Customizing Pry
Pry supports customization of the input, the output, the commands,
the hooks, the prompt, and the ‘print’ object (the “P” in REPL).
Global customization, which applies to all Pry sessions, is done
through invoking class accessors on the Pry class, the accessors
are:
Pry.input=Pry.output=Pry.commands=Pry.hooks=Pry.prompt=Pry.print=
Local customization (applied to a single Pry session) is done by
passing config hash options to Pry.start() or to Pry.new(); also the
same accessors as described above for the Pry class exist for a
Pry instance so that customization can occur at runtime.
Input
For input Pry accepts any object that implements the readline method. This
includes IO objects, StringIO, Readline, File and custom objects. Pry
initially defaults to using Readline for input.
Example: Setting global input
Setting Pry’s global input causes all subsequent Pry instances to use
this input by default:
Pry.input = StringIO.new(“@x = 10\nexit”)
Object.pry
Object.instance_variable_get(:@x) #=> 10
The above will execute the code in the StringIO
non-interactively. It gets all the input it needs from the StringIO
and then exits the Pry session. Note it is important to end the
session with ‘exit’ if you are running non-interactively or the Pry
session will hang as it loops indefinitely awaiting new input.
Example: Setting input for a specific session
The settings for a specific session override the global settings
(discussed above). There are two ways to set input for a specific pry session: At the
point the session is started, or within the session itself (at runtime):
At session start
Pry.start(Object, :input => StringIO.new(“@x = 10\nexit”))
Object.instance_variable_get(:@x) #=> 10
At runtime
If you want to set the input object within the session itself you use
the special _pry_ local variable which represents the Pry instance
managing the current session; inside the session we type:
pry.input = StringIO.new(“@x = 10\nexit”)
Note we can also set the input object for the parent Pry session (if
the current session is nested) like so:
pry.parent.input = StringIO.new(“@x = 10\nexit”)
Output
For output Pry accepts any object that implements the puts method. This
includes IO objects, StringIO, File and custom objects. Pry initially
defaults to using $stdout for output.
Example: Setting global output
Setting Pry’s global output causes all subsequent Pry instances to use
this output by default:
Pry.output = StringIO.new
Example: Setting output for a specific session
As per Input, given above, we set the local output as follows:
At session start
Pry.start(Object, :output => StringIO.new(“@x = 10\nexit”))
At runtime
pry.output = StringIO.new
Commands
Pry commands are not methods; they are commands that are intercepted
and executed before a Ruby eval takes place. Pry comes with a default
command set (Pry::Commands), but these commands can be augmented or overriden by
user-specified ones.
The Pry command API is quite sophisticated supporting features such as:
command set inheritance, importing of specific commands from another
command set, deletion of commands, calling of commands within other
commands, and so on.
A valid Pry command object must inherit from
Pry::CommandBase (or one of its subclasses) and use the special command API:
Example: Defining a command object and setting it globally
class MyCommands < Pry::CommandBase
command “greet”, “Greet the user.” do |name, age|
output.puts “Hello #{name.capitalize}, how does it feel being #{age}?”
end
end
Pry.commands = MyCommands
Then inside a pry session:
pry(main)> greet john 9
Hello John, how does it feel being 9?
=> nil
Example: Using a command object in a specific session
As in the case of input and output:
At session start:
Pry.start(self, :commands => MyCommands)
At runtime:
pry.commands = MyCommands
The command API
The command API is defined by the Pry::CommandBase class (hence why
all commands must inherit from it or a subclass). The API works as follows:
command method
The command method defines a new command, its parameter is the
name of the command and an optional second parameter is a description of
the command.
The associated block defines the action to be performed. The number of
parameters in the block determine the number of parameters that will
be sent to the command (from the Pry prompt) when it is invoked. Note
that all parameters that are received will be strings; if a parameter
is not received it will be set to nil.
command “hello” do |x, y, z|
puts “hello there #{x}, #{y}, and #{z}!”
end
Command aliases can also be defined - simply use an array of strings
for the command name - all these strings will be valid names for the
command.
command [“ls”, “dir”], “show a list of local vars” do
output.puts target.eval(“local_variables”)
end
delete method
The delete method deletes a command or a group of commands. It
can be useful when inheriting from another command set and you wish
to keep only a portion of the inherited commands.
class MyCommands < Pry::Commands
delete “show_method”, “show_imethod”
end
import_from method
The import_from method enables you to specifically select which
commands will be copied across from another command set, useful when
you only want a small number of commands and so inheriting and then
deleting would be inefficient. The first parameter to import_from
is the class to import from and the other paramters are the names of
the commands to import:
class MyCommands < Pry::CommandBase
import_from Pry::Commands, “ls”, “status”, “!”
end
run method
The run command invokes one command from within another.
