Decorators in Ruby (migrating from Python)
Question:
I’m spending today learning Ruby from a Python perspective. One thing I have completely failed to grapple with is an equivalent of decorators. To pare things down I’m trying to replicate a trivial Python decorator:
#! /usr/bin/env python
import math
def document(f):
def wrap(x):
print "I am going to square", x
f(x)
return wrap
@document
def square(x):
print math.pow(x, 2)
square(5)
Running this gives me:
I am going to square 5
25.0
So, I want to create a function square(x), but decorate it so it alerts me as to what it’s going to square before it does it. Let’s get rid of the sugar to make it more basic:
...
def square(x):
print math.pow(x, 2)
square = document(square)
...
So, how do I replicate this in Ruby? Here’s my first attempt:
#! /usr/bin/env ruby
def document(f)
def wrap(x)
puts "I am going to square", x
f(x)
end
return wrap
end
def square(x)
puts x**2
end
square = document(square)
square(5)
Running this generates:
./ruby_decorate.rb:8:in `document': wrong number of arguments (0 for 1) (ArgumentError)
from ./ruby_decorate.rb:15:in `<main>'
Which I guess is because parentheses aren’t mandatory and it’s taking my return wrap as an attempt to return wrap(). I know of no way to refer to a function without calling it.
I’ve tried various other things, but nothing gets me far.
Answers:
Your guess is right.
You best use alias to bind the original method to another name, and then define the new one to print something and call the old one. If you need to do this repeatedly, you can make a method that does this for any method (I had an example once, but cannot find it now).
PS: your code does not define a function within a function but another function on the same object (yes, this is an undocument feature of Ruby)
class A
def m
def n
end
end
end
defines both m and n on A.
NB: the way to refer to a function would be
A.method(:m)
This is a slightly unusual question, but interesting. I’d first strongly recommend that you don’t try and directly transfer your Python knowledge to Ruby – it’s better to learn the idioms of Ruby and apply them directly, rather than try to transfer Python directly. I’ve used both languages a lot, and they’re both best when following their own rules and conventions.
Having said all that, here’s some nifty code that you can use.
def with_document func_name, *args
puts "about to call #{func_name}(#{args.to_s[1...-1]})"
method(func_name).call *args
end
def square x
puts x**2
end
def multiply a, b
puts a*b
end
with_document :square, 5
with_document :multiply, 5, 3
this produces
about to call square(5)
25
about to call multiply(5, 3)
15
which I’m sure you’ll agree does the job.
Okay, found my code again that does decorators in Ruby. It uses alias to bind the original method to another name, and then define the new one to print something and call the old one. All this is done using eval, such that it can be reused like decorators in Python.
module Document
def document(symbol)
self.send :class_eval, """
alias :#{symbol}_old :#{symbol}
def #{symbol} *args
puts 'going to #{symbol} '+args.join(', ')
#{symbol}_old *args
end"""
end
end
class A
extend Document
def square(n)
puts n * n
end
def multiply(a,b)
puts a * b
end
document :square
document :multiply
end
a = A.new
a.square 5
a.multiply 3,4
Edit: here the same with a block (no string manipulation pain)
module Document
def document(symbol)
self.class_eval do
symbol_old = "#{symbol}_old".to_sym
alias_method symbol_old, symbol
define_method symbol do |*args|
puts "going to #{symbol} "+args.join(', ')
self.send symbol_old, *args
end
end
end
end
I believe the corresponding Ruby idiom would be the alias method chain, which is heavily used by Rails. This article also considers it as the Ruby-style decorator.
For your example it should look like this:
class Foo
def square(x)
puts x**2
end
def square_with_wrap(x)
puts "I am going to square", x
square_without_wrap(x)
end
alias_method_chain :square, :wrap
end
The alias_method_chain call renames square to square_without_wrap and makes square an alias for square_with_wrap.
I believe Ruby 1.8 doesn’t have this method built-in, so you would have to copy it from Rails, but 1.9 should include it.
