Python / Ruby Comparison
Table Of Contents
Ruby and Python are both agile, interpreted languages with an object oriented design and very large communities. Although the languages share some ideas, syntax elements and have almost the same features the two communities have nothing in common. Not only because the Ruby language is "made in Japan", also because there is some kind of hatred between both communities.
Ruby people hate Python because of missing blocks, indention as syntax element, no native regular expressions and the fact that Python has some functions like len(), abs() etc. which aren't object orientated in their mind. Python people on the other side don't like Ruby because it breaks every rule of The Zen Of Python, It has much too few braces, functions don't look like first-class functions and they miss multiple inheritance :)
But what's really the truth? Most of those points aren't absolutely true or make sense if you know why the developers have chosen that kind of implementation.
Important: this document describes the current stable version of both Python and Ruby. At the moment this is Python 2.5 and Ruby 1.8. If you think I'm wrong in one point send me a mail and I'll try to correct that.
Comparison Table
| Feature | Python | Ruby |
|---|---|---|
| Language Features | ||
| Namespaces | yes | yes |
| Constants | no* | yes* |
| Generators | yes | yes* |
| Iterators | yes | yes* |
| Coroutines | yes | yes |
| Continuations | no | yes |
| Classes | ||
| Multiple Inheritance | yes | no* |
| Interfaces | no* | no* |
| Class Includes* | no | yes |
| Nested Classes | yes | yes |
| Properties | yes | yes* |
| Operator Overloading | yes* | yes* |
| Functions | ||
| First-Class Functions | yes | yes* |
| Anonymous Functions | yes* | yes* |
| Keyword Arguments | yes | no* |
| Closures | yes | yes |
| Decorators | yes | no |
| Collection Objects | ||
| Tuples | yes | no |
| Lists | yes | yes |
| Hashes | yes | yes |
| Strings | ||
| String Type | yes | yes |
| Char Type | no | no |
| Symbol Type | no | yes |
| Immutable | yes | no* |
| Interned | yes | no* |
| Heredocs | no | yes |
| Multiline Strings | yes | yes |
| Unicode Support | yes | no |
| Regular Expressions | ||
| Regex Literal | no | yes |
| Named Groups | yes | no |
| Lookaheads | yes | yes |
| Lookbehinds | yes | no |
| Yes/No Pattern | yes | no |
| Unicode Support | yes | yes* |
Details
Often it's not possible to just say yes or no. So here are some explanations to the comparsion table above.
Interfaces in Python
Currently python doesn't ship an interface system (although some libraries implement interfaces). Maybe this will change with Python3.
Ruby Class Includes
Ruby doesn't provide multiple inheritance. But it's possible to mix-in a module into a class defintion which means that methods defined in that module are part of the class then. In combintation with defined functions a mixin can add new functionallity to a class. For example, if a class has a working .each method, mixing in the standard library's Enumerable module gives the class sort and find methods.
Python Operator Overloading
In Python you can overload each operator using an alias function name except and, or, not and the assignment operator for names (not attributes).
Ruby Operator Overloading
In Ruby you overload operators by defining a method with the operator as name. You can't overload some operators like the call operator or !, not, &&, and, ||, or or ., !=, .., ..., :: and some others.
Ruby Properties
Because instance variables aren't accessable from outside of a class Ruby uses getter and setter methods called variable_name and variable_name= to rebind them. In fact they are just a form of operator overloading. Because you can add, remove or modify those getter and setter methods it's very easy to implement properties although no Ruby programmer would call them properties.
Ruby First Class Functions
Ruby doesn't really have functions at all. Ruby binds functions to an object (if not defined in a class a function is bound to the module) and gives them a special syntax then. If you call a method using the name (you don't need braces) it really gets called and returns the result:
irb(main):001:0> def foo irb(main):002:1> 42 irb(main):003:1> end => nil irb(main):004:0> foo => 42 irb(main):005:0> def bar x irb(main):006:1> 42 - x irb(main):007:1> end => nil irb(main):008:0> bar 23 => 19
This makes it impossible to pass methods around. But you can get the object behind a method:
irb(main):009:0> bar_method = Class.method(:bar) => #<Method: Class(Object)#bar> irb(main):010:0> bar_method.call 23 => 19
The method Class.method looks up the name of the method in the current namespace and returns the object. You can then pass around that method object. So methods are objects too. But nevertheless we can't call that first-class functions. Sorry for that.
Also methods defined in methods aren't defined in the namespace of the method but in the parent namespace if the outer method is called (like in PHP). If you want an anonymous function you have to use Proc.new.
So: methods aren't functions. Functions are code blocks bound to an object (the Proc object), functions are first class, methods not.
Python Anonymous Functions
In Python functions are first-class functions which mean that the behave like any other object. If you create them you'll bind them to a variable in the current namespace. So it's on the one hand possible to create a function inside of another function so that you can call it only from that function. But that's not really anonmous since it has a name. One the other side you have the lambda expressions which allows you to bind expressions to anonymous functions.
