Hello,
I'm using Ruby 1.8.2 and I'm reading the book "Programming Ruby 2nd
edition". In this book they say, that Ruby implements private methods by
not allowing another receiver than "self". After reading this, I tried
the following:
class Test
private
def print_hello
puts "Hello everyone!"
end
end
t = Test.new
t.send( "print_hello" )
This program runs just fine and prints "Hello everyone!".
Is this behaviour to be expected?
Turing
--
Posted via http://www.ruby-forum.com/.
Frank Meyer wrote:
Hello,
I'm using Ruby 1.8.2 and I'm reading the book "Programming Ruby 2nd
edition". In this book they say, that Ruby implements private methods by
not allowing another receiver than "self". After reading this, I tried
the following:class Test
private
def print_hello
puts "Hello everyone!"
end
endt = Test.new
t.send( "print_hello" )This program runs just fine and prints "Hello everyone!".
Is this behaviour to be expected?
Yes - send trumps private. Private in Ruby is more of a guideline than a rule. The implementer is trying to tell you not to go there, but you can get around it if you really need to.
--
Alex
Yes. send bypasses protection levels, for reasons I'm not really clear
on. If I were to hazard a guess, it would be that what you send is
eval'ed inside the context of the object, thereby making self the
receiver, but that's a total guess.
Test.new.print_hello will cause the error you expect.
As a sidenote, upgrade your ruby 
Ben
On Thu, Aug 02, 2007, Frank Meyer wrote:
class Test
private
def print_hello
puts "Hello everyone!"
end
endt = Test.new
t.send( "print_hello" )This program runs just fine and prints "Hello everyone!".
Is this behaviour to be expected?
It's a guideline in any language.
I think people tend to think of 'private' as some sort of security
enforcement. It's not. It just means, "You aren't supposed to access
this method directly. If you go out of your way to do so, I am not
responsible for the consequences".
The advantage of allowing send to access private methods is that it
allows you to test private methods. 
Regards,
Dan
On Aug 1, 4:06 pm, Alex Young <a...@blackkettle.org> wrote:
Frank Meyer wrote:
> Hello,
> I'm using Ruby 1.8.2 and I'm reading the book "Programming Ruby 2nd
> edition". In this book they say, that Ruby implements private methods by
> not allowing another receiver than "self". After reading this, I tried
> the following:> class Test
> private
> def print_hello
> puts "Hello everyone!"
> end
> end> t = Test.new
> t.send( "print_hello" )> This program runs just fine and prints "Hello everyone!".
> Is this behaviour to be expected?
Yes - send trumps private. Private in Ruby is more of a guideline than
a rule.
Ben Bleything wrote:
class Test
private
def print_hello
puts "Hello everyone!"
end
endt = Test.new
t.send( "print_hello" )This program runs just fine and prints "Hello everyone!".
Is this behaviour to be expected?
Yes. send bypasses protection levels, for reasons I'm not really clear
on. If I were to hazard a guess, it would be that what you send is
eval'ed inside the context of the object, thereby making self the
receiver, but that's a total guess.Test.new.print_hello will cause the error you expect.
As a sidenote, upgrade your ruby
Ben
Ben,
It is always possible to run private methods, change class variables, etc. The way to do it is to reopen the class (or add an instance method) that calls the private method or sets/gets the variable we need. Since it's possible, why not make it easier? Ruby tries not to impose on what the user can do, so we have foo.send, foo.instance_variable_set, and foo.instance_variable_get.
Dan
On Thu, Aug 02, 2007, Frank Meyer wrote:
It is in C++ and Java, maybe even VB, if I recall... (I try not to
recall VB).
Well, maybe not a "security enforcement" but a visibility and/or
invocation restriction.
My JVM even gives me an IllegalAccessException when I try to access an
private Method at runtime via reflection.
On Aug 1, 5:42 pm, Daniel Berger <djber...@gmail.com> wrote:
On Aug 1, 4:06 pm, Alex Young <a...@blackkettle.org> wrote:
> Frank Meyer wrote:
> > Hello,
> > I'm using Ruby 1.8.2 and I'm reading the book "Programming Ruby 2nd
> > edition". In this book they say, that Ruby implements private methods by
> > not allowing another receiver than "self". After reading this, I tried
> > the following:> > class Test
> > private
> > def print_hello
> > puts "Hello everyone!"
> > end
> > end> > t = Test.new
> > t.send( "print_hello" )> > This program runs just fine and prints "Hello everyone!".
> > Is this behaviour to be expected?
