pytrix

--- name: pytrix description: "Knowledge base from \"Python Tricks: The Book\" by Dan Bader. Use when applying Bader's idioms for assertions, decorators, closures, namedtuples, comprehensions, generators, iterators, dict tricks, data-structure choice, and Pythonic style — while coding Python, studying the book, or referencing its concepts." allowed-tools: - Read - Grep argument-hint: [topic, idiom name, or chapter number] --- # Python Tricks: The Book **Author**: Dan Bader | **Pages**: ~297 | **Chapters**: 8 | **Generated**: 2026-06-07 ## How to Use This Skill - **Without arguments** — load core idioms below for reference. - **With a topic** — ask about `decorators`, `namedtuples`, `generators`, `dict merge`, etc.; I find and read the relevant chapter. - **With a chapter** — ask for `ch06`; I load that specific chapter file. - **Browse** — ask "what chapters do you have?" for the full index. When you ask about a topic not in Core Idioms below, I read the relevant chapter file before answering. --- ## Core Idioms & Mental Models **Assertions are self-checks, not validation (Ch 2).** `assert cond, msg` declares impossible conditions (bugs). Never validate data or guard auth with them — they vanish under `-O`. Never put a tuple in an assert (always truthy). **Trailing commas everywhere (Ch 2).** End every line in multi-line list/dict/set literals with a comma — clean diffs, no accidental string-concat bugs. **`with` + context managers for resources (Ch 2).** Implement `__enter__`/`__exit__` or `@contextlib.contextmanager`; guarantees cleanup via `try/finally`. **Underscores (Ch 2).** `_var` = internal hint (convention); `__var` = real name mangling (`_Class__var`); `__var__` = reserved; `_` = throwaway / last REPL result. **String formatting rule of thumb (Ch 2).** User-supplied → Template Strings (safe). Else → f-strings (3.6+) or `str.format()`. **Functions are first-class (Ch 3).** Assign, store, pass, return them. Closures capture enclosing scope — the basis of factories and decorators. **Decorators = `callable -> callable` (Ch 3).** `@deco` is sugar for `f = deco(f)`; stacking applies bottom-to-top; **always** `@functools.wraps(func)` to preserve metadata. Use for logging/timing/auth/caching. **`*args`/`**kwargs` collect; `*`/`**` unpack (Ch 3).** In a def they gather (tuple/dict); at a call they spread an iterable/dict. **Lambdas sparingly (Ch 3).** Fine as a one-off `key=` func; for anything complex use a named function or comprehension. **`is` vs `==` (Ch 4).** Identity vs equality. Use `is` only for `None`/singletons. **Always add `__repr__` (Ch 4).** Unambiguous, dev-facing, ideally re-creatable: `f'{self.__class__.__name__}({self.x!r})'`. `__str__` (readable) falls back to it. **Custom exception hierarchies (Ch 4).** Derive from `Exception`/built-ins; give a module a base error so callers catch the family with one `except`. **Shallow vs deep copy (Ch 4).** `list(x)`/`copy.copy` shares children; `copy.deepcopy` is fully independent. **ABCs enforce interfaces at instantiation (Ch 4).** `ABCMeta` + `@abstractmethod` beats `NotImplementedError`. **Method types (Ch 4).** instance (`self`), `@classmethod` (`cls`, alt constructors/factories), `@staticmethod` (neither, namespaced). **Class vs instance variables (Ch 4).** Class vars are shared; assigning `self.x` to a class-var name silently shadows it — a classic counter bug (`self.__class__.x += 1`). **Pick the right data structure (Ch 5).** `dict` (O(1)); `deque` for stacks/queues; `namedtuple`/`typing.NamedTuple` for records; `Counter` for multisets; `frozenset` as keys; `queue.PriorityQueue`/`heapq` for priority. Start with `list`, specialize later. **Pythonic loops (Ch 6).