What does Ruby have that Python doesn't, and vice versa?

There is a lot of discussions of Python vs Ruby, and I all find them completely unhelpful, because they all turn around why feature X sucks in language Y, or that claim language Y doesn't have X, although in fact it does. I also know exactly why I prefer Python, but that's also subjective, and wouldn't help anybody choosing, as they might not have the same tastes in development as I do.

It would therefore be interesting to list the differences, objectively. So no "Python's lambdas sucks". Instead explain what Ruby's lambdas can do that Python's can't. No subjectivity. Example code is good!

Don't have several differences in one answer, please. And vote up the ones you know are correct, and down those you know are incorrect (or are subjective). Also, differences in syntax is not interesting. We know Python does with indentation what Ruby does with brackets and ends, and that @ is called self in Python.

UPDATE: This is now a community wiki, so we can add the big differences here.

Ruby has a class reference in the class body

In Ruby you have a reference to the class (self) already in the class body. In Python you don't have a reference to the class until after the class construction is finished.

An example:

class Kaka
  puts self
end

self in this case is the class, and this code would print out "Kaka". There is no way to print out the class name or in other ways access the class from the class definition body in Python (outside method definitions).

All classes are mutable in Ruby

This lets you develop extensions to core classes. Here's an example of a rails extension:

class String
  def starts_with?(other)
    head = self[0, other.length]
    head == other
  end
end

Python (imagine there were no ''.startswith method):

def starts_with(s, prefix):
    return s[:len(prefix)] == prefix

You could use it on any sequence (not just strings). In order to use it you should import it explicitly e.g., from some_module import starts_with.

Ruby has Perl-like scripting features

Ruby has first class regexps, $-variables, the awk/perl line by line input loop and other features that make it more suited to writing small shell scripts that munge text files or act as glue code for other programs.

Ruby has first class continuations

Thanks to the callcc statement. In Python you can create continuations by various techniques, but there is no support built in to the language.

Ruby has blocks

With the "do" statement you can create a multi-line anonymous function in Ruby, which will be passed in as an argument into the method in front of do, and called from there. In Python you would instead do this either by passing a method or with generators.

Ruby:

amethod { |here|
    many=lines+of+code
    goes(here)
}

Python (Ruby blocks correspond to different constructs in Python):

with amethod() as here: # `amethod() is a context manager
    many=lines+of+code
    goes(here)

Or

for here in amethod(): # `amethod()` is an iterable
    many=lines+of+code
    goes(here)

Or

def function(here):
    many=lines+of+code
    goes(here)

amethod(function)     # `function` is a callback

Interestingly, the convenience statement in Ruby for calling a block is called "yield", which in Python will create a generator.

Ruby:

def themethod
    yield 5
end

themethod do |foo|
    puts foo
end

Python:

def themethod():
    yield 5

for foo in themethod():
    print foo

Although the principles are different, the result is strikingly similar.

Ruby supports functional style (pipe-like) programming more easily

myList.map(&:description).reject(&:empty?).join("\n")

Python:

descriptions = (f.description() for f in mylist)
"\n".join(filter(len, descriptions))

Python has built-in generators (which are used like Ruby blocks, as noted above)

Python has support for generators in the language. In Ruby 1.8 you can use the generator module which uses continuations to create a generator from a block. Or, you could just use a block/proc/lambda! Moreover, in Ruby 1.9 Fibers are, and can be used as, generators, and the Enumerator class is a built-in generator 4

docs.python.org has this generator example:

def reverse(data):
    for index in range(len(data)-1, -1, -1):
        yield data[index]

Contrast this with the above block examples.

Python has flexible name space handling

In Ruby, when you import a file with require, all the things defined in that file will end up in your global namespace. This causes namespace pollution. The solution to that is Rubys modules. But if you create a namespace with a module, then you have to use that namespace to access the contained classes.

In Python, the file is a module, and you can import its contained names with from themodule import *, thereby polluting the namespace if you want. But you can also import just selected names with from themodule import aname, another or you can simply import themodule and then access the names with themodule.aname. If you want more levels in your namespace you can have packages, which are directories with modules and an __init__.py file.

Python has docstrings

Docstrings are strings that are attached to modules, functions and methods and can be introspected at runtime. This helps for creating such things as the help command and automatic documentation.

def frobnicate(bar):
    """frobnicate takes a bar and frobnicates it

       >>> bar = Bar()
       >>> bar.is_frobnicated()
       False
       >>> frobnicate(bar)
       >>> bar.is_frobnicated()
       True
    """

Ruby's equivalent are similar to javadocs, and located above the method instead of within it. They can be retrieved at runtime from the files by using 1.9's Method#source_location example use

Python has multiple inheritance

Ruby does not ("on purpose" -- see Ruby's website, see here how it's done in Ruby). It does reuse the module concept as a type of abstract classes.