My Ruby-Skills have gotten a bit rusty, so I’m sorry if the code doesn’t actually work, but I’m sure it demonstrates the concept.
Python-like decorators can be implemented in Ruby. I won’t try to explain and give examples, because Yehuda Katz has already published a good blog post about decorators DSL in Ruby, so I highly recommend to read it:
UPDATE: I’ve got a couple of vote downs on this one, so let me explain further.
alias_method (and alias_method_chain) is NOT exactly the same concept as a decorator. It is just a way to re-define method implementation without using inheritance (so client code won’t notice a difference, still using the same method call). It could be useful. But also it could be error-prone. Anyone who used Gettext library for Ruby probably noticed that its ActiveRecord integration has been broken with each Rails major upgrade, because aliased version has been following the semantics of an old method.
The purpose of a decorator in general is NOT to change the internals of any given method and still be able to call the original one from a modified version, but to enhance the function behavior. The “entry/exit” use case, which is somewhat close to alias_method_chain, is only a simple demonstration. Another, more useful kind of a decorator could be @login_required, which checks authorization, and only runs the function if authorization was successful, or @trace(arg1, arg2, arg3), which could perform a set of tracing procedures (and be called with different arguments for different methods decoration).
IMO mooware has the best answer so far and it is the cleanest, simplest and most idiomatic. However he is making use of ‘alias_method_chain’ which is part of Rails, and not pure Ruby. Here is a rewrite using pure Ruby:
class Foo
def square(x)
puts x**2
end
alias_method :orig_square, :square
def square(x)
puts "I am going to square #{x}"
orig_square(x)
end
end
You can also accomplish the same thing using modules instead:
module Decorator
def square(x)
puts "I am going to square #{x}"
super
end
end
class Foo
def square(x)
puts x**2
end
end
# let's create an instance
foo = Foo.new
# let's decorate the 'square' method on the instance
foo.extend Decorator
# let's invoke the new decorated method
foo.square(5) #=> "I am going to square 5"
#=> 25
Ok, time for my attempt at an answer. I’m aiming here specifically at Pythoneers trying to reorganize their brains. Here’s some heavily documented code that (approximately) does what I was originally trying to do:
Decorating instance methods
#! /usr/bin/env ruby
# First, understand that decoration is not 'built in'. You have to make
# your class aware of the concept of decoration. Let's make a module for this.
module Documenter
def document(func_name) # This is the function that will DO the decoration: given a function, it'll extend it to have 'documentation' functionality.
new_name_for_old_function = "#{func_name}_old".to_sym # We extend the old function by 'replacing' it - but to do that, we need to preserve the old one so we can still call it from the snazzy new function.
alias_method(new_name_for_old_function, func_name) # This function, alias_method(), does what it says on the tin - allows us to call either function name to do the same thing. So now we have TWO references to the OLD crappy function. Note that alias_method is NOT a built-in function, but is a method of Class - that's one reason we're doing this from a module.
define_method(func_name) do |*args| # Here we're writing a new method with the name func_name. Yes, that means we're REPLACING the old method.
puts "about to call #{func_name}(#{args.join(', ')})" # ... do whatever extended functionality you want here ...
send(new_name_for_old_function, *args) # This is the same as `self.send`. `self` here is an instance of your extended class. As we had TWO references to the original method, we still have one left over, so we can call it here.
end
end
end
class Squarer # Drop any idea of doing things outside of classes. Your method to decorate has to be in a class/instance rather than floating globally, because the afore-used functions alias_method and define_method are not global.
extend Documenter # We have to give our class the ability to document its functions. Note we EXTEND, not INCLUDE - this gives Squarer, which is an INSTANCE of Class, the class method document() - we would use `include` if we wanted to give INSTANCES of Squarer the method `document`. <http://blog.jayfields.com/2006/05/ruby-extend-and-include.html>
def square(x) # Define our crappy undocumented function.