Ruby Anonymous Functions
Like in Python you can have anonymous functions in Ruby too. But not using the normal method syntax, you have to use the Proc.new method or the lambda method which work basically the same (there are a few differences from what I know). And in Ruby you also have code blocks which can store some code which you can call. That's some kind of anomyous function too.
Ruby Keyword Arguments
Ruby doesn't have keyword arguments but some libraries (as Rails) use hashes for that purpose. While it doesn't provide all features like in Python. For Ruby2 keyword arguments are on the todo list.
Constants in Ruby and Python
In fact neither Ruby nor Python have real constants. But it a variable starts with an upper case letter in Ruby the interpreter will warn everytime you bin the variable new. In Ruby constants have an own lookup which works different than the local variable / method lookup. In Python you just bind a value to a name.
Ruby Generators and Iterators
If you know iterators from other languages like Python or PHP you might wonder why it doesn't really exist as such. Most objects support methods like .each which is passed a code block that is executed for each item in the sequence (ie: implemented for arrays). In combination with continuations you can implement iterators and generators.
Ruby Strings
In Ruby strings are mutable (like in PHP or C). Because of that strings are a lot heaver than in Python for example where all strings are interned. So you have a second data type for symbols which work like Python strings, except that they don't provide string manipulation functions.
Ruby Unicode Regular Expressions
Because unicode isn't supported at the moment by Ruby it doesn't mean that you can't use them in regular expressions. In fact there is utf-8 support in the current Ruby Regular Expression engine.
Other Differences
There are also other differences between Ruby and Python. Here a list of things that are completely different :-)
Boolean Values
In Python you have a boolean data type called bool which is a subclass of int and which can either be True or False. Those two values are the only instances of this class, they behave like singletons. Every Python object can either be True or False which is defined by overriding the __nonzero__ method of an object. Per default all empty objects (empty lists, dicts, tuples, strings etc) are False. Same for 0 and 0.0.
In Ruby the situation is completely different. There is no boolean data type but there are two singletons (true, instance of TrueClass and false, instance of FalseClass). Everything but nil and false is true. It's also not overrideable.
Namespaces
Ruby and Python have different ideas about namespaces. In Python there is a stack of namespaces, everytime you assign a name it's saved in the highest namespace. In Ruby you have implicit method lookup, an own namespace for local variables, an @-prefix for instance variables, and @@ prefix for class variables and a $-prefix for global variables. And global variables are really global. Each module has access to them. That's something that doesn't really exist in Python where the only way to define a inter module global is to patch the special __builtin__ module.
Iterables
Ruby and Python have different concepts of Iteration. In Python an object is iterable if it defines a method __iter__ which returns in iterator. An iterator is an object with an __iter__ method that returns a reference to itself and a next method which returns the next item or raises StopIteration.
In Ruby you normally iterate passing a code block to the .each method of an object which is called with the item for each item in the sequence.
Whereas both work nearly the same for most of the users Python programmer will find the Ruby way of iteration darn limited because they are used to wrap iterators. (Python ships an module called itertools for example) In fact it's possible to emulate the Python iterator/generator system by using callcc. The generator.rb module in the Ruby distribution does that for example.
Object Orientation
Haaa! The best part. Because it's also the most discussed one. Python and Ruby are both object orientated. Nevertheless most people call Python not object orientated which isn't the case. But Python has a different idea of it. In Ruby the list of methods which are defined per default for the plainest object (Object) looks like this:
irb(main):001:0> Object.methods
=> ["inspect", "autoload?", "send", "class_eval", "clone",
"public_methods", "method", "protected_instance_methods", "__send__",
"private_method_defined?", "equal?", "freeze", "methods", "instance_eval",
"dup", "include?", "private_instance_methods", "instance_variables",
"extend", "protected_method_defined?", "const_defined?", "instance_of?",
"name", "eql?", "public_class_method", "object_id", "hash", "id", "new",
"singleton_methods", "taint", "frozen?", "instance_variable_get",
"constants", "kind_of?", "to_a", "ancestors", "display",
"private_class_method", "const_missing", "type", "autoload",
"<", "protected_methods", "<=>", "instance_methods", "==",
"method_defined?", "superclass", ">", "===", "instance_variable_set",
"instance_method", "const_get", "is_a?", ">=", "respond_to?", "to_s",
">=", "module_eval", "class_variables", "allocate", "class",
"public_instance_methods", "tainted?", "=~", "private_methods",
"public_method_defined?", "__id__", "nil?", "untaint", "included_modules",
"const_set"]
In Python like this:
>>> dir(object) ['__class__', '__delattr__', '__doc__', '__getattribute__', '__hash__', '__init__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__']
That in fact looks different. Let's have a look at the input line of both snippets (I used irb for the first, and the Python shell for the latter). In Ruby you can use the method methods which is defined for each object to get a list of methods. In Python I can use the builtin function dir() to get a list of all members of an object. It's possible to override the methods method in Ruby, but not the behavior of the dir() function which is just meant for object introspection.
