Aren't django signals a little like comefrom?

By : rohit

In computer programming, COMEFROM (or COME FROM) is an obscure control flow
structure used in some programming languages, primarily as a joke.

I never liked using signals. Recently I was asked that, and has no answer, but a little thinking led me to this question. Aren't signals a little like COMEFROM. If yes, aren't they as bad as COMEFROM? If you do not know what a COMEFROM is, [wikipedia to the rescue](http://en.wikipedia.org/wiki/COMEFROM)

Some hypothetical code using COMEFROM, again from wikipedia,

from goto import comefrom, label
comefrom .repeat
name = raw_input('what is your name? ')
if name:
print "Hello",name
label .repeat
print "Goodbye!"

And Some actual Django code using signals

class Post(models.Model):
name = models.CharField(...)
...
num_comments = models.PositiveIntegerField(default = 0)

class Comment(models.Model):
...
post = models.ForeignKey(Post)

def handle_num_comments(sender, **kwargs):
instance = kwargs['instance']
instance.post.num_comments+=1
instance.post.save()

from django.db.signals import post_save

post_save.connect(handle_num_comments, sender=Comment)

And the same code using COMEFROM

class Post(models.Model):
name = models.CharField(...)
...
num_comments = models.PositiveIntegerField(default = 0)

class Comment(models.Model):
...
post = models.ForeignKey(Post)

def save(self, *args, **kwargs):
super(Comment, self).save(*args, **kwargs)
instance = self
LABEL .post_save

def handle_num_comments(sender, **kwargs):
instance = kwargs['instance']
COMEFROM .post_save
instance.post.num_comments+=1
instance.post.save()

So isn't the signals code a little like COMEFROM, and why is it superior to COMEFROM?

--------------
* [There is a library to add goto and comefrom to Python](http://entrian.com/goto/)
* [Discussion about COMEFROM](http://www.c2.com/cgi/wiki?ComeFrom)

-------------
You should follow me on [twitter here](http://twitter.com/uswaretech).

Topics : django rambling

Comments

mjc 19th July, 2009

I remember in the 70's studying the semantics of programming languages and coming across this definition of the semantics of a label:
The condition true at a label is the union (logical or) of all the conditions true at the places where the next statement (via goto or falling through) is the label.
This seemed (1) true after it is pointed out, and (2) a lot like the COMEFROM.

K 19th July, 2009

I haven't looked at Django, but it seems like basic event handling to me. If so, you could say that all event handling is a lot like COMEFROM :-)

Douglas 19th July, 2009

The difference between COMEFROM and event handling/post_save, is that control does not return back to the label after COMFROM.

Example:

register_post_save_handler();
save();
// handler called here
now_this_is_called();

register_COMEFROM_post_save();
save();
// handler called here
this_may_not_get_called,_call_stack_gone();

Max Battcher 19th July, 2009

Although "GOTO is considered harmful", no one has proven that COMEFROM is harmful. In the case of INTERCAL, COMEFROM is a nightmare because it is the only real flow-control mechanism in the language and requires some mind-bending to actually accomplish more straightforward patterns. Django signals doesn't suffer from the same problems as COMEFROM, because ultimately it is not the only flow-control mechanism in Django/Python. Signals are merely a nice aspect-oriented/event-oriented support tool that is one in a large bag of useful tools.

bill jones 19th July, 2009

The similarity is superficial.

COMEFROM (and GOTO) are both static - in that they are defined when the code is written and cannot be changed at runtime. Signal-based connections can be created or removed at runtime.

Any number of signal handlers can respond to a single event, whereas COMEFROM/GOTO only transfer control flow from point A to point B (not B1, B2... Bn).

Also, I'm pretty sure your COMEFROM based translation of the signal example could not possibly work. For example, what's the actual entry point to the 'handle_num_comments' function? The fact that you can't create a corresponding example (that actually works) kind of proves the point that these structures are not equivalent.

æ‚‰å°¼ 19th July, 2009

yes, it's a little bit like comefrom...

Gonzalo Delgado 19th July, 2009

So you're saying the observer pattern is a bit like comefrom.. interesting.

Kelvin71 22nd Oct., 2009

Ask yourself these seven questions to determine what type of learner you are: If you would choose to read about it you are more of a visual learner, if you would prefer to hear about it you are a more auditory learner and if you would like to try it yourself you are more of a tactile learner. ,

Reactions

uswaretech 18th July, 2009

Arenâ€™t django signals a little like comefrom? New Post. http://bit.ly/tzHa4

This comment was originally posted on Twitter

webstartupgroup 18th July, 2009

StartupNews: Aren’t Django signals a little like COMEFROM? http://bit.ly/TjE7b

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Zak 19th July, 2009

It’s better because it doesn’t require messing around with, or even being aware of the internals of the code it’s being connected to. It’s actually a lot like a Common Lisp :before method, which I’ve found really bloody useful on a few occasions.