Learning Python from Ruby; Differences and Similarities

My suggestion: Don’t try to learn the differences. Learn how to approach the problem in Python. Just like there’s a Ruby approach to each problem (that works very well givin the limitations and strengths of the language), there’s a Python approach to the problem. they are both different. To get the best out of each language, you really should learn the language itself, and not just the “translation” from one to the other.

Now, with that said, the difference will help you adapt faster and make 1 off modifications to a Python program. And that’s fine for a start to get writing. But try to learn from other projects the why behind the architecture and design decisions rather than the how behind the semantics of the language…

I know little Ruby, but here are a few bullet points about the things you mentioned:

• nil, the value indicating lack of a value, would be None (note that you check for it like x is None or x is not None, not with == – or by coercion to boolean, see next point).
• None, zero-esque numbers (0, 0.0, 0j (complex number)) and empty collections ([], {}, set(), the empty string "", etc.) are considered falsy, everything else is considered truthy.
• For side effects, (for-)loop explicitly. For generating a new bunch of stuff without side-effects, use list comprehensions (or their relatives – generator expressions for lazy one-time iterators, dict/set comprehensions for the said collections).

Concerning looping: You have for, which operates on an iterable(! no counting), and while, which does what you would expect. The fromer is far more powerful, thanks to the extensive support for iterators. Not only nearly everything that can be an iterator instead of a list is an iterator (at least in Python 3 – in Python 2, you have both and the default is a list, sadly). The are numerous tools for working with iterators – zip iterates any number of iterables in parallel, enumerate gives you (index, item) (on any iterable, not just on lists), even slicing abritary (possibly large or infinite) iterables! I found that these make many many looping tasks much simpler. Needless to say, they integrate just fine with list comprehensions, generator expressions, etc.

In Ruby, instance variables and methods are completely unrelated, except when you explicitly relate them with attr_accessor or something like that.

In Python, methods are just a special class of attribute: one that is executable.

So for example:

>>> class foo:
...     x = 5
...     def y(): pass
... 
>>> f = foo()
>>> type(f.x)
<type 'int'>
>>> type(f.y)
<type 'instancemethod'>

That difference has a lot of implications, like for example that referring to f.x refers to the method object, rather than calling it. Also, as you can see, f.x is public by default, whereas in Ruby, instance variables are private by default.

I, like you, looked for inject and other functional methods when learning Python. I was disappointed to find that they weren’t all there, or that Python favored an imperative approach. That said, most of the constructs are there if you look. In some cases, a library will make things nicer.

A couple of highlights for me:

• The functional programming patterns you know from Ruby are available in Python. They just look a little different. For example, there’s a map function:

    def f(x):
        return x + 1
  
    map(f, [1, 2, 3]) # => [2, 3, 4]
  

  Similarly, there is a reduce function to fold over lists, etc.

  That said, Python lacks blocks and doesn’t have a streamlined syntax for chaining or composing functions. (For a nice way of doing this without blocks, check out Haskell’s rich syntax.)

• For one reason or another, the Python community seems to prefer imperative iteration for things that would, in Ruby, be done without mutation. For example, folds (i.e., inject), are often done with an imperative for loop instead of reduce:

    running_total = 0
    for n in [1, 2, 3]:
        running_total = running_total + n
  

  This isn’t just a convention, it’s also reinforced by the Python maintainers. For example, the Python 3 release notes explicitly favor for loops over reduce:

  Use functools.reduce() if you really need it; however, 99 percent of the time an explicit for loop is more readable.

• List comprehensions are a terse way to express complex functional operations (similar to Haskell’s list monad). These aren’t available in Ruby and may help in some scenarios. For example, a brute-force one-liner to find all the palindromes in a string (assuming you have a function p() that returns true for palindromes) looks like this:

    s = 'string-with-palindromes-like-abbalabba'
    l = len(s)
    [s[x:y] for x in range(l) for y in range(x,l+1) if p(s[x:y])]
  

• Methods in Python can be treated as context-free functions in many cases, which is something you’ll have to get used to from Ruby but can be quite powerful.

In case this helps, I wrote up more thoughts here in 2011: The ‘ugliness’ of Python. They may need updating in light of today’s focus on ML.