> Yes - send trumps private. Private in Ruby is more of a guideline than
> a rule.It's a guideline in any language.
I think people tend to think of 'private' as some sort of security
enforcement. It's not.
Daniel Berger wrote:
[ .. snip .. ]
The advantage of allowing send to access private methods is that it
allows you to test private methods.
Isn't that a Bad Practice (tm)? Private methods are mostly meant for the
internals of a class, and a test should really only test how the class
interacts with the rest of the world - right? (i.e. public methods)
I might've missed the whole point, so this post contains Many
Questionmarks.
Kindest regards, Jørgen P. Tjernø.
Right. I was just trying to answer the original question
Ben
On Thu, Aug 02, 2007, Dan Zwell wrote:
Ben Bleything wrote:
>Yes. send bypasses protection levels, for reasons I'm not really clear
>on. If I were to hazard a guess, it would be that what you send is
>eval'ed inside the context of the object, thereby making self the
>receiver, but that's a total guess.It is always possible to run private methods, change class variables,
etc. The way to do it is to reopen the class (or add an instance method)
that calls the private method or sets/gets the variable we need. Since
it's possible, why not make it easier? Ruby tries not to impose on what
the user can do, so we have foo.send, foo.instance_variable_set, and
foo.instance_variable_get.
It is a root of bad practices, but it may be necessary... I think a good
principe is : use public methods only of others's libraries, and use
instance_eval (etc) of yours when you need it, and if there is
collaboration between classes (like friendly classes of C++).
Thus you will be able to keep your model safe.
Le jeudi 02 août 2007 à 20:28 +0900, "Jørgen P. Tjernø" a écrit :
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1Daniel Berger wrote:
> [ .. snip .. ]
> The advantage of allowing send to access private methods is that it
> allows you to test private methods.
Isn't that a Bad Practice (tm)? Private methods are mostly meant for the
internals of a class, and a test should really only test how the class
interacts with the rest of the world - right? (i.e. public methods)I might've missed the whole point, so this post contains Many
Questionmarks.Kindest regards, Jørgen P. Tjernø.
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Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.orgiD8DBQFGsbDiUMzc1WGo4zgRAoJ3AJ9MtInhJTZS9Uq1ABXwxxN9rwkeVACffYBG
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--
Etienne Vallette d'Osia
Hi --
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1Daniel Berger wrote:
[ .. snip .. ]
The advantage of allowing send to access private methods is that it
allows you to test private methods.Isn't that a Bad Practice (tm)? Private methods are mostly meant for the
internals of a class, and a test should really only test how the class
interacts with the rest of the world - right? (i.e. public methods)
No; you definitely want to test your private methods too. First of
all, if you're writing a class, you need to test its internals;
nothing should be a "black box" for the tests. Second, private
methods are actually available to the world, even without send. They
just have to be called the right way:
puts "Hi"
class C
attr_accessor :x
end
C.class_eval { define_method "y" }
puts, attr_accessor, and define_method are all private, but you'd
certainly want to test them if you had written them.
David
On Thu, 2 Aug 2007, "Jørgen P. Tjernø" wrote:
--
* Books:
RAILS ROUTING (new! http://www.awprofessional.com/title/0321509242\)
RUBY FOR RAILS (http://www.manning.com/black\)
* Ruby/Rails training
& consulting: Ruby Power and Light, LLC (http://www.rubypal.com)
Hello everybody,
thanks for your explanations. And I didn't know that ruby allows the
reading and setting (and even creation) of instance variables.
What does class_eval do?
And actually I'm using Ruby 1.8.6 I confused the version number with
those in the book.
Turing
--
Posted via http://www.ruby-forum.com/.
Hi --
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Daniel Berger wrote:
>> [ .. snip .. ]
>> The advantage of allowing send to access private methods is that it
>> allows you to test private methods.
> Isn't that a Bad Practice (tm)? Private methods are mostly meant for the
> internals of a class, and a test should really only test how the class
> interacts with the rest of the world - right? (i.e. public methods)No; you definitely want to test your private methods too.
Conventional wisdom in the Agile/TDD community is that you shouldn't
be testing private methods. The reasoning goes something like this:
1. If you're doing TDD, you're sending messages to objects that other
objects in your system will send. The corresponding methods should be
public.
2. In TDD, private methods appear through refactoring, and therefore
are already tested implicitly through the tests of public methods.
3. When you feel the need to add tests on private methods (which have
appeared through refactoring), it should be considered a sign that a
new object is wanting to be born and should be extracted out into a
new class.
