Ruby for Pythonistas

It’s been 11 years since I first encountered Python, and it’s been my favourite programming language ever since. However, I’ve always been curious about the Ruby language. On the surface, they have a lot in common: both dynamic, object-oriented, interpreted, vm-based, and optimize for developer happiness rather than machine speed. However, they have quite different data models, syntax and community culture.

This post is a summary of my journey learning Ruby as a seasoned Python developer, and my thoughts comparing these two popular languages.

Disclaimer:

• I’m by no means a Ruby expert. My understand of Ruby so far has been quite superficial.
• Most of the points here are my notes while going through the excellent Ruby Koan exercises. A big shout out to them!

Arrays

In my opinion, Ruby arrays are a lot richer than Python lists.

Concatenation

Use the << operator on arrays:

a << 3

in Python:

a.append(3)

Access

To access an array, you can use the array index just like it is in almost every other language. However, Ruby’s Array class also implements convenient methods to access the first and last element of the array:

irb(main):004:0> a = [1, 2, :three]
=> [1, 2, :three]
irb(main):005:0> a[0]
=> 1
irb(main):006:0> a[-1]
=> :three
irb(main):007:0> a.last
=> :three
irb(main):008:0> a.first
=> 1

Slices and Ranges

Both Python and Ruby support slicing although the syntax are slightly different:

Ruby: arr[a, b], where a is the starting index and b is the size of the slice.

irb(main):012:0> a=[:one, :two, :three]
=> [:one, :two, :three]
irb(main):013:0> a[0,1]
=> [:one]

Python: arr[a:b], where a is the starting index and b is the ending index (exclusive)

>>> a=[1,2,3]
>>> a[0:1]
[1]

Python has range and xrange builtin functions.

>>> list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Ruby also has range, but it provides language construct for Range objects:

irb(main):010:0> (1..5).to_a
=> [1, 2, 3, 4, 5]

irb(main):011:0> (1...5).to_a
=> [1, 2, 3, 4]

It’s worth noting that double dot (..) creates an inclusive interval whereas triple dot (...) creates an exclusive interval. This threw me off at first, since somehow my brain associate ... with inclusive and .. being exclusive.

In Ruby, you can also pass a range object to an array, and that behaves more or less the same way as Python’s slicing:

irb(main):019:0> a[1...3]
=> [:two, :three]

Python’s slicing is also pretty flexible. Python’s special method __getitem__ is able to take a range object (or any object for that matter) and override the behaviour of [] operator.

Unpacking

One of the nice features of Python is list/tuple unpacking, e.g.,

>>> a=['one', 'two']
>>> one, two = a
>>> one
'one'
>>> two
'two'
>>>

IMO, Ruby does it better.

irb(main):020:0> a=[:one, :two]
=> [:one, :two]
irb(main):021:0> one, two = a
=> [:one, :two]
irb(main):022:0> one
=> :one
irb(main):023:0> two
=> :two

Left side doesn’t have to match the right side cardinally:

irb(main):024:0> one, two = [1, 2, 3, 4, 5]
=> [1, 2, 3, 4, 5]
irb(main):025:0> one
=> 1
irb(main):026:0> two
=> 2

irb(main):027:0> one, two = [1]
=> [1]
irb(main):029:0> one
=> 1
irb(main):030:0> two
=> nil

In Python, that’s a ValueError:

>>> one, two = [1,2,3,4,5]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  ValueError: too many values to unpack (expected 2)

>>> one, two = [1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  ValueError: not enough values to unpack (expected 2, got 1)

Ruby also supports “wildcard” match which in Python-land is only available after Python 3.

irb(main):031:0> first, *rest = [1,2,3,4,5]
=> [1, 2, 3, 4, 5]
irb(main):032:0> first
=> 1
irb(main):033:0> rest
=> [2, 3, 4, 5]

Both Languages support inline swapping:

irb(main):034:0> a, b = [1, 3]
=> [1, 3]
irb(main):035:0> a, b = b, a
=> [3, 1]
irb(main):036:0> a
=> 3
irb(main):037:0> b
=> 1

Hashes

Ruby hashes are equivalent of Python dictionaries. They can be constructed and accessed using similar syntax.

No KeyError

In Python, when accessing a key that doesn’t exist in the dictionary, you get a KeyError:

>>> x={'a': 1}
>>> x['b']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  KeyError: 'b'

In Ruby, you simply get nil:

irb(main):038:0> a={:a => 1, :b => 2}
=> {:a=>1, :b=>2}
irb(main):039:0> a[:c]
=> nil

However, doing a fetch will result in KeyError if it doesn’t exist:

irb(main):040:0> a.fetch(:c)
KeyError: key not found: :c
    from (irb):40:in `fetch'
    from (irb):40
    from /usr/bin/irb:11:in `<main>'

Default value

In Ruby, default value functionality is built-in with Hash class:

irb(main):041:0> x=Hash.new([])
=> {}
irb(main):042:0> x[:one]
=> []

In Python, we use collections.defaultdict:

>>> import collections
>>> x=collections.defaultdict(list)
>>> x["one"]
[]

Python’s defaultdict is more flexible, allowing the default value be obtained via a callable. Maybe there’s a way to achieve the same thing with Ruby but I haven’t found it.