It's pretty interesting that the amout of per default methods in Ruby is 73. That's really a huge number of methods which are taken, eg: method names you can't use for something else without destroying functionallity. For example you can't really name a method display for something else than printing the value of the object to an output buffer. Basically you would be able to do that but someone else working with that class would expect a different behavior.
Now let's have a look at the Python class. Because Python objects use the same namespace for methods and members in that list are members and methods. The underlines indicate that those methods are special and defined in the Python specification. But normally you wouldn't call them because there are other ways to do that: the builtin functions. Which do more than just calling the attribute methods etc.
For example those things do exactly the same:
>>> class Foo(object): pass ... >>> foo = Foo() >>> type(foo) <class '__main__.Foo'>
>>> Foo = type('Foo', (object,), {})
>>> foo = Foo.__call__()
>>> foo.__class__
<class '__main__.Foo'>
And that's basically where the difference between Ruby and Python is. In Ruby you would say: an object has a length if it has an method called length and is iterable if it has a method called each. In Python you would say: An object has a length if it has a method called __len__ which I can call using len(obj) and is iterable if it provides a method called __iter__ which returns a valid iterator.
Many people don't like those builtin accessor functions for the special methods in Python but once you learned to live with them you don't have an problem with them. In fact they do more than just calling those internal methods. For example the iter() function which returns the iterator of an object can give you an iterator also for object which don't have a method called __iter__ as long as it has a method called __len__ and a method to get an item for an index, called __getitem__.
Metaclasses
Ruby and Python have strong support for metaprogramming. And the support for metaprogramming is part of the language itself and not added by extension libraries like in PHP.
In Python the class of a new class is resolved after the class definition. Python looks for a __metaclass__ variable in either the class definition, the global namespace of the current module or if it finds nothing, it uses the metaclass of the first class the new class is inheriting from. If the user doesn't inherit from a class it uses the default metaclass which is classobj for current Python versions. In Python 3 this will become type.
In Ruby you have a special syntax to get the singleton of an object which you can use to modify the class of a class.
Of Methods and Functions
In Ruby it's easy. You define methods using the def keyword like you define functions and methods in Python. Methods aren't first-class but you can get the Method object behind the method easily. Additionally you can create anonymous functions using the Proc.new method which have a method call for calling it. Ruby provides an variable self which automatically points to the instance of the class the method is defined in.
In Python you just have functions. But the metaclass (the class of a class in fact) of classes wraps functions in method proxy objects. That means a function inside of a class (without applied decorators like classmethod or staticmethod) is passed an first parameter (usually called self) which points to the current instance of the class. That explicit first parameter is quite irritating at the beginning but makes it possible to pass functions and methods around without problems.
You can for example bind an instance method of an object to another class (self would still point to the instance), or a class method to an instance (self would point to the class), or you can bind the function behind the instance method to another class and self would point to that other class etc.
Syntax
If you compare Ruby and Python code they look similar. In fact they don't have anything in common but some keywords they share. In Ruby the syntax is much more dynamic. Because you can call functions without parenthesis Ruby has a very low amount of statements (like print/import/exec in Python) but functions. Also Ruby has much more possibilities defining a string (including recursive string patterns and heredocs). There is also a difference in expressions. In Ruby everything an expression whereas Python doesn't see assignments as expression. (A lambda in Python can't assign something to an attribute for example)
The Python syntax is defined in the language reference which means that Python doesn't rely on the original C-Implementation. The Python syntax is also very strict in comparison with Ruby where the meaning of an syntax element (for example heredocs vs. lshift operator, fancy strings vs modulo operator, regular expressions vs. division operator) is often guessed (the Ruby interpreter keeps track of defined methods and variables so that it can lower the amount of wrong matches).
Performance
That's a hot topic. Because Python and Ruby are awfully slow in comparison with other languages. But if you compare Python with Ruby, Python wins. Currently. Together with Murphy from the German Ruby forum we found out that some parts of Ruby are badly implemented (for example the .index method of strings is always slower than a regular expression) which means that not the dynamic nature of Ruby leads to a slow language. Also there is no bytecode caching at the moment which will be there with Ruby2. So I think that in the next few years Ruby has a good chance to get faster than the current Python C implementation.
The Python community on the other side knows about a project called pypy which tries to write a Python interpreter in Python itself which is able to translate a restricted Python subset (called rPython) to other languages like C, javascript or into .net assemblies. While the interpreted version of pypy is slower than the C implementation for the moment, compiled rPython code is faster.
Documentation
Documentation is very important for the success of a language. And often people complain (at least I hear people complain) about the Ruby documentation. While that's true for the C code of Ruby, the Ruby documentation for the standard library very good. There is not a single document about the Ruby Specification in English. It's impossible to find out what's a bug in the parser and what not because nobody ever wrote down how the parser is supposted to parse. But now, where Ruby is part of the web2.0 movement I suppose that will improve in the next two years.
On the other side is Python. The documentation is complete for both the language itself, the standard library and inline comments in the C code.
Community
Ruby and Python have a very active and friendly community (although the Ruby community a bit more active and a bit more friendy :D)