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04 — Collections

One line: Ruby's Array, Hash, Range, Set, Struct and Data are Python's list, dict, range, set, namedtuple and frozen dataclass — with nil where Python raises, insertion order where Python has hash-table order, and one <=> method where Python has six.

The containers a Python programmer already knows are all here, and most of a Python mental model carries over: arrays index from zero and count backwards from −1, hashes keep insertion order, sets do the algebra, ranges slice, and records compare by value. The chapter is about where that model breaks. The first break is the missing case: a[10] and h[:zz] answer nil where Python's list and dict raise, and fetch is the method that raises. The second is the default: a Hash carries one, and the difference between a value given to Hash.new (returned, never stored), a block (stored), and a mutable object (shared by every miss) is the trap that Hash.new([]) and Array.new(3, []) set for a newcomer, exactly as [[]] * 3 and dict.fromkeys(keys, []) do in Python. The third is that a Range is an object with two ends rather than a list of numbers — inclusive or exclusive, endless, made of Strings — where Python's range is a half-open integer sequence.

The second half of the chapter is about identity and order. A Set is a core class in 4.0, written in C, and it keeps insertion order where a Python set walks its hash table; in both languages an element is identified by hash and eql? (__hash__ and __eq__), but Ruby lets a class in that defines neither and Python refuses one that defines only __eq__. Destructuring is the same syntax with different rules at the edges: Ruby fills a missing value with nil and drops an extra one, unpacks only Arrays (and to_ary), and lets a block spread an Array across its parameters; Python raises ValueError, unpacks any iterable, and never spreads. Struct and Data are the two records — one mutable, one not — beside namedtuple and the two flavours of dataclass. And Comparable is the mixin that turns one <=> method into <, ==, between? and clamp, where Python's functools.total_ordering turns __eq__ and __lt__ into the other four — and where nil from <=> and NotImplemented from __lt__ are the two ways to say "these do not compare".

Lesson Level The one thing
Past the end, an array gives nil, not an error 101 a[10] is nil and a[5] = x pads with nil; Python raises on both.
A missing key is nil, and Hash.new([]) shares its default 101 Three kinds of default, and which one stores.
Two dots include the end, three dots exclude it 101 A Range is two ends and a flag, not a list; cover? is not include?.
Set is a core class in 4.0 and keeps insertion order 201 No require, C underneath, insertion order kept, elements by hash/eql?.
Assignment destructures, and blocks auto-splat 201 Missing is nil, extra is dropped, only Arrays unpack, blocks spread.
Struct is mutable, Data is not 201 Two records that compare by value; with instead of a setter.
Define <=> and Comparable gives you the rest 201 One method buys six; nil means "does not compare" and sort raises on it.

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