puts x**2
end
document(:square) # this is the same as `self.document`. `self` here is the CLASS. Because we EXTENDED it, we have access to `document` from the class rather than an instance. `square()` is now jazzed up for every instance of Squarer.
def cube(x) # Yes, the Squarer class has got a bit to big for its boots
puts x**3
end
document(:cube)
end
# Now you can play with squarers all day long, blissfully unaware of its ability to `document` itself.
squarer = Squarer.new
squarer.square(5)
squarer.cube(5)
Still confused? I wouldn’t be surprised; this has taken me almost a whole DAY. Some other things you should know:
- The first thing, which is CRUCIAL, is to read this: http://www.softiesonrails.com/2007/8/15/ruby-101-methods-and-messages. When you call ‘foo’ in Ruby, what you’re actually doing is sending a message to its owner: “please call your method ‘foo'”. You just can’t get a direct hold on functions in Ruby in the way you can in Python; they’re slippery and elusive. You can only see them as though shadows on a cave wall; you can only reference them through strings/symbols that happen to be their name. Try and think of every method call ‘object.foo(args)’ you do in Ruby as the equivalent of this in Python: ‘object.getattribute(‘foo’)(args)’.
- Stop writing any function/method definitions outside of modules/classes.
- Accept from the get-go that this learning experience is going to be brain-melting, and take your time. If Ruby isn’t making sense, punch a wall, go make a cup of coffee, or take a night’s sleep.
Decorating class methods
The above code decorates instance methods. What if you want to decorate methods directly on the class? If you read http://www.rubyfleebie.com/understanding-class-methods-in-ruby, you find there are three methods for creating class methods — but only one of them works for us here.
That is the anonymous class << self technique. Let’s do the above but so we can call square() and cube() without instantiating it:
class Squarer
class << self # class methods go in here
extend Documenter
def square(x)
puts x**2
end
document(:square)
def cube(x)
puts x**3
end
document(:cube)
end
end
Squarer.square(5)
Squarer.cube(5)
Have fun!
Here’s another approach that eliminates the problem with conflicts between names of aliased methods (NOTE my other solution using modules for decoration is a good alternative too as it also avoids conflicts):
module Documenter
def document(func_name)
old_method = instance_method(func_name)
define_method(func_name) do |*args|
puts "about to call #{func_name}(#{args.join(', ')})"
old_method.bind(self).call(*args)
end
end
end
The above code works because the old_method local variable is kept alive in the new ‘hello’ method by fact of define_method block being a closure.
In Ruby you can mimic Python’s syntax for decorators like this:
def document
decorate_next_def {|name, to_decorate|
print "I am going to square", x
to_decorate
}
end
document
def square(x)
print math.pow(x, 2)
end
Though you need some lib for that. I’ve written here how to implement such functionality (when I was trying to find there something in Rython that is missing in Ruby).
Michael Fairley demonstrated this at RailsConf 2012. Code is available here on Github. Simple usage examples:
class Math
extend MethodDecorators
+Memoized
def fib(n)
if n <= 1
1
else
fib(n - 1) * fib(n - 2)
end
end
end
# or using an instance of a Decorator to pass options
class ExternalService
extend MethodDecorators
+Retry.new(3)
def request
...
end
end
What you might achieve with decorators in Python, you achieve with blocks in Ruby. (I cannot believe how many answers are on this page, without a single yield statement!)
def wrap(x)
puts "I am going to square #{x}"
yield x
end
def square(x)
x**2
end
>> wrap(2) { |x| square(x) }
=> I am going to square 2
=> 4
The concept is similar. With the decorator in Python, you’re essentially passing the function “square” to be called from within “wrap”. With the block in Ruby, I’m passing not the function itself, but a block of code inside of which the function is invoked, and that block of code is executed within the context of “wrap”, where the yield statement is.
