Restrictons of Python Compared to Ruby: Lambda'S

Restrictons of Python compared to Ruby: lambda's

I don't think you're really asking about lambdas, but inline functions.

This is genuinely one of Python's seriously annoying limitations: you can't define a function (a real function, not just an expression) inline; you have to give it a name. This is very frustrating, since every other modern scripting language does this and it's often very painful to have to move functions out-of-line. It's also frustrating because I have a feeling Python bytecode can represent this trivially--it's just the language syntax that can't.

Javascript:

responses = {
        "resp1": {
                "start": function() { ...  },
                "stop": function() { ... },
        },
        "resp2": {
                "start": function() { ...  },
                "stop": function() { ... },
        },
        ...
}
responses["resp1"]["start"]();

Lua:

responses = {
        resp1 = {
                start = function() ...  end;
                end = function() ...  end;
        };
        ...
}
responses.resp1.start();

Ruby:

responses = {
    "resp1" => {
        "start" => lambda { },
        "stop" => lambda { },
    },
}
responses["resp1"]["start"].call

Python:

def resp1_start():
    pass
def resp1_stop():
    pass
responses = {
    "resp1": {
        "start": resp1_start,
        "stop": resp1_stop,
    },
}
responses["resp1"]["start"]()

Note that JavaScript and Lua don't have lambdas: they have no reason to exist, since inline functions cover them in a much more natural and general way.

I'd probably rate this as the single most annoying day-to-day Python limitation.

Python lambda vs Ruby lambda

Python lambdas versus functions

Python lambdas are fully functional function objects, they simply lack names and documentation (docstrings), and can only return expressions (no statements). So both named functions and lambdas have closures and can be passed around like objects.

Many Python programmers would prefer not to use Python lambdas because of their drawbacks and no plusses other than being able to provide them as arguments directly in function calls as opposed to creating a named function on a different line.

For Python, other than constraints in lamba creation, there is no difference between a function created by a lambda and a function created by a def statement. Both are the same type of object:

>>> def foo(): return None
>>> bar = lambda: None
>>> import dis
>>> dis.dis(foo)
  1           0 LOAD_CONST               0 (None)
              3 RETURN_VALUE        
>>> dis.dis(bar)
  1           0 LOAD_CONST               0 (None)
              3 RETURN_VALUE

Ruby lambdas for functional programming

Ruby lambdas are, as I understand it, the best practice way to do functional programming by passing around functions, as they provide closures - and they should be called with the .call method. Ruby lambdas can contain complex/multiline expressions.

Conclusion

To compare, both are fully functional, provide lexical scoping (closures), are anonymous, and lack documentation.

To contrast, Python lambdas return a single expression, while Ruby lambdas can contain multiple lines. Python lambdas are expected to be called directly with possible arguments passed within parens, (), while Ruby lambda best practices is to use the .call method.

Based on the tradeoffs and recognized best practices for each domain, one would expect those who program in Ruby to prefer lambdas, especially for functional programming, and those who use Python to prefer named functions.

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.

Blocks of code in Python

Functions are the first-class members in Python:

def add(x, y):
    return x + y

a = add          # Bind
b = a(34, 1)     # Call

So you can pass functions around all you want. You can do the same with any callable object in Python.

Where and when to use Lambda?

It's true, you don't need anonymous functions (or lambdas, or whatever you want to call them). But there are a lot of things you don't need. You don't need classes—just pass all the instance variables around to ordinary functions. Then

class Foo
  attr_accessor :bar, :baz
  def frob(x)
    bar = baz*x
  end
end

would become

def new_Foo(bar,baz)
  [bar,baz]
end

def bar(foo)
  foo[0]
end
# Other attribute accessors stripped for brevity's sake

def frob(foo,x)
  foo[0] = foo[1]*x
end

Similarly, you don't need any loops except for loop...end with if and break. I could go on and on.¹ But you want to program with classes in Ruby. You want to be able to use while loops, or maybe even array.each { |x| ... }, and you want to be able to use unless instead of if not.

Just like these features, anonymous functions are there to help you express things elegantly, concisely, and sensibly. Being able to write some_function(lambda { |x,y| x + f(y) }) is much nicer than having to write

def temp(x,y)
  x + f(y)
end
some_function temp

It's much bulkier to have to break off the flow of code to write out a deffed function, which then has to be given a useless name, when it's just as clear to write the operation in-line. It's true that there's nowhere you must use a lambda, but there are lots of places I'd much rather use a lambda.

