| Andrew Cooke | Contents | Latest | RSS | Twitter | Previous | Next

C[omp]ute

Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

Personal Projects

Lepl parser for Python.

Colorless Green.

Photography around Santiago.

SVG experiment.

Professional Portfolio

Calibration of seismometers.

Data access via web services.

Cache rewrite.

Extending OpenSSH.

C-ORM: docs, API.

Last 100 entries

Nussbaum on Anger; Credit Card Processing for Cat Soft LLC; Discover new movies on demand in our online cinema; Tasting; Credit Card Processing for Cat Soft LLC; Apple + Kiwi Jam; Hit Me; Increase Efficiency with GPS Vehicle Tracking for Cat Soft LLC; Sudoku - CSP + Chaos; Recycling Electronics In Santiago; Vector Displays in OpenGL; Call Center Services for Cat Soft LLC; And Anti-Aliased; OpenGL - Render via Intermediate Texture; And Garmin Connect; Using Garmin Forerunner 230 With Linux; Payroll Service Quotes for Cat Soft LLC; (Beating Dead Horse) StackOverflow; Current State of Justice in China; Now Is Cat Soft LLC's Chance To Save Up To 32% On Mail; Axiom of Determinacy; Ewww; Fee Chaos Book; Course on Differential Geometry; Increase Efficiency with GPS Vehicle Tracking for Cat Soft LLC; Okay, but...; Sparse Matrices, Deep Learning; Sounds Bad; Applebaum Rape; Tomato Chutney v4; Have to add...; Culturally Liberal and Nothing More; Weird Finite / Infinite Result; Your diamond is a beaten up mess; Maths Books; Good Bike Route from Providencia / Las Condes to Panul\; Iain Pears (Author of Complex Plots); Plum Jam; Excellent; More Recently; For a moment I forgot StackOverflow sucked; A Few Weeks On...; Chilean Book Recommendations; How To Write Shared Libraries; Jenny Erpenbeck (Author); Dijkstra, Coins, Tables; Python libraries error on OpenSuse; Deserving Trump; And Smugness; McCloskey Economics Trilogy; cmocka - Mocks for C; Concept Creep (Americans); Futhark - OpenCL Language; Moved / Gone; Fan and USB issues; Burgers in Santiago; The Origin of Icosahedral Symmetry in Viruses; autoenum on PyPI; Jars Explains; Tomato Chutney v3; REST; US Elections and Gender: 24 Point Swing; PPPoE on OpenSuse Leap 42.1; SuperMicro X10SDV-TLN4F/F with Opensuse Leap 42.1; Big Data AI Could Be Very Bad Indeed....; Cornering; Postcapitalism (Paul Mason); Black Science Fiction; Git is not a CDN; Mining of Massive Data Sets; Rachel Kaadzi Ghansah; How great republics meet their end; Raspberry, Strawberry and Banana Jam; Interesting Dead Areas of Math; Later Taste; For Sale; Death By Bean; It's Good!; Tomato Chutney v2; Time ATAC MX 2 Pedals - First Impressions; Online Chilean Crafts; Intellectual Variety; Taste + Texture; Time Invariance and Gauge Symmetry; Jodorowsky; Tomato Chutney; Analysis of Support for Trump; Indian SF; TP-Link TL-WR841N DNS TCP Bug; TP-Link TL-WR841N as Wireless Bridge; Sending Email On Time; Maybe run a command; Sterile Neutrinos; Strawberry and Banana Jam; The Best Of All Possible Worlds; Kenzaburo Oe: The Changeling; Peach Jam; Taste Test; Strawberry and Raspberry Jam; flac to mp3 on OpenSuse 42.1; Also, Sebald

© 2006-2015 Andrew Cooke (site) / post authors (content).

Dynamic Dispatch in Python

From: andrew cooke <andrew@...>

Date: Mon, 9 May 2011 17:51:36 -0400

This is an example of how the pytyp type library can be useful.  It's a piece
of code from another part of the pytyp library that rewrites JSON data as
Python classes.

