RubyGotchas

Last edited 2014-11-26

by Dave Aronson,T. Rex of Codosaurus, LLC

Though "engineered to maximize programmer happiness", with the "principle of least surprise", Ruby still has gotchas.

This presentation will proceed from newbie trivial gotchas, to more advanced and confusing gotchas.

We = 2class Fixnum def rb; self; endend

We <3 .rb => true

But = 3still = 1perfect = 4

But - still .rb < perfect => true

Ruby can be surprising!

Don't quote me on this, but . . . .

x = 3

puts 'x = #{x}\nx'

x = #{x}\nx

puts "x = #{x}\nx"

x = 3

x

String interpolation (including special chars like \n) fails with 'single' quotes -- it requires "double" quotes.

(Just like in most languages with string interpolation.)

To avoid: use doubles whenever practical.

Only two things are false (falsey): false, and nil.

Everything else is true (truthy), even 0 (false in C), "" (false in JS), [], etc.

(Trips up people from C, JS, etc. where some of these are false.)

It's twue! It's twue!

true ? "true" : "false" => "true"false ? "true" : "false" => "false"nil ? "true" : "false" => "false"

1 ? "true" : "false" => "true"0 ? "true" : "false" => "true""false"? "true" : "false" => "true""" ? "true" : "false" => "true"[] ? "true" : "false" => "true"

Symbols != strings.Even if same, printed.Remember which one to use for what (args).Ideally, take either: "Be liberal in what you accept, and conservative in what you send." (Postel's Law)

str = "string"

sym = :string

puts str

string

puts sym

string

str == sym

=> false

Hang him in effigy(String him up, symbolically)

bash> irb-1.9

str = "string"

=> "string"

str[2]

=> "r"

# makes sense *now*

bash> irb-1.8str = "string" => "string"str[2] => 114# ??? ascii code!str[2..2] => "r"# that's better!str[2,1] => "r"# works too....

String... or nothing!

FOO = 5 => 5 FOO = 7(irb):3: warning: already initialized constant FOO => 7 FOO => 7

Constants Aren't (Part 1/2)

(Initial uppercase means constant, in Ruby.)

Try to change a constant.Ooooh, you get a WARNING! BFD.

Even freezing doesn't work for Fixnums.

It does work for arrays (sort of) and most other objects . . . he said foreshadowingly.

FOO

=> 7

FOO.freeze

=> 7

FOO += 2

(irb):5: warning: already

initialized constant FOO

=> 9

FOO

=> 9

Constants Aren't (Part 2/2)

Some are more equal than others

Effectively:

== is the usual(same value)

.eql? is value and class(1 is Fixnum, 1.0 is Float)

.equal? is same object

It's actually much hairier;see docs on class Object

1 == 1.0 => true1.eql? 1.0 => false

a = "foo"b = "foo"a == b => truea.eql? b => truea.equal? b => falsea.equal? a => true

Effectively:

=== is "case equality", as in case statements. A better name (IMHO) might be ".describes?", or overload ".includes?"!

Again, it's actually much hairier; see the docs on class Object.

Gets people from languages where === is identity, or same value and class.

=== != ==!

1 === 1 => trueFixnum === 1 => true1 === Fixnum => false

Class === ClassObject === ObjectClass === ObjectObject === Class => all trueFixnum === Fixnum => false

(1..3) === 2 => true2 === (1..3) => false

&& has higher precedencethan =, sox = true && falsemeansx = (true && false)

and has lower precedence, so x = true and falsemeans(x = true) and false

Ruby Style Guide: Use && / || for boolean expressions, [use] and / or for control flow.

x = true && false

=> false

x

=> false

# OK so far, but:

x = true and false

=> false

x

=> true

Return value is false but variable is true!Why the mismatch?!

and != &&

|| has higher precedencethan =, sox = false || truemeansx = (false || true)

or has lower precedence so x = false or truemeans(x = false) or true

Also, && is higher than ||, but and and or are equal, so they are evaluated left-to-right!

x = false || true

=> true

x

=> true

# OK so far, but:

x = false or true

=> true

x

=> false

Return value is true but variable is false!Why the mismatch?!

or != ||

Whitespace-insensitive?NOT ALWAYS!With multiple args:- No parens, no problem.- Parens w/o space, OK.- Parens and space, NO!Parser thinks it's an expression, as one arg, but (1, 2) is not a valid Ruby expression!

