Physics Hacker

This is the second part of two minimal examples, on how to interact with the Facebook REST API. Last weeks part was in python, this time we do the exact same thing in erlang. (Note that there are python and erlang libs in various stages of completion, but my focus is on minimal examples and understanding here.)

The code below calls the function users.getinfo in the “call” method. The request send to the Facebook REST server has to be signed according to Facebook’s signing algorithm this is done in “get_rest_url”.

The result will be requested to be transmitted as JSON encoded text. I am using mochijson2 to decode the result.

-module(fb).
-export([call/1, test/0]).

-define(SECRET, "a1b1c1e1d1f1......"
-define(API_KEY, "a2b2c2e2d2f2......"
-define(REST_SERVER, "api.facebook.com").
-define(URL_START, "/restserver.php?").

get_rest_url(Args) ->
  % this implements the the facebook signing recipe
  % the link is in the text above
  SortedArgs = lists:sort(Args),
  Fun = fun({Key, Value}) -> Key ++ "=" ++ Value end,
  StringArgs = lists:map(Fun, SortedArgs),
  String = lists:flatten(StringArgs, ?SECRET),
  <<M:128>> = erlang:md5(String),
  Sig = lists:flatten(io_lib:format("~32.16.0b",[M])),  % string rep of hexdigest
  StringArgs2 = lists:map(fun(X) -> X ++ "&" end, StringArgs),
  lists:flatten(["http://", ?REST_SERVER, ?URL_START, StringArgs2, "sig=", Sig]).

call(Args) ->
  {Megaseconds,Seconds,Microseconds} = erlang:now(),
  CallId = integer_to_list(Megaseconds)
        ++ integer_to_list(Seconds)
        ++ integer_to_list(Microseconds),
  Args2 = lists:append(Args, [{"api_key", ?API_KEY},   % my application key
                             {"call_id", CallId},      % just any value that increases
                             {"format",  "JSON"}]),    % request JSON formatted reply
  Url = fb:get_rest_url(Args2),
  {ok, {{HttpVer, Code, Msg}, Headers, Body}} =
                    http:request(get, {Url, [{"User-Agent", "test "}]}, [], []),
  mochijson2:decode(Body).

test() ->
  call([{"method","users.getinfo"},
        {"v","1.0"},
        {"uids","1234567..."},
        {"fields","first_name"}]).

you would run the above like this:

erl -pa /some/path/to/mochiweb/ebin/
Erlang R13B04 (erts-5.7.5) [source] [smp:4:4] [rq:4] [async-threads:0] [kernel-poll:false]

Eshell V5.7.5  (abort with ^G)
1> inets:start().
ok
2> c(fb).
./fb.erl:28: Warning: variable 'Code' is unused
./fb.erl:28: Warning: variable 'Headers' is unused
./fb.erl:28: Warning: variable 'HttpVer' is unused
./fb.erl:28: Warning: variable 'Msg' is unused
{ok,fb}
3> fb:test().
[{struct,[{<<"first_name">>,<<"Joe">>},
          {<<"uid">>,123456...}]}]
4>

This is the first part of 2 minimal examples, on how to interact with the Facebook REST API. This part will be an example in python, the next part will do the exact same thing in erlang. (Note that there are python and erlang libs in various stages of completion, but my focus is on minimal examples and understanding here.)

The code below calls the function users.getinfo in the “call” method. The request send to the Facebook REST server has to be signed according to Facebook’s signing algorithm this is done in “get_rest_url”.

The result will be requested to be transmitted as JSON encoded text. I am using cjson to decode the result, but you could use any of the python JSON libs (Note that from python 2.6 onward there is a json lib in the standard lib).

__secret__     = 'a1b1c1e1d1f1......'
__api_key__    = 'a2b2c2e2d2f2......'
__restserver__ = 'api.facebook.com'
__url_start__  = '/restserver.php?'

import time
import hashlib
import httplib
import cjson

def get_rest_url(d):
  # this implements the recipe you can find on this page:
  # http://wiki.developers.facebook.com/index.php/How_Facebook_Authenticates_Your_Application
  l = d.items()
  l.sort()
  l = [str(x[0])+'='+str(x[1]) for x in l]
  sig = hashlib.md5(''.join(l) + __secret__).hexdigest()
  url_list = [ __url_start__ ]
  url_list.extend([x+'&' for x in l])
  url_list.append('sig='+sig)
  return ''.join(url_list)

def call(d):
  # make sure certain keys are set
  d.update({'api_key': __api_key__,   # my application key
            'call_id': time.time(),   # just any value that increases
            'format' : 'JSON'})       # request JSON formatted reply
  url = get_rest_url(d)
  connection = httplib.HTTPConnection(__restserver__)
  connection.request('GET',url)
  response = connection.getresponse()
  return cjson.decode(response.read())

def main():
  d = {'method':'users.getinfo',
       'v':'1.0',
       'uids':'1234567...',    #use ur own userid for testing
       'fields':'contact_email,first_name',
      }
  print call(d)
if __name__ == '__main__':
  main()

Here is a quick example of how one can create and fill a simple TTree in PyRoot. I find this especially usefull when the input data are in form of a flat text file as python can do string parsing very well.

This script creates a C-style struct and uses the addresses of the elements inside to this to fill the root tree.

#!/usr/bin/env python
from ROOT import TTree, TFile, AddressOf, gROOT

def main():
  # Make a tree
  f = TFile('myTest.root','RECREATE')
  t = TTree('MyTree','My test tree')

  # Create a struct
  gROOT.ProcessLine(\
    "struct MyStruct{\
      Int_t someInt;\
      Double_t someDouble;\
    };")
  from ROOT import MyStruct

  # Create branches in the tree
  s = MyStruct()
  t.Branch('rootInt',AddressOf(s,'someInt'),'someInt/I')
  t.Branch('rootDouble',AddressOf(s,'someDouble'),'someDouble/D')

  # Fill tree
  for i in range(100):
    s.someInt = i
    s.someDouble = i
    t.Fill()

  f.Write()
  f.Close()

if __name__=='__main__':main()

This time we do the same minimal histogram example, but using python. Again we assume that the user provides a file example.dat, that contains integer values in ascii format seperated by newlines.

call:

python historam.py example.dat

source for “histogram.py”:

import ROOT, sys

def readDataToHist(fileName):
  f = open(fileName);
  x_min = 0;
  x_max = 10;
  histo = ROOT.TH1I("h1","Histogram Title", 100,x_min,x_max);
  for line in f:
    raw = line[:-1]
    y = int(raw)
    histo.Fill(y);
  return histo;

def histogram(fileName):
  ROOT.gROOT.Reset()
  c1 = ROOT.TCanvas("c1","c1",800,800);
  histo = readDataToHist(fileName);
  c1.cd(1);
  histo.Draw();
  #c1.Update();
  c1.Print("histogram.pdf","Portrait pdf");

if __name__ == "__main__" :
  histogram(sys.argv[1])