IMPORTANT: I don't expect anyone to understand the code below.  That's not the
point!  Please read on to see what the point is...

The code below is a complex function.  It contains a lot of logic for how to
rewrite different JSON objects.  Lists, for example, should be kept as lists,
but we need to rewrite the contents of the lists (so this rewriting function
needs to call itself recursively).

Hash maps (dicts), however, may be rewritten as Python classes.  That's the
"useful" part of this particular library - it simplifies Python code that
works with JSON.  So when this function finds a dict it needs to compare it
with the specification that the user has entered and do something useful (in
simple terms you can say to the routine "at this point in the JSON data,
convert the data into a class Foo", for example).

Hang on - we're almost at the clever bit...

So this is a complicated function that has to do different things when working
on different types.  Typically in Python you handle a case like this by having
a set of subclasses that all implement the same interface.  And then the
correct action is taken when you call the method on a certain instance.

That's normal OO dispatch - you're chosing a particular method based on the
type of an object.

What pytyp adds here is the ability to dispatch on other function arguments,
instead of "the object before the dot".  So in the call:

   my_function(bar, baz)

you can call different functions depending on the type of baz!  For example:

  my_function(bar, 2)  =>  my_int_function(bar, 2)

  my_function(bar, 'two')  =>  my_str_function(bar, 'two')

When that's applied to this complicated function used to rewrite JSON it lets
us separate out the "routing", which depends on the type of the data, so that
different types are handled by different methods:


  class Transcode:

      @overload
      def __call__(self, value, spec):
	  if isinstance(value, spec):
	      return value
	  else:
	      type_error(value, spec)

      @__call__.intercept
      def map_instance(self, value:Mapping, spec:Sub(Ins)):
	  (varargs, varkw, dict_spec) = class_to_dict_spec(spec)
	  new_value = transcode(value, dict_spec)
	  args = new_value.pop(Rec.OptKey(varargs), []) if varargs else []
	  kargs = new_value.pop(Rec.OptKey(varkw), {}) if varkw else {}
	  args.extend(new_value.pop(index) 
		      for index in sorted(key 
			  for key in new_value.keys() 
			  if isinstance(Rec.OptKey.unpack(key), int)))
	  kargs.update((Rec.OptKey.unpack(key), value) 
		       for (key, value) in new_value.items())    
	  return spec._abc_class(*args, **kargs)

      @__call__.intercept
      def other_instance(self, value, spec:Sub(Ins)):
	  if isinstance(value, spec):
	      return value
	  else:
	      type_error(value, spec)

      @__call__.intercept
      def sequence(self, value, spec:Sub(Seq)):
	  return list(spec._for_each(value, lambda c, vsn: (transcode(v, s) for (v, s, n) in vsn)))

      @__call__.intercept
      def record(self, value, spec:Sub(Rec)):
	  if spec._int_keys():
	      return tuple(spec._for_each(value, 
			  lambda c, vsn: (transcode(v, s) 
					  for (v, s, n) in sorted(vsn, 
					      key=lambda vsn: Rec.OptKey.unpack(vsn[2])))))
	  else:
	      return dict(spec._for_each(value, 
			  lambda c, vsn: ((n, transcode(v, s)) for (v, s, n) in vsn)))

      @__call__.intercept
      def alternative(self, value, spec:Sub(Alt)):
	  def alt(c, vsn):
	      error = None
	      for (v, s, _) in vsn:
		  try:
		      return transcode(v, s)
		  except TypeError as e:
		      error = e
	      raise error
	  print(spec, 'is alt!', value)
	  return spec._for_each(value, alt)

  transcode = Transcode()


As I said, you don't need to understand all that.  All you need to see is that
when you call the transcode() function, what is actually called can be any of
those methods above.  The method Transcode.sequence() handles lists of data,
while the method Transcode.map_instance() is the "core" that rewrites maps as
instance of particular classes.  And the method Transcode.__call__() is the
default method that's called if none of the other, specialised methods match
up with the specifications.

Andrew

Comment on this post