(All work fine w/ 1 arg.)

def method(arg1, arg2); end

method 1, 2 => nil

method(1, 2) => nil

method (1, 2)syntax error, unexpected ',', expecting ')'method (1, 2) ^

Don't be so sensitive! (Part 1/4)

def method; 42; endnum = 21method/num => 2method / num => 2method/ num => 2method /numSyntaxError: unterminated regexp

"method /num" is an unended regex or string! Ruby thinks you might be giving an argument to method meth.General principle: use BALANCED whitespace; both sides or neither.

Don't be so sensitive! (Part 2/4)

"one -1" makes Ruby think you might be giving an argument (of -1) to method one. (Same for +1 . . . or even *1!)Again: use BALANCED whitespace; both sides or neither.

def one 1endone - 1 => 0one-1 => 0one- 1 => 0one -1ArgumentError: wrong number of arguments (1 for 0)

Don't be so sensitive! (Part 3/4)

dbl = ->(x) { x * 2 } => #<Proc:... (lambda)>

dbl = ->x{ x * 2 } => #<Proc:... (lambda)>

dbl = -> x { x * 2 } => #<Proc:... (lambda)>

two = -> { 2 } => #<Proc:... (lambda)>

dbl = -> (x) { x * 2 }syntax error, unexpected tLPAREN_ARG, expecting keyword_do_LAMBDA or tLAMBEG

two = -> () { 2 }same syntax error

"Stabby" lambdas (1.9+)

Parentheses optional

Space before/after args without parens, OK.

Space after parens, OK.

Again, space before parens, NO!

UPDATE: Fixed in 2.0!

Don't be so sensitive! (Part 4/4)

class Foo attr_reader :value def initialize(v) value = v end def set_val(v) @value = v endendf = Foo.new(3)f.value => nil # not 3?!f.set_val 5 => 5f.value => 5

'Ang onto yer @!

Naked value becomes a temporary local variable!

Solution: remember the @! (Or "self.".)

Gets people from Java/C++, not so much Python (which needs "self." too).

"You keep on using that variable. I don't think it means what you think it means."

What the fillintheblank? We didn't change Parent’s @@value before checking it, nor Child’s at all! . . . Or did we?@@ variables are shared with subclasses -- not just that they exist, but the variables themselves! Declaring Child’s @@value changed Parent’s, and inc’ing Parent’s changed Child’s.IMHO, best just forget them.

class Parent @@value = 6 def self.value @@value end def self.inc_value @@value += 1 endend

class Child < Parent @@value = 42end

Parent.value => 42 # wtf?Parent.inc_valueChild.value => 43 # wtf?!

Look out, it’s an @@!

class Parent def initialize puts "Parent init" endend

class NoInitChild < Parentend

NoInitChild.new Parent init

class NormalChild < Parent def initialize puts "NormalChild init" endend

NormalChild.new "NormalChild init"

With init(ialize) or without it

class SuperChild < Parent def initialize puts "SuperChild" super puts "init" endend

SuperChild.newSuperChildParent initinit

Parent's initialize runs automagically only if child has none. Else, parent's must be called to run.

Superman vs. the Invisible Man

Child2.new.add 1, 2, 3, 5ArgumentError: wrong number of arguments (4 for 2)Child2.new.add 1, 2 => 3Child4.new.add 1, 2, 3, 5 => 11

super with no arg list sends what caller gotsuper with explicit args sends those argsto send NO args, use empty parens: super()

class Parent def add *args args.inject :+ endend

class Child2 < Parent def add arg1, arg2 super arg1, arg2 endend

class Child4 < Parent def add a1, a2, a3, a4 super # no args! endend

When will it end? (Or start?)

str = "One\nTwo\nThree"

str =~ /^Two$/ => 4

str =~ /\ATwo\Z/ => nil

str =~ /\AOne/ => 0

str =~ /Three\Z/ => 8

In "standard" regexps:^ is start and $ is end...of the whole string.Ruby’s regexes default to multiline, so:^ is start and $ is end...of any line!\A is start and \Z is end of the whole string. (Or \z to include any newline… which is another gotcha!)

[].any? => false[1].any? => true[:foo, :bar].any? => true

# ok so far, BUT:

[nil].any? => false[false].any? => false[false, nil].any? => false

.any? does not mean “any elements?”!

With block: “do anymake the block true?”

Without: “are any truthy?”Has implicit block:{ |element| element }

getting .any?

Variables declared in blocks passed to iterators (e.g., times or each) are undefined at the top of each iteration!

Iterators call the block repeatedly, so vars are out of scope again after each call.

Built-in looping constructs (e.g., while or for) are OK.