Unlike with decorators, the Ruby block being passed doesn’t need a function to be part of it. The above could have been simply:
def wrap(x)
puts "I am going to square #{x}"
yield x
end
>> wrap(4) { |x| x**2 }
=> I am going to square 4
=> 16
I’m spending today learning Ruby from a Python perspective. One thing I have completely failed to grapple with is an equivalent of decorators. To pare things down I’m trying to replicate a trivial Python decorator:
#! /usr/bin/env python
import math
def document(f):
def wrap(x):
print "I am going to square", x
f(x)
return wrap
@document
def square(x):
print math.pow(x, 2)
square(5)
Running this gives me:
I am going to square 5
25.0
So, I want to create a function square(x), but decorate it so it alerts me as to what it’s going to square before it does it. Let’s get rid of the sugar to make it more basic:
...
def square(x):
print math.pow(x, 2)
square = document(square)
...
So, how do I replicate this in Ruby? Here’s my first attempt:
#! /usr/bin/env ruby
def document(f)
def wrap(x)
puts "I am going to square", x
f(x)
end
return wrap
end
def square(x)
puts x**2
end
square = document(square)
square(5)
Running this generates:
./ruby_decorate.rb:8:in `document': wrong number of arguments (0 for 1) (ArgumentError)
from ./ruby_decorate.rb:15:in `<main>'
Which I guess is because parentheses aren’t mandatory and it’s taking my return wrap as an attempt to return wrap(). I know of no way to refer to a function without calling it.
I’ve tried various other things, but nothing gets me far.
Your guess is right.
You best use alias to bind the original method to another name, and then define the new one to print something and call the old one. If you need to do this repeatedly, you can make a method that does this for any method (I had an example once, but cannot find it now).
PS: your code does not define a function within a function but another function on the same object (yes, this is an undocument feature of Ruby)
class A
def m
def n
end
end
end
defines both m and n on A.
NB: the way to refer to a function would be
A.method(:m)
This is a slightly unusual question, but interesting. I’d first strongly recommend that you don’t try and directly transfer your Python knowledge to Ruby – it’s better to learn the idioms of Ruby and apply them directly, rather than try to transfer Python directly. I’ve used both languages a lot, and they’re both best when following their own rules and conventions.
Having said all that, here’s some nifty code that you can use.
def with_document func_name, *args
puts "about to call #{func_name}(#{args.to_s[1...-1]})"
method(func_name).call *args
end
def square x
puts x**2
end
def multiply a, b
puts a*b
end
with_document :square, 5
with_document :multiply, 5, 3
this produces
about to call square(5)
25
about to call multiply(5, 3)
15
which I’m sure you’ll agree does the job.
Okay, found my code again that does decorators in Ruby. It uses alias to bind the original method to another name, and then define the new one to print something and call the old one. All this is done using eval, such that it can be reused like decorators in Python.
module Document
def document(symbol)
self.send :class_eval, """
alias :#{symbol}_old :#{symbol}
def #{symbol} *args
puts 'going to #{symbol} '+args.join(', ')
#{symbol}_old *args
end"""
end
end
class A
extend Document
def square(n)
puts n * n
end
def multiply(a,b)
puts a * b
end
document :square
document :multiply
end
a = A.new
a.square 5
a.multiply 3,4
Edit: here the same with a block (no string manipulation pain)
module Document
def document(symbol)
self.class_eval do
symbol_old = "#{symbol}_old".to_sym
alias_method symbol_old, symbol
define_method symbol do |*args|
puts "going to #{symbol} "+args.join(', ')
self.send symbol_old, *args
end
end
end
end
I believe the corresponding Ruby idiom would be the alias method chain, which is heavily used by Rails. This article also considers it as the Ruby-style decorator.
For your example it should look like this:
class Foo
def square(x)
puts x**2
end
def square_with_wrap(x)
puts "I am going to square", x
square_without_wrap(x)
end
alias_method_chain :square, :wrap
end
The alias_method_chain call renames square to square_without_wrap and makes square an alias for square_with_wrap.
I believe Ruby 1.8 doesn’t have this method built-in, so you would have to copy it from Rails, but 1.9 should include it.
