So what is “binding.pry” exactly?

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The very first feature that you learn about Pry is undoubtedly “binding.pry”. You put it here, there and everywhere; you’re happier than you’ve ever been before. In simple words: “binding.pry” makes your life a lot easier. But, have you ever wondered what “binding.pry” is? It appears so simple that you don’t even think of the details. When I first started to use Pry, I simply cargo-culted it. “binding.pry” seemed to me like some mysterious sorcery. Obviously, that’s not true, but when you learn something new things always appear more mysterious than they really are. By the end of this article you should have a solid understanding of one of the oldest and most interesting features of Pry.

The story of a little expression with incredible power

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We can use “binding.pry” almost anywhere. For instance, when you launch Pry via the command-line interface its default context is always “main”¹. Pry displays this information in the prompt (“[1] pry(main)>”). The prompt is dynamic: it always displays the current context.

The following snippets demonstrate a number possible places you can insert a “binding.pry”. The only difference is the context.

“binding.pry” divided into “binding” and “pry”

It’s time to slightly lift the veil. I’m sure a few of you know about the Binding class from Ruby; but I’m also sure an even larger number have never heard of it. It’s not really a feature that every Ruby programmer needs to be familiar with — but this is exactly what powers “binding.pry”. Let’s analyse this further below.

“binding”

“binding” is a method. It always returns a new instance of the Binding class.

binding.class #=> Binding
binding #=> #<Binding:0xa9dcefc>
binding #=> #<Binding:0xa9ee490>

You can invoke it anywhere you want, because it is a method of the Kernel module and Kernel methods are available on every object. One more important thing to know is that the Binding class can’t be instantiated directly². The “binding” method is the only interface.

So what exactly is a “binding”? A binding is a like a “snapshot” of everything available at the moment of instantiation: current value of “self”, local variables, methods, instance variables and more. Think of it as a room in the house full of items. There can be many rooms. Each room has its own set of items and a window. In Pry internals such a place is called a context. But foremost, in order to get an item from a room, we need to pry open a window and creep into a house, so we can see what’s available around.

Thanks to the Binding class, the so-called window can be implemented with a single method call. Let’s call it “Room#window” and put a teddy bear in it. Our aim is to take out that bear.

You can also ask Maria to take back the teddy bear. Thanks to “binding” we are able to evaluate Room’s code in the context of Maria, so no-one will cry later.

“pry”

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Onwards to the other part of “binding.pry” expression, “pry”. It turns out that you can invoke “pry” almost on every Ruby object. That's possible because it is defined on Object, the ancestor of every Ruby class. The method being invoked on a random object, starts a new Pry session in the context of that object.

[1] pry(main)> ['andrew', 'alexander', 'vladimir'].pry
[1] pry(#<Array>)> map &:capitalize
=> ["Andrew", "Alexander", "Vladimir"]
[2] pry(#<Array>)> exit
=> nil
[2] pry(main)> "do you hear me?".pry
[1] pry("do you hear me?")> upcase
=> "DO YOU HEAR ME?"
[2] pry("do you hear me?")> exit
=> nil
[3] pry(main)>

Remembering the examples in the beginning of this article and looking at the snippet above might put an idea into your head: we can get rid of constant typing of “binding” and just write “pry” instead, can't we? It turns out, we can. However, “binding.pry” and “pry” are not interchangeable. They start Pry sessions in different contexts.

The following example uses “binding.pry”. A Pry session is started in the context of a Song instance and the “music” local variable is accessible.

[1] pry(#<Song>)> music
=> "rock & roll"

Let’s slighlty modify the “Song#play” method and use “pry” this time. Again, the session starts in the context of the Song instance, but things work a bit different, now. In this case the “music” local variable is not accessible. Although we invoke “pry” in the scope of “Song#play” method, the Pry read-eval-print-loop starts not where you might expect. The local variable is unreachable, because Pry does not operate on the binding of the “Song#play” method. Instead, it operates directly on the instance, implicitly accessible via “self”.

[1] pry(#<Song>)> music
NameError: undefined local variable or method `music' for #<Song:0xbd2fe00>
from (pry):7:in `__pry__'
[2] pry(#<Song>)> whereami
Inside #<Song>.
[3] pry(#<Song>)> object_id == $old_id
=> true

The “self” keyword has also acquired its “pry” method from Object (not literally, since it’s just a placeholder for other Ruby objects that have that method defined). So “self.pry” and “pry” are totally equal.