(Or declare vars before block.)

3.times do |loop_num| sum ||= 0 sum += 1 puts sumend111

for loop_num in 1..3 sum ||= 0 sum += 1 puts sumend123

(Un)Def Leppard

arr = ["one", "two", "three"]arr.freezearr << "four"RuntimeError: can't modify frozen Arrayarr[0] = "eno"RuntimeError: can't modify frozen Arrayarr[0].object_id => 1234567890arr[0].reverse!arr => ["eno", "two", "three"]arr[0].object_id => 1234567890

Freezing an array (or a

hash) freezes it, not the

items it contains.

Strings can be modified

in place. This way, you

can modify a given slot in

a frozen Array of Strings.

Freeze (Ar)ray

Changing Fixnum to new value means new object. They can't be modified in place! So, can’t modify a frozen Array of Fixnums.

(Fixnums and Integers have no bang-methods to demo trying with.)

BTW: a Fixnum's object_id is value * 2 + 1.

1 is 1 … and ever more shall be so!

arr = [1, 2, 3, 4]arr.freeze => [1, 2, 3, 4]arr << 5RuntimeError: can't modify frozen Arrayarr[0] += 2RuntimeError: can't modify frozen Array

1.object_id => 33.object_id => 7

str = "foo"str.upcase => ”FOO”str => ”foo”str.upcase! => ”FOO”str => ”FOO”

# Now that it’s already FOO:

str.upcase! => nil # ?!str => ”FOO”

Well-known semi-gotcha: bang versions of methods are dangerous; usually may modify receiver.

DO NOT RELY ON THEM RETURNING SAME VALUE AS NON-BANG VERSION!

Many return nil if no change needed!

(to! || ! to!) == ?

Initial value given as object is same object for each slot (if modded in place, not reassigned as with = or +=).

Initial value given as block gets evaluated separately for each slot. Use this to create new vars for each.

An Array of New Gotchas

class Person attr_accessor :nameend

people = Array.new(3, Person.new)people[0].name = "Alice"people[1].name = "Bob"people[0].name => "Bob"# should have been "Alice"!

people = Array.new(3) { Person.new }people[0].name = "Alice"people[1].name = "Bob"people[0].name => "Alice"

Mostly same problem (and solution) as Arrays.

WARNING: creates new object on any access to empty slot! May create excessive number of new objects; ruins checking “real” contents or count (nil-checking, .size, etc.).

langs = Hash.new []langs[:jane] << "Java"langs[:rachel] << "Ruby"langs[:jane] => ["Java", "Ruby"]langs[:rachel] => ["Java", "Ruby"]

langs = Hash.new { |h, k| h[k] = [] }langs[:jane] << "Java"langs[:rachel] << "Ruby"langs[:jane] => ["Java"]langs[:rachel] => ["Ruby"]

Making a Hash of it

/* JAVA: */try { throw new MyException("blah");} catch(MyException e) { fix_it();}

# RUBY:index = catch(:idx) { arr.each_with_index do |v, i| throw :idx, i if v == target end -1}

begin raise MyException.new "blah"rescue MyException => e fix_itend

Rescue Me, Throw a Line, I'll Try to Catch It!

In Ruby, throw and catch are NOT for exceptions!

They are advanced flow control, to exit deep nesting.

Ruby uses raise and rescue for exceptions.

- Watch out for these gotchas as you code.

- If Ruby behaves badly, refer to these slides.

- Available at http://bit.ly/RubyGotchas

- If your gotcha isn’t listed, tell me; maybe I’ll add it!

I’m Gonna Getcha Getcha Getcha Getcha!

Add gotchas:- to_s vs. to_str- need to coordinate method_missing and respond_to_missing?- rescue from a StandardError, not an Exception- instance_eval with calls in local scope- private data isn’t really, and not at all w/ class methods- private/protected not same as in other languages- diffs in lambda/proc/block/method, WRT break/next/return/etc.- braces vs. do-end (TL;DR: braces high precedence, do-end low)- attribute=(val) always returns the argument, no matter the code- Proxies are always truthy, even if the target is not- class Foo::Bar, defined outside Module Foo, won’t see inside Foo- in debugging, “next” goes into loops but skips over blocks- vars introduced in a loop (not block!) are visible outside it (?)- private methods are accessible by the instance, not the whole class

Rails gotchas?Screencast series -- maybe text, audio, and screencast versions?

The Someday-Maybe List

questions.any? ; gotchas[:more].any?Contact information / shameless plug:

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