Ruby solves a lot of the lambda-using cases with blocks: all the functions like each, map, and open which can take a block as an argument are basically taking a special-cased anonymous function. array.map { |x| f(x) + g(x) } is the same as array.map(&lambda { |x| f(x) + g(x) }) (where the & just makes the lambda "special" in the same way that the bare block is). Again, you could write out a separate deffed function every time—but why would you want to?

Languages other than Ruby which support that style of programming don't have blocks, but often support a lighter-weight lambda syntax, such as Haskell's \x -> f x + g x, or C#'s x => f(x) + g(x);². Any time I have a function which needs to take some abstract behavior, such as map, or each, or on_clicked, I'm going to be thankful for the ability to pass in a lambda instead of a named function, because it's just that much easier. Eventually, you stop thinking of them as somehow special—they're about as exciting as literal syntax for arrays instead of empty().append(1).append(2).append(3). Just another useful part of the language.

1: In the degenerate case, you really only need eight instructions: +-<>[].,. <> move an imaginary "pointer" along an array; +- increment and decrement the integer in the current cell; [] perform a loop-while-non-zero; and ., do input and output. In fact, you really only need just one instruction, such as subleq a b c (subtract a from b and jump to c if the result is less than or equal to zero).

2: I've never actually used C#, so if that syntax is wrong, feel free to correct it.

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.

what is the Python equivalent of Ruby's yield?

Looking more into ruby's yield, it looks like you want something like contextlib.contextmanager:

from contextlib import contextmanager

def razz_the_jazz():
    print gval

@contextmanager
def quietude(level):
    global gval
    saved_gval = gval
    gval = level

    try:
        yield
    finally:
        gval = saved_gval

gval = 1

with quietude(3):
     razz_the_jazz()

razz_the_jazz()

This script outputs:

3
1

indicating that our context manager did reset gval in the global namespace. Of course, I wouldn't use this context manager since it only works in the global namespace. (It won't work with locals in a function) for example.

This is basically a limitation of how assignment creates a new reference to an object and that you can never mutate an object by assignment to it directly. (The only way to mutate an object is to assign to one of it's attributes or via __setitem__ (a[x] = whatever))

Is Ruby's code block same as C#'s lambda expression?

Ruby actually has 4 constructs that are all extremely similar

The Block

The idea behind blocks is sort of a way to implement really light weight strategy patterns. A block will define a coroutine on the function, which the function can delegate control to with the yield keyword. We use blocks for just about everything in ruby, including pretty much all the looping constructs or anywhere you would use using in c#. Anything outside the block is in scope for the block, however the inverse is not true, with the exception that return inside the block will return the outer scope. They look like this

def foo
  yield 'called foo'
end

#usage
foo {|msg| puts msg} #idiomatic for one liners

foo do |msg| #idiomatic for multiline blocks
  puts msg
end

Proc

A proc is basically taking a block and passing it around as a parameter. One extremely interesting use of this is that you can pass a proc in as a replacement for a block in another method. Ruby has a special character for proc coercion which is &, and a special rule that if the last param in a method signature starts with an &, it will be a proc representation of the block for the method call. Finally, there is a builtin method called block_given?, which will return true if the current method has a block defined. It looks like this

def foo(&block)
  return block
end

b = foo {puts 'hi'}
b.call # hi

To go a little deeper with this, there is a really neat trick that rails added to Symbol (and got merged into core ruby in 1.9). Basically, that & coercion does its magic by calling to_proc on whatever it is next to. So the rails guys added a Symbol#to_proc that would call itself on whatever is passed in. That lets you write some really terse code for any aggregation style function that is just calling a method on every object in a list

class Foo
  def bar
    'this is from bar'
  end
end

list = [Foo.new, Foo.new, Foo.new]

list.map {|foo| foo.bar} # returns ['this is from bar', 'this is from bar', 'this is from bar']
list.map &:bar # returns _exactly_ the same thing

More advanced stuff, but imo that really illustrates the sort of magic you can do with procs

Lambdas

The purpose of a lambda is pretty much the same in ruby as it is in c#, a way to create an inline function to either pass around, or use internally. Like blocks and procs, lambdas are closures, but unlike the first two it enforces arity, and return from a lambda exits the lambda, not the containing scope. You create one by passing a block to the lambda method, or to -> in ruby 1.9

l = lambda {|msg| puts msg} #ruby 1.8
l = -> {|msg| puts msg} #ruby 1.9

l.call('foo') # => foo

Methods

Only serious ruby geeks really understand this one :) A method is a way to turn an existing function into something you can put in a variable. You get a method by calling the method function, and passing in a symbol as the method name. You can re bind a method, or you can coerce it into a proc if you want to show off. A way to re-write the previous method would be

l = lambda &method(:puts)
l.call('foo')