My Ruby-Skills have gotten a bit rusty, so I’m sorry if the code doesn’t actually work, but I’m sure it demonstrates the concept.
Python-like decorators can be implemented in Ruby. I won’t try to explain and give examples, because Yehuda Katz has already published a good blog post about decorators DSL in Ruby, so I highly recommend to read it:
UPDATE: I’ve got a couple of vote downs on this one, so let me explain further.
alias_method (and alias_method_chain) is NOT exactly the same concept as a decorator. It is just a way to re-define method implementation without using inheritance (so client code won’t notice a difference, still using the same method call). It could be useful. But also it could be error-prone. Anyone who used Gettext library for Ruby probably noticed that its ActiveRecord integration has been broken with each Rails major upgrade, because aliased version has been following the semantics of an old method.
The purpose of a decorator in general is NOT to change the internals of any given method and still be able to call the original one from a modified version, but to enhance the function behavior. The “entry/exit” use case, which is somewhat close to alias_method_chain, is only a simple demonstration. Another, more useful kind of a decorator could be @login_required, which checks authorization, and only runs the function if authorization was successful, or @trace(arg1, arg2, arg3), which could perform a set of tracing procedures (and be called with different arguments for different methods decoration).
IMO mooware has the best answer so far and it is the cleanest, simplest and most idiomatic. However he is making use of ‘alias_method_chain’ which is part of Rails, and not pure Ruby. Here is a rewrite using pure Ruby:
class Foo
def square(x)
puts x**2
end
alias_method :orig_square, :square
def square(x)
puts "I am going to square #{x}"
orig_square(x)
end
end
You can also accomplish the same thing using modules instead:
module Decorator
def square(x)
puts "I am going to square #{x}"
super
end
end
class Foo
def square(x)
puts x**2
end
end
# let's create an instance
foo = Foo.new
# let's decorate the 'square' method on the instance
foo.extend Decorator
# let's invoke the new decorated method
foo.square(5) #=> "I am going to square 5"
#=> 25
Ok, time for my attempt at an answer. I’m aiming here specifically at Pythoneers trying to reorganize their brains. Here’s some heavily documented code that (approximately) does what I was originally trying to do:
Decorating instance methods
#! /usr/bin/env ruby
# First, understand that decoration is not 'built in'. You have to make
# your class aware of the concept of decoration. Let's make a module for this.
module Documenter
def document(func_name) # This is the function that will DO the decoration: given a function, it'll extend it to have 'documentation' functionality.
new_name_for_old_function = "#{func_name}_old".to_sym # We extend the old function by 'replacing' it - but to do that, we need to preserve the old one so we can still call it from the snazzy new function.
alias_method(new_name_for_old_function, func_name) # This function, alias_method(), does what it says on the tin - allows us to call either function name to do the same thing. So now we have TWO references to the OLD crappy function. Note that alias_method is NOT a built-in function, but is a method of Class - that's one reason we're doing this from a module.
define_method(func_name) do |*args| # Here we're writing a new method with the name func_name. Yes, that means we're REPLACING the old method.
puts "about to call #{func_name}(#{args.join(', ')})" # ... do whatever extended functionality you want here ...
send(new_name_for_old_function, *args) # This is the same as `self.send`. `self` here is an instance of your extended class. As we had TWO references to the original method, we still have one left over, so we can call it here.
end
end
end
class Squarer # Drop any idea of doing things outside of classes. Your method to decorate has to be in a class/instance rather than floating globally, because the afore-used functions alias_method and define_method are not global.
extend Documenter # We have to give our class the ability to document its functions. Note we EXTEND, not INCLUDE - this gives Squarer, which is an INSTANCE of Class, the class method document() - we would use `include` if we wanted to give INSTANCES of Squarer the method `document`. <http://blog.jayfields.com/2006/05/ruby-extend-and-include.html>
def square(x) # Define our crappy undocumented function.
