Learning Python from Ruby; Differences and Similarities

I know Ruby very well. I believe that I may need to learn Python presently. For those who know both, what concepts are similar between the two, and what are different?

I’m looking for a list similar to a primer I wrote for Learning Lua for JavaScripters: simple things like whitespace significance and looping constructs; the name of nil in Python, and what values are considered “truthy”; is it idiomatic to use the equivalent of map and each, or are mumble somethingaboutlistcomprehensions mumble the norm?

If I get a good variety of answers I’m happy to aggregate them into a community wiki. Or else you all can fight and crib from each other to try to create the one true comprehensive list.

Edit: To be clear, my goal is “proper” and idiomatic Python. If there is a Python equivalent of inject, but nobody uses it because there is a better/different way to achieve the common functionality of iterating a list and accumulating a result along the way, I want to know how you do things. Perhaps I’ll update this question with a list of common goals, how you achieve them in Ruby, and ask what the equivalent is in Python.

Answer

Here are some key differences to me:

  1. Ruby has blocks; Python does not.

  2. 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.

  3. 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.

  4. 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.

  5. Python supports tuples; Ruby doesn’t. In Ruby, you have to use arrays to simulate tuples.

  6. Ruby supports switch/case statements; Python does not.

  7. Ruby supports the standard expr ? val1 : val2 ternary operator; Python does not.

  8. 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.

  9. 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:
      ...
    
  10. 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.