There is one minor case when you can’t invoke “#pry”, though. Unfortunately, Pry doesn’t support instances of BasicObject, so you can’t pry into them. The reason is that BasicObject is the superclass of Object, and it sits even higher in the ancestry chain. It means that its instances don’t have the “pry” method. We can’t just move “Object#pry” to “BasicObject#pry”, because BasicObject instances don’t have the “binding” method, on which “Object#pry” relies.

basic = BasicObject.new #=> #<BasicObject:0x512c30c>
basic.pry #=> NoMethodError: undefined method `pry' for #<BasicObject:0xa258618>

Remember, you get “binding” from the Kernel module and Object is the only class, which includes it. This is by Ruby’s design. Some people want to see the binding support for BasicObject, but this is unlikely to happen in the near future. In Pry lobby interviews there were attempts to work around this, but they didn’t go too far.

Although Pry can’t support BasicObject instances, it supports BasicObject class itself, because Class instances do have bindings.

[1] pry(main)> BasicObject.pry
[1] pry(BasicObject)> __id__
=> 81839890

Another interesting feature of the “pry” method is that it supports arguments. So pry, self.pry and pry(self) are the exact same expressions. In fact, it accepts any Ruby objects, not only “self”. Just imagine, it’s only a one method, but how much power it is endowed with!

In the example below you can clearly see that “pry” is not like the “cd” command. It doesn’t store a chain of bindings (known as a binding stack). Instead, it always creates a new session, with its own binding stack (look at the “[1]” from the prompt, it doesn’t continue the previous counter, but starts a new one, without modifying the existing counter).

[1] pry(main)> pry 1337
[1] pry(1337)> pry ''
[1] pry("")> pry :awesome!
[1] pry(:awesome!)>
[1] pry(:awesome!)> nesting
Nesting status:
--
0. :awesome! (Pry top level)
[2] pry(:awesome!)> exit
=> nil
[1] pry("")> nesting
Nesting status:
--
0. "" (Pry top level)

The “pry” method also accepts the second argument: a hash of options. Most of the time you don’t need it. However, it won’t hurt you to know slightly more than an average Pry user, because you’re the special one. You can check the full list of options in the “pry_instance.rb” file (as of Pry v0.9.12.2). That list is quite big, so let me show you some of the insteresting options: “:output” and “:extra_sticky_locals”.

In the next example I did a couple of things. Firstly, I redirected all the output from a nested Pry session to a local variable called “output_history” and then printed its contents. Secondly, I injected a new sticky local variable. Sticky variables are shared across all Pry sessions and they’re accessible in any context. Just bear in mind that since the “pry” method creates a new Pry session, the hash options don’t affect the parent session: they’re valid only for the new session.

[1] pry(main)> output_history = StringIO.new
=> #<StringIO:0xab8978c>
[2] pry(main)> :universe.pry :output => output_history, :extra_sticky_locals => { :time => Time.now }
[1] pry(:universe)> whereami
[2] pry(:universe)> ls
[3] pry(:universe)> time
[4] pry(:universe)> Help me out!
[5] pry(:universe)> exit
=> nil
[3] pry(main)> puts output_history.string
Inside :universe.
Comparable#methods: <  <=  >  >=  between?
Symbol#methods: 
  <=>  =~       capitalize  empty?    inspect  match               size   swapcase  to_sym
  ==   []       casecmp     encoding  intern   next                slice  to_proc   upcase
  ===  __pry__  downcase    id2name   length   pretty_print_cycle  succ   to_s    
locals: _  __  _dir_  _ex_  _file_  _in_  _out_  _pry_  time
=> 2013-05-25 14:22:34 +0300
NoMethodError: undefined method `out!' for :universe:Symbol
from (pry):3:in `__pry__'
=> nil
[4] pry(main)> time
NameError: undefined local variable or method `time' for main:Object
from (pry):8:in `__pry__'

“binding” and “pry” pulled together

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So now, since you know a lot more about bindings and prys, it’s time to answer the main question. How does the Pry REPL know where to start a read-eval-print-loop? Why does “binding.pry” work? In simple words, when you invoke “pry” on an object, Pry gets the binding of that object and starts a REPL in its context. The mechanism for retrieving a binding is simple and robust. Its name is “Pry.binding_for”.

b = Pry.binding_for(:universe) #=> #<Binding:0xb2e7ad8>
b.eval('self') #=> :universe

You can pass any Ruby object to it. If you pass a Binding instance or a top-level binding, it returns it. But with other parameters it works a little bit different. It calls __binding__ method on “:universe” and it returns the corresponding binding. The question is where does “:universe” obtain this method? It is defined on Object, too.

Another supplementary method is used for retrieving a binding. Whenever you call “pry” on something, Pry internally creates a new method on that object called __pry__.

