What Does Ruby Have That Python Doesn'T, and Vice Versa

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

You can have code in the class definition in both Ruby and Python. However, in Ruby you have a reference to the class (self). In Python you don't have a reference to the class, as the class isn't defined yet.

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.

Learning Python from Ruby; Differences and Similarities

Here are some key differences to me:

Ruby has blocks; Python does not.
Python has functions; Ruby does not. In Python, you can take any function or method and pass it to another function. In Ruby, everything is a method, and methods can't be directly passed. Instead, you have to wrap them in Proc's to pass them.
Ruby and Python both support closures, but in different ways. In Python, you can define a function inside another function. The inner function has read access to variables from the outer function, but not write access. In Ruby, you define closures using blocks. The closures have full read and write access to variables from the outer scope.
Python has list comprehensions, which are pretty expressive. For example, if you have a list of numbers, you can write
```
[x*x for x in values if x > 15]
```
to get a new list of the squares of all values greater than 15. In Ruby, you'd have to write the following:
```
values.select {|v| v > 15}.map {|v| v * v}
```
The Ruby code doesn't feel as compact. It's also not as efficient since it first converts the values array into a shorter intermediate array containing the values greater than 15. Then, it takes the intermediate array and generates a final array containing the squares of the intermediates. The intermediate array is then thrown out. So, Ruby ends up with 3 arrays in memory during the computation; Python only needs the input list and the resulting list.
Python also supplies similar map comprehensions.
Python supports tuples; Ruby doesn't. In Ruby, you have to use arrays to simulate tuples.
Ruby supports switch/case statements; Python does not.
~~Ruby supports the standard expr ? val1 : val2 ternary operator; Python does not.~~
Ruby supports only single inheritance. If you need to mimic multiple inheritance, you can define modules and use mix-ins to pull the module methods into classes. Python supports multiple inheritance rather than module mix-ins.
Python supports only single-line lambda functions. Ruby blocks, which are kind of/sort of lambda functions, can be arbitrarily big. Because of this, Ruby code is typically written in a more functional style than Python code. For example, to loop over a list in Ruby, you typically do
```
collection.each do |value|
  ...
end
```
The block works very much like a function being passed to collection.each. If you were to do the same thing in Python, you'd have to define a named inner function and then pass that to the collection each method (if list supported this method):
```
def some_operation(value):
  ...

collection.each(some_operation)
```
That doesn't flow very nicely. So, typically the following non-functional approach would be used in Python:
```
for value in collection:
  ...
```
Using resources in a safe way is quite different between the two languages. Here, the problem is that you want to allocate some resource (open a file, obtain a database cursor, etc), perform some arbitrary operation on it, and then close it in a safe manner even if an exception occurs.
In Ruby, because blocks are so easy to use (see #9), you would typically code this pattern as a method that takes a block for the arbitrary operation to perform on the resource.
In Python, passing in a function for the arbitrary action is a little clunkier since you have to write a named, inner function (see #9). Instead, Python uses a with statement for safe resource handling. See How do I correctly clean up a Python object? for more details.

python and ruby - for what to use it?

They are good for mostly for rapid prototyping, quick development, dynamic programs, web applications and scripts. They're general purpose languages, so you can use them for pretty much everything you want. You'll have smaller development times (compared to, say, Java or C++), but worse performance and less static error-checking.

You can also develop desktop apps on them, but there may be some minor complications on shipping (since you'll usually have to ship the interpreter too).

You shouldn't do critical code or heavy computations on them - if you need these things, make them on a faster language (like C) and make a binding for the code. I believe Python is better for this than Ruby, but I could be wrong. (OTOH, Ruby has a stronger metaprogramming)

Why would one want to use ruby over python or vice versa?

I just came across this comparison, b/w Ruby and Python, which is in terms of performance and memory management.

A fair comparison can be found here. Further, I tend to agree with all the three answers above.

Why does Ruby have Rails while Python has no central framework?

As I see it, Rails put Ruby on the map. The simple fact is that before Rails, Ruby was a minor esoteric language, with very little adoption. Ruby owes its success to Rails. As such, Rails has a central place in the Ruby ecosystem. As slim points out, there are other web frameworks, but it's going to be very difficult to overtake Rails as the leader.

Python on the other hand, had a very different adoption curve. Before Rails, Python was much more widely used than Ruby, and so had a number of competing web frameworks, each slowly building their constituencies. Django has done a good job consolidating support, and becoming the leader in the Python web framework world, but it will never be the One True Framework simply because of the way the community developed.

Does Ruby have a dir method, similar to Python?

There is fairly popular gem pry, which has method called ls that can provide you I believe with something, similar to the dir in Python.

Please advise on Ruby vs Python, for someone who likes LISP a lot

I'd go with Ruby. It's got all kinds of metaprogramming and duck punching hacks that make it really easy to extend. Features like blocks may not seem like much at first, but they make for some really clean syntax if you use them right. Open classes can be debugging hell if you screw them up, but if you're a responsible programmer, you can do things like 2.days.from_now (example from Rails) really easily (Python can do this too, I think, but with a bit more pain)

PS: Check out "Why Ruby is an acceptable LISP".

What Does Ruby Have That Python Doesn'T, and Vice Versa