Gotcha: The default value in Ruby’s Hash though is shared among all instances:

irb(main):043:0> x=Hash.new([])
=> {}
irb(main):044:0> x[:one] << "1"
=> ["1"]
irb(main):045:0> x[:two] << "2"
=> ["1", "2"]
irb(main):046:0> x[:two]
=> ["1", "2"]

This could lead to subtle bugs if not careful.

A safer approach is to use block initialization:

irb(main):047:0> x=Hash.new { |hash, key| hash[key] = [] }
=> {}
irb(main):048:0> x[:one] << 1
=> [1]
irb(main):049:0> x[:two] << 2
=> [2]
irb(main):050:0> x[:two]
=> [2]

Block is one of my favourite Ruby language features.

Strings

Strings are similar in both languages, except Ruby strings are mutable, while Pythons’ are immutable. Ruby also has more ways to escape quotes:

irb(main):051:0> %(I can use ' and " here no problem)
=> "I can use ' and \" here no problem"

irb(main):052:0> %!and here '"!
=> "and here '\""

irb(main):053:0> %{or here '"}
=> "or here '\""

irb(main):054:0> %{multi
irb(main):055:0" line
irb(main):056:0" strings%}
=> "multi\nline\nstrings%"

You can use regex as string index to extract the matched substring:

irb(main):065:0> "j'ai 34 ans"[/\d+/]
=> "34"

Symbols

Symbols are a unique in Ruby that’s not present in Python (although I wish). It has permeated into the design of other modern languages, like Clojure and Elixir. You can think of symbols as a way to do free-form enums, a way to name something, as oppose to just some free form texts.

You can get all symbols in the current scope:

irb(main):060:0> Symbol.all_symbols
[... long list of global symbols ...]

You can use symbols to refer to functions:

irb(main):062:0> ''.respond_to? :to_i
=> true

You can dynamically create a symbol from strings:

irb(main):063:0> 'abc'.to_sym
=> :abc

Methods

Default values

Same as Python:

def foo(a, b='default') 

Variadic arguments

Same as Python:

def foo(a, *c)

Inside the method, c is available as an Array.

Invocation and Return

Ruby method invocation does not require parentheses, unless it’s it results in ambiguity. e.g., you can omit parens in:

def foo(arg)
  ...
end

foo :bar

but you have to use parens to disambiguate in situations like:

def foo(arg)
end

def bar(arg)
end

bar(foo arg)

In Python, parens are mandatory.

In Ruby, almost everything is an expression. In the case of a method definition, the last expression becomes the return value of the method. Some people call it implicit returns, and people have mixed feelings about it. Personally I like the everything-is-an-expression model and using the last expression as the return value feels natural. In Python, you have to use return, otherwise, it implicitly returns None.

Keyword arguments

Ruby 1.x doesn’t have keyword arguments as a language feature. This is a little disappointing. However, it’s idiomatic in Ruby to have a method accept a hash, and use symbols to simulate keyword arguments:

irb(main):068:0> def foo(args)
irb(main):069:1>   puts args
irb(main):070:1> end
=> :foo
irb(main):071:0> foo(a: 5, b: 6)
{:a=>5, :b=>6}

Since you can omit {} in Hash construction, this code is almost like Python’s keyword arguments. However, you have to do argument validation yourself.

In Ruby 2.x, this pattern has been elevated as a language feature, so now Ruby has proper keyword argument support. However, I have not seen an equivalent of Python’s keyword-only argument feature.

Constants

Ruby symbols start with capital letters are “constants”, so classes are also “constants”:

irb(main):074:0> class foo
irb(main):075:1> end
SyntaxError: (irb):74: class/module name must be CONSTANT
        from /usr/bin/irb:11:in `<main>'

Ruby constants are more enforced than their counterpart in Python. (well, technically, Python doesn’t have constants, only by convention, all cap symbols are considered constants.)

irb(main):076:0> A=1
=> 1
irb(main):077:0> A=2
(irb):77: warning: already initialized constant A
(irb):76: warning: previous definition of A was here
=> 2

Truthiness

Truthiness in Ruby is a lot different from Python. Python has the concept of “falsy”, in which 0, [], {}, '' are all evaluated to False. In Ruby, however, only false is false, and everything else is treated as true:

def is_true?(value)
  if value
    :true
  else
    :false
  end
end

irb(main):098:0> is_true? 0
=> :true
irb(main):099:0> is_true? '0'
=> :true
irb(main):100:0> is_true? ''
=> :true
irb(main):101:0> is_true? []
=> :true
irb(main):103:0> is_true?({})
=> :true
irb(main):104:0> is_true?(false)
=> :false

Exceptions

Ruby’s exception hierarchy is a lot like Python’s, at least in name:

RuntimeError < StandardError < Exception

To handle exceptions, you use begin...rescue...ensure rather than try...except...finally.