The first parameter is the name of the command to invoke
and the remainder of the parameters will be passed on to the command
being invoked:
class MyCommands < Pry::Commands
command “ls_with_hello” do
output.puts “hello!”
run “ls”
end
end
alias_command method
The alias_command method creates an alias of a command. The first
parameter is the name of the new command, the second parameter is the
name of the command to be aliased; an optional third parameter is the
description to use for the alias. If no description is provided then
the description of the original command is used.
class MyCommands < Pry::Commands
alias_command “help2”, “help”, “An alias of help”
end
desc method
The desc method is used to give a command a new description. The
first parameter is the name of the command, the second parameter is
the description.
class MyCommands < Pry::Commands
desc “ls”, “a new description”
end
Utility methods for commands
All commands can access the special output and target methods. The
output method returns the output object for the active pry session.
Ensuring that your commands invoke puts on this rather than using
the top-level puts will ensure that all your session output goes to
the same place.
The target method returns the Binding object the Pry session is currently
active on - useful when your commands need to manipulate or examine
the state of the object. E.g, the “ls” command is implemented as follows
command “ls” do
output.puts target.eval(“local_variables + instance_variables”).inspect
end
The opts hash
These are miscellaneous variables that may be useful to your commands:
opts[:val]- The line of input that invoked the command.opts[:eval_string]- The cumulative lines of input for multi-line input.opts[:nesting]- Lowlevel session nesting information.opts[:commands]- Lowlevel data of all Pry commands.
(see commands.rb for examples of how some of these options are used)
The help command
The Pry::CommandBase class automatically defines a help command
for you. Typing help in a Pry session will show a list of commands
to the user followed by their descriptions. Passing a parameter to
help with the command name will just return the description of that
specific command. If a description is left out it will automatically
be given the description “No description.”.
If the description is explicitly set to "" then this command will
not be displayed in help.
Hooks
Currently Pry supports just two hooks: before_session and
after_session. These hooks are invoked before a Pry session starts
and after a session ends respectively. The default hooks used are
stored in the Pry::DEFAULT_HOOKS and just output the text "Beginning and
Pry session for ""Ending Pry session for ".
Example: Setting global hooks
All subsequent Pry instances will use these hooks as default:
Pry.hooks = {
:before_session => proc { |out, obj| out.puts “Opened #{obj}” },
:after_session => proc { |out, obj| out.puts “Closed #{obj}” }
}
5.pry
Inside the session:
Opened 5
pry(5)> exit
Closed 5
Note that the before_session and after_session procs receive the
current session’s output object and session receiver as parameters.
Example: Setting hooks for a specific session
Like all the other customization options, the global default (as
explained above) can be overriden for a specific session, either at
session start or during runtime.
At session start
Pry.start(self, :hooks => { :before_session => proc { puts “hello world!” },
:after_session => proc { puts “goodbye world!” }
})
At runtime
pry.hooks = { :before_session => proc { puts “puts "hello world!” } }
Prompts
The Pry prompt is used by Readline and other input objects that
accept a prompt. Pry can accept two prompt-types for every prompt; the
‘main prompt’ and the ‘wait prompt’. The main prompt is always used
for the first line of input; the wait prompt is used in multi-line
input to indicate that the current expression is incomplete and more lines of
input are required. The default Prompt used by Pry is stored in the
Pry::DEFAULT_PROMPT constant.
A valid Pry prompt is either a single Proc object or a two element
array of Proc objects. When an array is used the first element is
the ‘main prompt’ and the last element is the ‘wait prompt’. When a
single Proc object is used it will be used for both the main prompt
and the wait prompt.
Example: Setting global prompt
The prompt Proc objects are passed the receiver of the Pry session
and the nesting level of that session as parameters (they can simply
ignore these if they do not need them).
# Using one proc for both main and wait prompts
Pry.prompt = proc { |obj, nest_level| “#{obj}:#{nest_level}> ” }
# Alternatively, provide two procs; one for main and one for wait
Pry.prompt = [ proc { “ENTER INPUT> ” }, proc { “MORE INPUT REQUIRED!* ” }]
Example: Setting the prompt for a specific session
At session start
Pry.start(self, :prompt => [proc { “ENTER INPUT> ” },
proc { “MORE INPUT REQUIRED!* ” }])
At runtime
pry.prompt = [proc { “ENTER INPUT> ” },
proc { “MORE INPUT REQUIRED!* ” }]
The Print phase of Pry’s READ-EVAL-PRINT-LOOP can be customized. The
default action is stored in the Pry::DEFAULT_PRINT constant and it
simply outputs the value of the current expression preceded by a => (or the first
line of the backtrace if the value is an Exception object.)
The print object should be a Proc and the parameters passed to the
Proc are the output object for the current session and the ‘value’
returned by the current expression.
Example: Setting global print object
Let’s define a print object that displays the full backtrace of any
exception and precedes the output of a value by the text "Output is: ":
Pry.print = proc do |output, value|
case value
when Exception
output.puts value.backtrace
else
output.puts “Output is: #{value}”
end
end
Example: Setting the print object for a specific session
At session start
Pry.start(self, :print => proc do |output, value|
case value
when Exception
output.puts value.backtrace
else
output.puts “Output is: #{value.inspect}”
end
end)
At runtime
pry.print = proc do |output, value|
case value
when Exception
output.puts value.backtrace
else
output.puts “Output is: #{value.inspect}”
end
end