Python and Ruby are both object oriented languages that provide a smooth transition from procedural to OO programming styles. Smalltalk, by contrast, is object only – you can’t do anything until you understand objects, inheritance and the sizable Smalltalk class hierarchy. By providing procedural training wheels, Python and Ruby “fix” one of the features that may have kept Smalltalk out of the mainstream. The two languages differ by approaching this solution from opposite directions.
Python is a hybrid language. It has functions for procedural programming and objects for OO programming. Python bridges the two worlds by allowing functions and methods to interconvert using the explicit “self” parameter of every method def. When a function is inserted into an object, the first argument automagically becomes a reference to the receiver.
Ruby is a pure OO language that can masquerade as a procedural one. It has no functions, only method calls. In a Ruby method the receiver, also called self, is a hidden argument like “this” in C++. A “def” statement outside of a class definition, which is a function in Python, is actually a method call in Ruby. These ersatz functions become private methods of class Object, the root of the Ruby class hierarchy. Procedural programming is neatly solved from the other direction – everything is an object. If the user doesn’t grok objects yet, they can just pretend that “def” is a function definition and still get useful work done.
Ruby’s OO purity provides a number features that Python lacks or is still working toward: a unified type/class hierarchy, metaclasses, the ability to subclass everything, and uniform method invocation (none of this len() is a function but items() is a method rubbish). Ruby, like Smalltalk, only supports single inheritance, but it does have a very powerful mixin concept: a class definition may include a module, which inserts that module’s methods, constants, etc. into the class.
Ruby, again like Smalltalk, provides closures and code blocks and uses them to the same good effect. The Ruby collection classes and iterators are outstanding, much more powerful and elegant than the ad hoc solutions that Python is sprouting (lambdas and list comprehensions).
Ruby’s syntax and design philosophy are heavily influenced by Perl. It has a lot of syntactic variability. Statement modifiers (if, unless, while, until, etc.) may appear at the end of any statement. Some key words are optional (the “then” in an “if” statement for example). Parentheses may sometimes be elided in method calls. The receiver of a method may usually be elided. Many, many things are lifted directly from Perl. Built in regular expressions, $_ and friends, here documents, the single-quoted / double-quoted string distinction, $ and @ prefixes to distinguish different kinds of names and so forth.
If you like Perl, you will like Ruby and be right at home with its syntax. If you like Smalltalk, you will like Ruby and be right at home with its semantics. If you like Python, you may or may not be put off by the huge difference in design philosophy between Python and Ruby/Perl.
Ruby is much more complex than Python but its features, for the most part, hang together well. Ruby is well designed and full of neat ideas that might be mined for P3K. I’m not sure how many Python programmers will be attracted to it though – it hasn’t won me over (yet). But it is worthy of serious study and could be a real threat to Perl.
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