Map/Reduce/Filter

Ruby the language itself is not a functional language, although it provides machinery for you to program in a functional way, such as using high-order functions.

When first learning Ruby, I was looking for my familiar friends map/reduce/filter but couldn’t find any. Then I realized they’re called something completely different:

In Ruby, filter is achieved using select/find_all:

irb(main):105:0> [1,2,3,4,5].select { |x| x % 2 == 0 }
=> [2, 4]
irb(main):106:0> [1,2,3,4,5].find_all { |x| x % 2 == 0 }
=> [2, 4]

map is equivalent to collect/map:

irb(main):107:0> [1,2,3,4,5].collect { |x| x * 2 }
=> [2, 4, 6, 8, 10]

reduce is not reduce, nor is it called fold, but is weirdly named inject:

irb(main):108:0> [1,2,3,4,5].inject(0) { |x, y| x + y }
=> 15

Blocks

As I eluded to before, blocks are one of my favourite Ruby language features. It’s comparable to Python’s context managers, but it can be invoked without using a keyword and can be added to any method calls. It feels more natural.

Similar to Python (using contextlib.contextmanager), you can invoke a block by yield to the caller:

def do_with_logging
    log "start"
    yield
    log "end"
end

do_with_logging {
    # serious business
}

A Ruby method is also able to determine if it’s called with a block being passed by using block_given? method.

def do_it
    if block_given?
        yield
    else
        put "no block"
    end
end

Classes

Ruby classes are defined in a similar way as Python.

self

Ruby’s self can reference different things in different scopes. For example, inside a class definition, self refers to the class object (kind of like when you define a classmethod in Python), but inside an instance method, self refers to the instance of the class.

private

Python doesn’t have private members or methods. With Ruby however, you can define “private” methods if you put private in front of your method definition.

class Foo
    private def foo
        puts "private"
    end
end

Outside callers can’t call foo. Members of the same class can call foo if only it is called with self being the implicit receiver:

class Foo
    private ...

    def use_foo
        foo  # this is fine
        self.foo  # Not allowed
    end
end

Ruby’s instance variables are private with @ prefix:

class Foo
    def initialize(name)
        @name = name
    end
end

To access @name you can define getters and/or settings:

class Foo
    attr_accessor :name
    # or attr_reader :name to make it readonly
    ...
end

Although the private restriction can be circumvented by metaprogramming:

foo = Foo.new
foo.instance_variable_get("@name")

Overall, I think Ruby encourages more encapsulation and better design.

Define class methods

You can refer to the class by name when defining methods to make class methods:

class Foo
    def Foo.bar
    end
end

Alternatively, remember we said before that self inside a class definition refers to the class itself? So this works too:

class Foo
    def self.bar
    end
end

Another way to write class method uses an inner class:

class Foo
    class << self
        def bar
        end
    end
end

This is good if you want to group class methods together.

Open class

A powerful (yet controversial) feature of Ruby is that you can “amend” any Ruby classes (even the builtin ones) during runtime. It’s like monkey-patching on steroids.

class ::Integer
    def even?
        self % 2 == 0
    end
end

Now suddenly 2.even? is a thing. It certainly makes writing your own DSL a lot easier, but it may lead to magical code that’s hard to track down.

Inheritance

Ruby uses A < B to mean class A inherits from class B. super is available to refer to the same-named methods in the super class. Because Ruby is single inheritance, there’s no ambiguity of super here.

Mixin

Ruby’s class can only have a single parent, but you can “mixin” behaviour into your class by include other modules. Compared to Python, which does support multiple inheritance, and extra attention has to be paid to avoid diamond inheritance problem.

I feel the Ruby design is more thought-out. As oppose to give more ropes to developers to hang themselves, it makes the use case of multiple inheritance more clear (only for mixins).

Message Passing

Ruby’s object-oriented model is based on the idea of message passing. As opposed to invoking using obj.method, you can pass a message method to object obj:

obj.send :method

obj.__send__ :method

Use respond_to? to test if a receiver can handle such message:

if obj.respond_to? :method
    ...
end

obj can implement method_mssing? method to implement generic method dispatcher. With Python, you can achieve the same thing with __getattribute__ magic method.