In Python, there is a difference between functions and bound methods.
>>> def foo():
... print "foo"
...
>>> class A:
... def bar( self ):
... print "bar"
...
>>> a = A()
>>> foo
<function foo at 0x00A98D70>
>>> a.bar
<bound method A.bar of <__main__.A instance at 0x00A9BC88>>
>>>
Bound methods have been "bound" (how descriptive) to an instance, and that instance will be passed as the first argument whenever the method is called.
Callables that are attributes of a class (as opposed to an instance) are still unbound, though, so you can modify the class definition whenever you want:
>>> def fooFighters( self ):
... print "fooFighters"
...
>>> A.fooFighters = fooFighters
>>> a2 = A()
>>> a2.fooFighters
<bound method A.fooFighters of <__main__.A instance at 0x00A9BEB8>>
>>> a2.fooFighters()
fooFighters
Previously defined instances are updated as well (as long as they haven't overridden the attribute themselves):
>>> a.fooFighters()
fooFighters
The problem comes when you want to attach a method to a single instance:
>>> def barFighters( self ):
... print "barFighters"
...
>>> a.barFighters = barFighters
>>> a.barFighters()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: barFighters() takes exactly 1 argument (0 given)
The function is not automatically bound when it's attached directly to an instance:
>>> a.barFighters
<function barFighters at 0x00A98EF0>
To bind it, we can use the MethodType function in the types module:
>>> import types
>>> a.barFighters = types.MethodType( barFighters, a )
>>> a.barFighters
<bound method ?.barFighters of <__main__.A instance at 0x00A9BC88>>
>>> a.barFighters()
barFighters
This time other instances of the class have not been affected:
>>> a2.barFighters()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: A instance has no attribute 'barFighters'
More information can be found by reading about descriptors and metaclass programming.
This explanation is based on a commented Ruby script from a friend of mine. If you want to improve the script, feel free to update it at the link.
First, note that when Ruby calls out to a shell, it typically calls /bin/sh
, not Bash. Some Bash syntax is not supported by /bin/sh
on all systems.
Here are ways to execute a shell script:
cmd = "echo 'hi'" # Sample string that can be used
Kernel#`
, commonly called backticks – `cmd`
This is like many other languages, including Bash, PHP, and Perl.
Returns the result (i.e. standard output) of the shell command.
Docs: http://ruby-doc.org/core/Kernel.html#method-i-60
value = `echo 'hi'`
value = `#{cmd}`
Built-in syntax, %x( cmd )
Following the x
character is a delimiter, which can be any character.
If the delimiter is one of the characters (
, [
, {
, or <
,
the literal consists of the characters up to the matching closing delimiter,
taking account of nested delimiter pairs. For all other delimiters, the
literal comprises the characters up to the next occurrence of the
delimiter character. String interpolation #{ ... }
is allowed.
Returns the result (i.e. standard output) of the shell command, just like the backticks.
Docs: https://docs.ruby-lang.org/en/master/syntax/literals_rdoc.html#label-Percent+Strings
value = %x( echo 'hi' )
value = %x[ #{cmd} ]
Kernel#system
Executes the given command in a subshell.
Returns true
if the command was found and run successfully, false
otherwise.
Docs: http://ruby-doc.org/core/Kernel.html#method-i-system
wasGood = system( "echo 'hi'" )
wasGood = system( cmd )
Kernel#exec
Replaces the current process by running the given external command.
Returns none, the current process is replaced and never continues.
Docs: http://ruby-doc.org/core/Kernel.html#method-i-exec
exec( "echo 'hi'" )
exec( cmd ) # Note: this will never be reached because of the line above
Here's some extra advice:
$?
, which is the same as $CHILD_STATUS
, accesses the status of the last system executed command if you use the backticks, system()
or %x{}
.
You can then access the exitstatus
and pid
properties:
$?.exitstatus
For more reading see:
Best Answer
This is really late, but here's how you can find where a method is defined:
http://gist.github.com/76951
If you're on Ruby 1.9+, you can use
source_location
Note that this won't work on everything, like native compiled code. The Method class has some neat functions, too, like Method#owner which returns the file where the method is defined.
EDIT: Also see the
__file__
and__line__
and notes for REE in the other answer, they're handy too. -- wg