[1] pry(main)> :universe.__pry__
NoMethodError: undefined method `__pry__' for :universe:Symbol
from (pry):1:in `__pry__'
[2] pry(main)> :universe.pry
[1] pry(:universe)> :universe.__pry__
=> #<Binding:0xadf1a74>
[2] pry(:universe)> 
=> nil
[3] pry(main)> :universe.__pry__
=> #<Binding:0x91a27c4>
[4] pry(main)> :universe.__pry__.eval('self')
=> :universe

This is not a rocket science, it simply retrieves a binding of an instance (recall “Room#window”. “__pry__” is exactly the same). Albeit this method is not for public use, you can do some interesting things with help of it. Namely, once “__pry__” is defined, you can peek into objects without prying into them. It’s kind of a gateway for accessing an object’s internals.

[1] pry(main)> 1337.pry
[1] pry(1337)> @leet_number = :so_leet
=> :so_leet
[2] pry(1337)> exit
=> nil
[2] pry(main)> 1337.__pry__.eval('@leet_number')
=> :so_leet

Additionally, as I already mentioned, Pry defines the “__binding__” method on every object. It’s already there, so you don’t need to call “pry” on it.

[1] pry(main)> :universe.__binding__.eval 'upcase'
=> :UNIVERSE

Why are there two exactly the same methods? Because they are not the same. “__pry__” is even more internal than “__binding__”, as the latter utilises the former. So “__binding__” is more powerful. There is no “__pry__” for classes an modules, but there is always “__binding__”. Do you remember that Pry can start its session in the context of a Class? This is exactly what “__binding__” is for.

A = Class.new
A.__pry__ #=> NoMethodError
A.__binding__ #=> #<Binding:0xb0e8570>

A.pry
exit # Exits from the nested session.
A.__pry__ #=> NoMethodError, still undefined.

So “__pry__” is for instances. For the rest there’s “__binding__”. The context of evaluation in this case is “self“, as always. Modules also behave as Classes.

[1] pry(main)> M = Module.new
=> M
[2] pry(main)> M.__binding__.eval('def magnifico; :splendid end')
=> nil
[3] pry(main)> include M
=> Object
[4] pry(main)> magnifico
=> :splendid

This is it. Imagine a REPL. Imagine a context. Tie them together.

[1] pry(main)> loop do
             |   print '>> '
             |   puts "=> #{ TOPLEVEL_BINDING.eval(gets) }"
             | end
>> def hello; :hi end
=>
>> hello
=> hi

Then, just spice up everything with bindings and you are ready to be famous.

A historical note

Roughly speaking, in the early days of Pry everything worked exactly the same. The API was different, though. There was no “binding.pry” and you had to “Pry.into(object)”. Nowadays the mechanism has become more robust and easier to use. I encourage you to check Pry as it was 3 years ago. It was only 125 lines of code. It’s interesting that the README claimed that “Pry does not pretend to be a replacement for IRB”. And for the full-featured replacement it was recommended to use ripl.

Conclusion

“binding.pry” is a very powerful expression. “binding” is powerful by itself, but if it is assisted with Pry, it reveals unbelievable possibilities. When Pry starts, it always starts in some context. The default context is “main” (just like in IRB). But with help of bindings it can load itself wherever a binding is available. It extends binding’s features with its own ones, allowing excellent introspection experience, and in the end you get the best Ruby debugging tool ever existed.

Homework

If you want to become more advanced with Pry and bindings, I offer you three tasks. The first one is very simple and it’s suitable for every reader of this article. The next one is simple, too, but it requires some thinking. The last one is really hard. It’s for passionate Pry users only. It makes you to poke around the Pry source code, so please, value your time.

Task 1

Figure out why the msg local variable is not accessible. How to inspect its value?

Task 2

Run the following snippet in your Pry console.

Without using Pry commands (e.g. “cd”), how to get the value of the count local variable?

Task 3

How to swap the top-level binding with a fresh one?

Note that “count = nil” is not the correct solution, as it doesn’t swap the current binding with a fresh one. Focus on the very bindings, not on garbage collection and other unrelated things that you might thought of. To complete this, you only need to swap the current binding object.

Additional reading

Some people can write better than me. These article are not about Pry, but by reading them you’ll be able to understand “binding.pry” even deeper. I heartily recommend to check them out.

Did you know?

Did you know that IRB has basic support for bindings, too? It's not really enjoyable to work with them there, though.

irb(main):001:0> irb "do you hear me?"
irb#1(do you hear me?):001:0> upcase
=> "DO YOU HEAR ME?"

Acknowledgements

Huge thanks to a friend of mine and the creator of Pry, John Mair, for helping me to polish my language.

Big thanks to Duncan Beevers for the criticism. I love to be criticised and Duncan is very sincere.


¹ — More information on “main” can be found in the “What is the Ruby top-level” by John Mair.

² — that is, we can’t create our own Binding instances using the “#new” method, like we can do it with many other Ruby standard classes.

Binding.new #=> NoMethodError: undefined method `new' for Binding:Class

Binding is somewhat similar to Symbol: there is no need in their instantiation. A binding, just like a symbol is already here, ready at hand.