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


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

Small Success With Go!; Re: Quick message - This link is broken; Adding Reverb To The Echo Chamber; Sox Audio Tools; Would This Have Been OK?; Honesty only important economically before institutions develop; Stegangraphy via PS4; OpenCL Mess; More Book Recommendations; Good Explanation of Difference Between Majority + Minority; Musical Chairs - Who's The Privileged White Guy; I can see straight men watching this conversation and laffing; When it's Actually a Source of Indignation and Disgust; Meta Thread Defending POC Causes POC To Close Account; Indigenous People Of Chile; Curry Recipe; Interesting Link On Marginality; A Nuclear Launch Ordered, 1962; More Book Recs (Better Person); It's Nuanced, And I Tried, So Back Off; Marx; The Negative Of Positive; Jenny Holzer Rocks; Huge Article on Cultural Evolution and More; "Ignoring language theory"; Negative Finger Counting; Week 12; Communication Via Telecomm Bids; Finding Suspects Via Relatives' DNA From Non-Crime Databases; Statistics and Information Theory; Ice OK in USA; On The Other Hand; (Current Understanding Of) Chilean Taxes / Contributions; M John Harrison; Playing Games on a Cloud GPU; China Gamifies Real Life; Can't Help Thinking It's Thoughtcrime; Mefi Quotes; Spray Painting Bike Frame; Weeks 10 + 11; Change: No Longer Possible To Merge Metadata; Books on Old Age; Health Tree Maps; MRA - Men's Rights Activists; Writing Good C++14; Risk Assessment - Fukushima; The Future of Advertising and Surveillance; Travelling With Betaferon; I think I know what I dislike so much about Metafilter; Weeks 8 + 9; More; Pastamore - Bad Italian in Vitacura; History Books; Iraq + The (UK) Governing Elite; Answering Some Hard Questions; Pinochet: The Dictator's Shadow; An Outsider's Guide To Julia Packages; Nobody gives a shit; Lepton Decay Irregularity; An Easier Way; Julia's BinDeps (aka How To Install Cairo); Good Example Of Good Police Work (And Anonymity Being Hard); Best Santiago Burgers; Also; Michael Emmerich (Vibrator Translator) Interview (Japanese Books); Clarice Lispector (Brazillian Writer); Books On Evolution; Looks like Ara (Modular Phone) is dead; Index - Translations From Chile; More Emotion in Chilean Wines; Week 7; Aeon Magazine (Science-ish); QM, Deutsch, Constructor Theory; Interesting Talk Transcripts; Interesting Suggestion Of Election Fraud; "Hard" Books; Articles or Papers on depolarizing the US; Textbook for "QM as complex probabilities"; SFO Get Libor Trader (14 years); Why Are There Still So Many Jobs?; Navier Stokes Incomplete; More on Benford; FBI Claimed Vandalism; Architectural Tessellation; Also: Go, Blake's 7; Delusions of Gender (book); Crypto AG DID work with NSA / GCHQ; UNUMS (Universal Number Format); MOOCs (Massive Open Online Courses); Interesting Looking Game; Euler's Theorem for Polynomials; Weeks 3-6; Reddit Comment; Differential Cryptanalysis For Dummies; Japanese Graphic Design; Books To Be Re-Read; And Today I Learned Bugs Need Clear Examples; Factoring a 67 bit prime in your head; Islamic Geometric Art; Useful Julia Backtraces from Tasks; Nothing, however, is lost with less discomfort than that which, when lost, cannot be missed

© 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:

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

      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 {}
		      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)

      def other_instance(self, value, spec:Sub(Ins)):
	  if isinstance(value, spec):
	      return value
	      type_error(value, spec)

      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)))

      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])))))
	      return dict(spec._for_each(value, 
			  lambda c, vsn: ((n, transcode(v, s)) for (v, s, n) in vsn)))

      def alternative(self, value, spec:Sub(Alt)):
	  def alt(c, vsn):
	      error = None
	      for (v, s, _) in vsn:
		      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.


Comment on this post