What is happening here is that you are creating a method for puts, coercing it into a proc, passing that in as a replacement for a block for the lambda method, which in turn returns you the lambda

Feel free to ask about anything that isn't clear (writing this really late on a weeknight without an irb, hopefully it isn't pure gibberish)

EDIT: To address questions in the comments

list.map &:bar Can I use this syntax
with a code block that takes more than
one argument? Say I have hash = { 0 =>
"hello", 1 => "world" }, and I want to
select the elements that has 0 as the
key. Maybe not a good example. – Bryan
Shen

Gonna go kind of deep here, but to really understand how it works you need to understand how ruby method calls work.

Basically, ruby doesn't have a concept of invoking a method, what happens is that objects pass messages to each other. The obj.method arg syntax you use is really just sugar around the more explicit form, which is obj.send :method, arg, and is functionally equivalent to the first syntax. This is a fundamental concept in the language, and is why things like method_missing and respond_to? make sense, in the first case you are just handling an unrecognized message, the second you are checking to see if it is listening for that message.

The other thing to know is the rather esoteric "splat" operator, *. Depending on where its used, it actually does very different things.

def foo(bar, *baz)

In a method call, if it is the last parameter, splat will make that parameter glob up all additional parameters passed in to the function (sort of like params in C#)

obj.foo(bar, *[biz, baz])

When in a method call (or anything else that takes argument lists), it will turn an array into a bare argument list. The snippet below is equivilent to the snippet above.

obj.foo(bar, biz, baz)

Now, with send and * in mind, Symbol#to_proc is basically implemented like this

class Symbol
  def to_proc
    Proc.new { |obj, *args| obj.send(self, *args) }
  end
end

So, &:sym is going to make a new proc, that calls .send :sym on the first argument passed to it. If any additional args are passed, they are globbed up into an array called args, and then splatted into the send method call.

I notice that & is used in three
places: def foo(&block), list.map
&:bar, and l = lambda &method(:puts).
Do they share the same meaning? –
Bryan Shen

Yes, they do. An & will call to_proc on what ever it is beside. In the case of the method definition it has a special meaning when on the last parameter, where you are pulling in the co-routine defined as a block, and turning that into a proc. Method definitions are actually one of the most complex parts of the language, there are a huge amount of tricks and special meanings that can be in the parameters, and the placement of the parameters.

b = {0 => "df", 1 => "kl"} p b.select
{|key, value| key.zero? } I tried to
transform this to p b.select &:zero?,
but it failed. I guess that's because
the number of parameters for the code
block is two, but &:zero? can only
take one param. Is there any way I can
do that? – Bryan Shen

This should be addressed earlier, unfortunately you can't do it with this trick.

"A method is a way to turn an existing
function into something you can put in
a variable." why is l = method(:puts)
not sufficient? What does lambda &
mean in this context? – Bryan Shen

That example was exceptionally contrived, I just wanted to show equivalent code to the example before it, where I was passing a proc to the lambda method. I will take some time later and re-write that bit, but you are correct, method(:puts) is totally sufficient. What I was trying to show is that you can use &method(:puts) anywhere that would take a block. A better example would be this

['hello', 'world'].each &method(:puts) # => hello\nworld

l = -> {|msg| puts msg} #ruby 1.9:
this doesn't work for me. After I
checked Jörg's answer, I think it
should be l = -> (msg) {puts msg}. Or
maybe i'm using an incorrect version
of Ruby? Mine is ruby 1.9.1p738 –
Bryan Shen

Like I said in the post, I didn't have an irb available when I was writing the answer, and you are right, I goofed that (spend the vast majority of my time in 1.8.7, so I am not used to the new syntax yet)

There is no space between the stabby bit and the parens. Try l = ->(msg) {puts msg}. There was actually a lot of resistance to this syntax, since it is so different from everything else in the language.

Difference between lambda and def method_name in Ruby

Lambdas:

are variations of Procs,
can be converted to/from blocks,
do not begin a new closure scope (meaning you can access variables defined outside the scope of your lambda, unlike def),
can be passed around as variables.

I recommend checking out this article that explain procs, blocks, and lambdas.

Edit: This link is outdated. For future reference, try this article

Restrictons of Python Compared to Ruby: Lambda'S