Python has list/dict comprehensions

Python:

res = [x*x for x in range(1, 10)]

Ruby:

res = (0..9).map { |x| x * x }

Python:

>>> (x*x for x in range(10))
<generator object <genexpr> at 0xb7c1ccd4>
>>> list(_)
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Ruby:

p = proc { |x| x * x }
(0..9).map(&p)

Python 2.7+:

>>> {x:str(y*y) for x,y in {1:2, 3:4}.items()}
{1: '4', 3: '16'}

Ruby:

>> Hash[{1=>2, 3=>4}.map{|x,y| [x,(y*y).to_s]}]
=> {1=>"4", 3=>"16"}

Python has decorators

Things similar to decorators can also be created in Ruby, and it can also be argued that they aren't as necessary as in Python.

Syntax differences

Ruby requires "end" or "}" to close all of its scopes, while Python uses white-space only. There have been recent attempts in Ruby to allow for whitespace only indentation http://github.com/michaeledgar/seamless

Ruby has the concepts of blocks, which are essentially syntactic sugar around a section of code; they are a way to create closures and pass them to another method which may or may not use the block. A block can be invoked later on through a yield statement.

For example, a simple definition of an each method on Array might be something like:

class Array
  def each
    for i in self  
      yield(i)     # If a block has been passed, control will be passed here.
    end  
  end  
end  

Then you can invoke this like so:

# Add five to each element.
[1, 2, 3, 4].each{ |e| puts e + 5 }
> [6, 7, 8, 9]

Python has anonymous functions/closures/lambdas, but it doesn't quite have blocks since it's missing some of the useful syntactic sugar. However, there's at least one way to get it in an ad-hoc fashion. See, for example, here.

Python Example

Functions are first-class variables in Python. You can declare a function, pass it around as an object, and overwrite it:

def func(): print "hello"
def another_func(f): f()
another_func(func)

def func2(): print "goodbye"
func = func2

This is a fundamental feature of modern scripting languages. JavaScript and Lua do this, too. Ruby doesn't treat functions this way; naming a function calls it.

Of course, there are ways to do these things in Ruby, but they're not first-class operations. For example, you can wrap a function with Proc.new to treat it as a variable--but then it's no longer a function; it's an object with a "call" method.

Ruby's functions aren't first-class objects

Ruby functions aren't first-class objects. Functions must be wrapped in an object to pass them around; the resulting object can't be treated like a function. Functions can't be assigned in a first-class manner; instead, a function in its container object must be called to modify them.

def func; p "Hello" end
def another_func(f); method(f)[] end
another_func(:func)      # => "Hello"

def func2; print "Goodbye!"
self.class.send(:define_method, :func, method(:func2))
func                     # => "Goodbye!"

method(:func).owner      # => Object
func                     # => "Goodbye!"
self.func                # => "Goodbye!"    

Ultimately all answers are going to be subjective at some level, and the answers posted so far pretty much prove that you can't point to any one feature that isn't doable in the other language in an equally nice (if not similar) way, since both languages are very concise and expressive.

I like Python's syntax. However, you have to dig a bit deeper than syntax to find the true beauty of Ruby. There is zenlike beauty in Ruby's consistency. While no trivial example can possibly explain this completely, I'll try to come up with one here just to explain what I mean.

Reverse the words in this string:

sentence = "backwards is sentence This"

When you think about how you would do it, you'd do the following:

1. Split the sentence up into words
2. Reverse the words
3. Re-join the words back into a string

In Ruby, you'd do this:

sentence.split.reverse.join ' '

Exactly as you think about it, in the same sequence, one method call after another.

In python, it would look more like this:

" ".join(reversed(sentence.split()))

It's not hard to understand, but it doesn't quite have the same flow. The subject (sentence) is buried in the middle. The operations are a mix of functions and object methods. This is a trivial example, but one discovers many different examples when really working with and understanding Ruby, especially on non-trivial tasks.

Python has a "we're all adults here" mentality. Thus, you'll find that Ruby has things like constants while Python doesn't (although Ruby's constants only raise a warning). The Python way of thinking is that if you want to make something constant, you should put the variable names in all caps and not change it.

For example, Ruby:

>> PI = 3.14
=> 3.14
>> PI += 1
(irb):2: warning: already initialized constant PI
=> 4.14

Python:

>>> PI = 3.14
>>> PI += 1
>>> PI
4.1400000000000006

You can import only specific functions from a module in Python. In Ruby, you import the whole list of methods. You could "unimport" them in Ruby, but it's not what it's all about.

EDIT:

let's take this Ruby module :

module Whatever
  def method1
  end

  def method2
  end
end

if you include it in your code :

include Whatever

you'll see that both method1 and method2 have been added to your namespace. You can't import only method1. You either import them both or you don't import them at all. In Python you can import only the methods of your choosing. If this would have a name maybe it would be called selective importing?