Ruby use case for nil, equivalent to Python None or JavaScript undefined -

how ruby's nil manifest in code? example, in python might use none default argument when refers argument, in ruby can refer other arguments in arg list (see this question). in js, undefined pops more because can't specify default arguments @ all. can give example of how rubynone pops , how it's dealt with?

i'm not looking example using nil. preferably real code snippet had use nil reason or other.

ruby's nil , python's none pretty equivalent (they represent absence of value), people coming python may find surprising behaviors. first, ruby returns nil in situations python raises exception:

accessing arrays , hashes:

[1, 2, 3][999] # nil. [].fetch(0) raises indexerror {"a" => 1, "b" => 2}["nonexistent"] # nil. {}.fetch("nonexistent") raises indexerror 

instance instance variables:

class myclass   def hello     @thisdoesnotexist   end end  myclass.new.hello #=> nil 

the second remarkable fact nil object lots of methods, can convert integer, float or string:

nil.to_i # 0 nil.to_f # 0.0 # integer(nil) or float(nil) raise typeerror. nil.to_s # "" 

unsurprisingly, python programmers tend dislike permissive behaviour (i did @ first), it's ok once used it.

i not sure answers question, 2 cents.