Ruby cheatsheet for pythonistas

Marcos Dione — Thu, 31 Mar 2016 17:33:50 GMT

In a shallow but long yak shaving streak, I ended up learning Ruby (again). Coming from a deep Python background (but also Perl and others), I sat to write down a cheatsheet so I can come back to it time and again:

module Foo  # this is the root of this namespace
# everything defined here must be referenced as Foo::thing, as in
Foo::CamelCase::real_method()

:symbol  # symbols are the simplest objects, wich only have a name and a unique value
:'symbol with spaces!'

"#{interpolated_expression}"  # how to iterpolate expressions in strings

/regular_expression/  # very Perl-ish

generator { |item| block }  # this is related to yield

%q{quote words}  # à la perl!
%w{words}  # same?

def classless_method_aka_function(default=:value)  # Ruby calls these methods too
    block  # ruby custom indents by 2!
end

method_call :without :parens

class CamelCase < SuperClass  # simple inheritance
    include Bar  # this is a mixin;
    # Bar is a module and the class 'inherits' all its 'methods'

    public :real_method, :assign_method=
    protected :mutator_method!
    private :query_method?

    self  # here is the class!
    def real_method(*args)  # splat argument, can be anywhere
        # no **kwargs?
        super  # this calls SuperClass::real_method(*args)
        # comapre with
        super()  # this calls SuperClass::real_method()!

        local_variable
        @instance_variable  # always private
        @@class_variable  # always private
        $global_variable

        return self
        # alternatively
        self
        # as the implicit return value is the last statement executed
        # and all statements produce a value
    end

    def assign_method=()
        # conventionally for this kind of syntactic sugar:
        # When the interpreter sees the message "name" followed by " =",
        # it automatically ignores the space before the equal sign
        # and reads the single message "name=" -
        # a call to the method whose name is name=
    end

    class << self
        # this is in metaclass context!
    end

    protected
    def mutator_method!(a, *more, b)
        # conventionally this modifies the instance
    end

    private
    def query_method?()
        # conventionally returns true/false
    end
end

# extending classes
class CamelCase  # do I need to respecify the inheritance here?
    def more_methods ()
    end
end

obj.send(:method_name_as_symbol, args, ...)

begin
    raise 'exceptions can be strings'
rescue OneType => e  # bind the exception to e
    # rescued
rescue AnotherType
    # also
ensure
    # finally
else
    # fallback
end

=begin
Long
comment
blocks!
=end

statement; statement

long \
line

# everything is true except
false
# and
nil

variable ||= default_value

`shell`

AConstant  # technically class names are constants
# so do module names
A_CONSTANT  # conventionally; always public
# The Ruby interpreter does not actually enforce the constancy of constants,
# but it does issue a warning if a program changes the value of a constant

# case is an expression
foo = case
    when true then 100
    when false then 200
    else 300
end

do |args; another_local_variable|
    # args are local variables of this block
    # whose scope ends with the block
    # and which can eclipse another variable of the same name
    # in the containing scope

    # another_local_variable is declared local but does not
    # consume parameters
end

{ |args| ... }  # another block, conventionally single lined

# Matz says that any method can be called with a block as an implicit argument.
# Inside the method, you can call the block using the yield keyword with a value.

# Matz is Joe Ruby

# yield is not what python does
# see http://rubylearning.com/satishtalim/ruby_blocks.html
# block_given?

a= []  # array
a[^0] == nil

ENV  # hash holding envvars
ARGV  # array with CL arguments

(1..10)  # range
(0...10)  # python like
5 === (1..10)  # true, 'case equality operator'

{ :symbol => 'value' } == { symbol: 'value' }  # hashes, not blocks :)

lambda { ... }  # convert a block into a Proc object

# you cannot pass methods into other methods (but you can pass Procs into methods),
# and methods cannot return other methods (but they can return Procs).

load 'foo.rb'  # #include like
require 'foo'  # import, uses $:
require_relative 'bar'  # import from local dir

It's not complete; in particular, I didn't want to go into depth on what yield does (hint: not what does in Python). I hope it's useful to others. I strongly recommend to read this tutorial.

Also, brace yourselves; my impression is that Ruby is not as well documented as we're used in Python.

.:: Marcos Dione/StyXman's glob ::. (Posts about ruby)

Ruby cheatsheet for pythonistas