** Iterate directly; `enumerate()` for index, `dict.items()` for k/v, `range(a, n, s)` for stepped. Avoid `range(len(...))`. **Comprehensions & generators (Ch 6).** `[expr for x in xs if cond]` is sugar for a build-up loop (also set/dict). Generators (`yield`) and genexprs `(...)` are lazy, memory-efficient iterators; chain them into pipelines. Keep nesting ≤ 1 level. **Dict tricks (Ch 7).** `d.get(k, default)` over membership checks (EAFP); `{key: fn}.get(k, default)()` for switch/case; `sorted(items, key=...)` for custom order; `{**a, **b}` to merge (right wins). Keys identified by equality **and** hash (`True == 1 == 1.0`). **Productivity (Ch 8).** `dir()`/`help()`/`dis.dis()` to explore from the REPL; always isolate deps in a `venv`. --- ## Chapter Index | # | Title | Key Idioms | |---|-------|-----------| | [ch01](chapters/ch01-introduction.md) | Introduction | Python Trick philosophy, Pythonic | | [ch02](chapters/ch02-cleaner-python.md) | Patterns for Cleaner Python | assertions, trailing commas, context managers, underscores, string formatting | | [ch03](chapters/ch03-effective-functions.md) | Effective Functions | first-class functions, closures, decorators, `*args`/`**kwargs`, lambdas | | [ch04](chapters/ch04-classes-oop.md) | Classes & OOP | `is`/`==`, `__repr__`, custom exceptions, copy, ABCs, namedtuples, method types | | [ch05](chapters/ch05-data-structures.md) | Common Data Structures | dict/array/record/set/stack/queue/PQ choice | | [ch06](chapters/ch06-looping-iteration.md) | Looping & Iteration | loops, comprehensions, slicing, iterator protocol, generators, chains | | [ch07](chapters/ch07-dictionary-tricks.md) | Dictionary Tricks | `get()`, sorting, dispatch tables, merge, key identity | | [ch08](chapters/ch08-pythonic-productivity.md) | Pythonic Productivity | `dir`/`help`, virtualenv, bytecode/`dis` | ## Topic Index - **ABC / abstract base class** → ch04 - **argument unpacking** → ch03 - **array / bytes / bytearray** → ch05 - **assertions** → ch02 - **bytecode / dis** → ch08 - **class vs instance variables** → ch04 - **classmethod / staticmethod** → ch04 - **closures** → ch03 - **comprehensions** → ch06 - **context managers / `with`** → ch02 - **copy (shallow/deep)** → ch04 - **Counter / multiset** → ch05 - **decorators** → ch03, ch02 - **defaultdict** → ch05, ch07 - **deque / stack / queue** → ch05 - **dict default / get** → ch07 - **dict merge** → ch07 - **dict dispatch / switch-case** → ch07 - **exceptions (custom)** → ch04 - **first-class functions** → ch03, ch07 - **f-strings / string formatting** → ch02 - **functools.wraps** → ch03 - **generators / generator expressions** → ch06 - **is vs ==** → ch04 - **iterator protocol** → ch06 - **key funcs / sorting** → ch07 - **lambdas** → ch03 - **name mangling / underscores** → ch02 - **namedtuple / NamedTuple** → ch04, ch05 - **priority queue / heapq** → ch05 - **`__repr__` / `__str__`** → ch04 - **slicing** → ch06 - **`*args` / `**kwargs`** → ch03 - **virtualenv** → ch08 - **yield** → ch06 - **Zen of Python** → ch02 ## Supporting Files - [glossary.md](glossary.md) — all key terms with definitions - [patterns.md](patterns.md) — all techniques with when/how/trade-offs - [cheatsheet.md](cheatsheet.md) — quick-reference tables and decision guides --- ## Scope & Limits Covers the book content only (Python 2/3-era idioms; some notes flag 3.5+/3.6+ features). For current Python versions, dataclasses, and modern typing, combine with up-to-date docs. For implementation in your codebase, pair with project-specific tooling.