My sense is that some of this thinking is a product of the fact that
testing privates in Java means using reflection, resulting in
refactoring inefficiencies. In Ruby, testing privates is fairly easy
and we don't really have the refactoring tools that the Java community
has, so refactoring in Ruby tends to be much more manual anyhow.
That said, I think the OO design questions that get raised are worthy
of exploration when you feel the need to test something private.
WDYT?
On 8/2/07, dblack@rubypal.com <dblack@rubypal.com> wrote:
On Thu, 2 Aug 2007, "Jørgen P. Tjernø" wrote:
First of
all, if you're writing a class, you need to test its internals;
nothing should be a "black box" for the tests. Second, private
methods are actually available to the world, even without send. They
just have to be called the right way:puts "Hi"
class C
attr_accessor :x
end
C.class_eval { define_method "y" }puts, attr_accessor, and define_method are all private, but you'd
certainly want to test them if you had written them.David
--
* Books:
RAILS ROUTING (new! http://www.awprofessional.com/title/0321509242\)
RUBY FOR RAILS (http://www.manning.com/black\)
* Ruby/Rails training
& consulting: Ruby Power and Light, LLC (http://www.rubypal.com)
thanks for your explanations. And I didn't know that ruby allows the
reading and setting (and even creation) of instance variables.
Yup. That's what @something variables are.
What does class_eval do?
It's like instance_eval, but evaluates in the context of a class
instead. Hark:
str = "foo"
str.instance_eval {length}
=> 3 (same as calling str.length)
str.class.class_eval {def foo; return 'bar'; end}
str.foo
=> 'bar' # effectively the same as re-opening the class
I'm sure there are other subtleties involved, but that's the top-level
view.
Ben
On Fri, Aug 03, 2007, Frank Meyer wrote:
I've written some interesting use cases concerning send here. FYI: my
understanding is that 1.9 will change the semantics of send so that it
no longer accesses private methods.
Regards,
Dan
Hi --
Hi --
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1Daniel Berger wrote:
[ .. snip .. ]
The advantage of allowing send to access private methods is that it
allows you to test private methods.Isn't that a Bad Practice (tm)? Private methods are mostly meant for the
internals of a class, and a test should really only test how the class
interacts with the rest of the world - right? (i.e. public methods)No; you definitely want to test your private methods too.
Conventional wisdom in the Agile/TDD community is that you shouldn't
be testing private methods. The reasoning goes something like this:1. If you're doing TDD, you're sending messages to objects that other
objects in your system will send. The corresponding methods should be
public.2. In TDD, private methods appear through refactoring, and therefore
are already tested implicitly through the tests of public methods.3. When you feel the need to add tests on private methods (which have
appeared through refactoring), it should be considered a sign that a
new object is wanting to be born and should be extracted out into a
new class.My sense is that some of this thinking is a product of the fact that
testing privates in Java means using reflection, resulting in
refactoring inefficiencies. In Ruby, testing privates is fairly easy
and we don't really have the refactoring tools that the Java community
has, so refactoring in Ruby tends to be much more manual anyhow.That said, I think the OO design questions that get raised are worthy
of exploration when you feel the need to test something private.WDYT?
My tendency is to think that the public/private distinction in Ruby
does not overlap very closely with the line between methods you're
likely to use from outside and methods you're not, mainly for two
reasons.
First, private instance methods of Kernel, like raise and sprintf:
these are not behind the black curtain, but are private so as to be
callable in a receiverless way. I'd expect them to be tested.
Second, quick and frequent scope and self-scope changes:
class C
define_method ...
end
etc. define_method, like raise and sprintf, won't go away because of
refactoring; it's a private method, but part of the interface one is
expected to use (rather than simply a by-product of the implementation
of some other such method).
That said, it may be that there are private methods one wouldn't test,
namely those that are really part of the black box, where all you
really need to know is whether the black box produces the right
answer. But I think that's just a subset of private methods,
certainly in core/standard Ruby and quite possibly in other Ruby code.
David
On Fri, 3 Aug 2007, David Chelimsky wrote:
On 8/2/07, dblack@rubypal.com <dblack@rubypal.com> wrote:
On Thu, 2 Aug 2007, "Jørgen P. Tjernø" wrote:
--
* Books:
RAILS ROUTING (new! http://www.awprofessional.com/title/0321509242\)
RUBY FOR RAILS (http://www.manning.com/black\)
* Ruby/Rails training
& consulting: Ruby Power and Light, LLC (http://www.rubypal.com)