puts x**2
end
document(:square) # this is the same as `self.document`. `self` here is the CLASS. Because we EXTENDED it, we have access to `document` from the class rather than an instance. `square()` is now jazzed up for every instance of Squarer.
def cube(x) # Yes, the Squarer class has got a bit to big for its boots
puts x**3
end
document(:cube)
end
# Now you can play with squarers all day long, blissfully unaware of its ability to `document` itself.
squarer = Squarer.new
squarer.square(5)
squarer.cube(5)
Still confused? I wouldn’t be surprised; this has taken me almost a whole DAY. Some other things you should know:
- The first thing, which is CRUCIAL, is to read this: http://www.softiesonrails.com/2007/8/15/ruby-101-methods-and-messages. When you call ‘foo’ in Ruby, what you’re actually doing is sending a message to its owner: “please call your method ‘foo'”. You just can’t get a direct hold on functions in Ruby in the way you can in Python; they’re slippery and elusive. You can only see them as though shadows on a cave wall; you can only reference them through strings/symbols that happen to be their name. Try and think of every method call ‘object.foo(args)’ you do in Ruby as the equivalent of this in Python: ‘object.getattribute(‘foo’)(args)’.
- Stop writing any function/method definitions outside of modules/classes.
- Accept from the get-go that this learning experience is going to be brain-melting, and take your time. If Ruby isn’t making sense, punch a wall, go make a cup of coffee, or take a night’s sleep.
Decorating class methods
The above code decorates instance methods. What if you want to decorate methods directly on the class? If you read http://www.rubyfleebie.com/understanding-class-methods-in-ruby, you find there are three methods for creating class methods — but only one of them works for us here.
That is the anonymous class << self technique. Let’s do the above but so we can call square() and cube() without instantiating it:
class Squarer
class << self # class methods go in here
extend Documenter
def square(x)
puts x**2
end
document(:square)
def cube(x)
puts x**3
end
document(:cube)
end
end
Squarer.square(5)
Squarer.cube(5)
Have fun!
Here’s another approach that eliminates the problem with conflicts between names of aliased methods (NOTE my other solution using modules for decoration is a good alternative too as it also avoids conflicts):
module Documenter
def document(func_name)
old_method = instance_method(func_name)
define_method(func_name) do |*args|
puts "about to call #{func_name}(#{args.join(', ')})"
old_method.bind(self).call(*args)
end
end
end
The above code works because the old_method local variable is kept alive in the new ‘hello’ method by fact of define_method block being a closure.
In Ruby you can mimic Python’s syntax for decorators like this:
def document
decorate_next_def {|name, to_decorate|
print "I am going to square", x
to_decorate
}
end
document
def square(x)
print math.pow(x, 2)
end
Though you need some lib for that. I’ve written here how to implement such functionality (when I was trying to find there something in Rython that is missing in Ruby).
Michael Fairley demonstrated this at RailsConf 2012. Code is available here on Github. Simple usage examples:
class Math
extend MethodDecorators
+Memoized
def fib(n)
if n <= 1
1
else
fib(n - 1) * fib(n - 2)
end
end
end
# or using an instance of a Decorator to pass options
class ExternalService
extend MethodDecorators
+Retry.new(3)
def request
...
end
end
What you might achieve with decorators in Python, you achieve with blocks in Ruby. (I cannot believe how many answers are on this page, without a single yield statement!)
def wrap(x)
puts "I am going to square #{x}"
yield x
end
def square(x)
x**2
end
>> wrap(2) { |x| square(x) }
=> I am going to square 2
=> 4
The concept is similar. With the decorator in Python, you’re essentially passing the function “square” to be called from within “wrap”. With the block in Ruby, I’m passing not the function itself, but a block of code inside of which the function is invoked, and that block of code is executed within the context of “wrap”, where the yield statement is.
Unlike with decorators, the Ruby block being passed doesn’t need a function to be part of it. The above could have been simply:
def wrap(x)
puts "I am going to square #{x}"
yield x
end
>> wrap(4) { |x| x**2 }
=> I am going to square 4
=> 16