01 — Objects and values

One line: Every Ruby value is an object with a class, an identity and a set of messages it answers — and the surprises for a Python programmer are which values count as false, what a symbol is, four equality methods where Python has two, strings that change in place, and a freeze that protects one object at a time.

This chapter is the ground the rest of the library stands on. A literal is a receiver (1.class, 3.times), an operator is a method (1.+(2)), nil is an object with methods, and the top level of a script is an object called main. From there the chapter takes the ideas a Python programmer already holds — truthiness, identity, equality, references, immutability — and prints where each one carries over and where it breaks: only nil and false are falsy, so 0 and "" are true; a symbol is a name with exactly one object per spelling, which Python covers with interning and Enum; ==, eql?, equal? and === are four different questions, and Python folds the second into the first; assignment shares objects in both languages, but Python's += on a list mutates while Ruby's always rebinds.

The last three pages are about change. A Ruby string is a mutable buffer, so << keeps the object and += makes a new one, where a Python str cannot change at all; freeze makes an object immutable but is shallow, Ractor.make_shareable freezes all the way down, and the magic comment freezes a file's literals, where Python gets immutability from the type and its frozen dataclass has a back door; and duck typing — respond_to?, the implicit conversions to_str/to_ary/to_proc/to_int, and Array()/Integer() — is the same philosophy Python spells hasattr, __iter__/__call__/__index__ and list()/int(), with a Protocol that Ruby lacks. Every page runs a Ruby program and a Python twin that print the same numbered rows, so each comparison is a measurement.

Lesson	Level	The one thing
Everything is an object, even 1 and nil	101	1.class, nil.to_a and 1.+(2) all work, Class.class is Class, and self at the top level is an object called main
Only nil and false are falsy	101	!!0, !!"" and !![] are all true, an or-default replaces only nil and false, and no class can opt in to falsiness
A symbol is a name, not a string	101	:a.equal?(:a) is true while two "a" literals are two objects; symbols name methods, key hashes and answer to_proc
Ruby has four kinds of equality	201	equal? is identity, == is value, eql? refuses 1 == 1.0 (so Hash keeps 1 and 1.0 apart), and === is what case/when calls
A variable is a reference, not a box	101	b = a; b << x is seen through a, b += x is not, a method can mutate but not rebind, and dup copies one level
freeze is shallow and raises FrozenError	201	a frozen array still holds mutable strings, Ractor.make_shareable freezes deeply, and # frozen_string_literal: true freezes a file's literals
Strings are mutable; << keeps the object	101	s << "x" keeps equal? true, s += "x" makes a new object, upcase! returns nil when nothing changed, and a literal is chilled
Duck typing asks respond_to?, not class	201	respond_to?(:each), is_a? versus instance_of?, the to_str/to_ary/to_proc/to_int protocol, and strict Array()/Integer()

Read more

• Object ↗ — the methods every object has, including equal?, ==, eql?, ===, freeze, dup, respond_to? and is_a?
• Symbol ↗ and NilClass ↗ — the two classes that have no Python counterpart in the shape Ruby gives them
• Data model ↗ — Python's account of objects, identity, __eq__, __hash__, __bool__ and the dunder protocols the twins call
• Python crosswalk ↗ — the Python library's one-table map, for the reader going the other way