](lst: list[T]) -> T: replaces TypeVar boilerplate (60% reduction)
• type statement (3.12+): type Vector = list[float] replaces TypeAlias
• TypeIs (3.13) preferred over TypeGuard — narrows both branches of conditional
• TaskGroup (3.11+) replaces asyncio.gather() — structured concurrency with auto-cancellation on failure
• except* (3.11+) for handling ExceptionGroup from concurrent failures
• Never swallow CancelledError; call task.result() outside the async with block
• tomllib in stdlib (3.11+) for TOML parsing — no external dependency needed
• ReadOnly TypedDict items (3.13) for immutable typed dict fields

Performance

• Profile first: cProfile for call counts, py-spy/scalene for line-level profiling, tracemalloc for memory leaks
• TaskGroup for structured async I/O concurrency; ProcessPoolExecutor for CPU-bound offloading
• Connection pooling mandatory for databases and HTTP — never open/close per request
• Eager-load related data to prevent N+1 queries; use selectinload() in SQLAlchemy
• uvloop for 2-4x async event loop throughput (drop-in replacement for asyncio loop)
• Pre-allocate collections when size is known: list(range(n)), bytearray(n)
• Avoid global lookups in tight loops — assign to local variable first
• Free-threaded mode (3.13t) shows ~80% improvement for multi-threaded CPU tasks — experimental, not for production

Type Safety

• mypy --strict for new projects; gradual adoption with --disallow-untyped-defs for existing
• Protocol for structural subtyping — don't force users to inherit from your base class
• ParamSpec + Concatenate for typing decorators that modify function signatures
• Self type (3.11+) for fluent method chaining returns
• TypedDict for typed dictionaries with known keys; Unpack for kwargs typing
• Run mypy in CI with zero tolerance for type errors on new code

Anti-Patterns

• ❌ Mutable default arguments: def f(items=[]) — shared across calls
• ❌ Bare except: — catches KeyboardInterrupt, SystemExit; use except Exception:
• ❌ eval() / exec() on untrusted input — injection vulnerability
• ❌ Inconsistent return types: sometimes None, sometimes value — use Optional[T] explicitly
• ❌ isinstance() chains instead of polymorphism or match/case
• ❌ Ignoring with statements for file/connection/lock resources
• ❌ Cargo-cult patterns: @property getters that just return an attribute
• ❌ LBYL (Look Before You Leap) — prefer EAFP (Easier to Ask Forgiveness)