Adding some­thing else to my plate is prob­a­bly not a very good idea, but what the heck, I can make it sleep an­oth­er three years if I lose in­ter­est.

So: I am writ­ing a re­al spread­sheet in python.

Prob­a­bly nev­er go­ing to be use­ful for cor­po­ra­tions, but it should be at least as fea­ture­ful as Google's and it should be amaz­ing­ly smal­l.

Here are the com­po­nents:

• Trax­ter: my spread­­sheet-­­for­­mu­la-­­like-lan­guage with de­pen­­den­­cy track­­ing that com­piles to python.

• PyQt (hey, it has a grid wid­get)

• Python (Of course)

The sta­tus right now:

• It's al­­most as func­­tion­al as it was 2.5 years ago

• Ex­­cept for bro­ken rel­a­­tive cel­l­s.

• But with the be­gin­n­ing of a re­al for­­mu­la lan­guage.

• With au­­to­­mat­ic re­­cal­cu­la­­tion and cyclic de­pen­­den­­cy check­­s.

• Adding things is dead sim­­ple. Here's the im­­ple­­men­­ta­­tion of SUM (not up­­load­­ed yet, though):

def sum(*args):
  ac=0
  for v in args:
    ac+=v
  return ac

All the range stuff hap­pens be­hind the scenes (although you may get a func­tion called with thou­sands of args... I won­der how well Python han­dles that).

You can check it at the google code project (Use the SVN).

I apol­o­gize in ad­vance for any ug­ly am­a­teurism in this post. It's my first at­tempt at a do­main spe­cif­ic lan­guage :-)

Yes­ter­day I post­ed about us­ing Py­Cells to write a spread­sheet in Python.

Sad­ly, I can't fig­ure out the prob­lem with my code, and the Py­Cells mail­ing list seems to be pret­ty much dead.

So, I start­ed think­ing... what oth­er ways are to achieve my goal? And de­cid­ed to go me­dieval on this prob­lem.

By that I mean that I will do it the most tra­di­tion­al way pos­si­ble... with a twist.

The tra­di­tion­al way is, of course, to write one or more of lex­er/­parser/in­ter­preter/­com­pil­er for the for­mu­la lan­guage.

Mind you, I don't in­tend to do any­thing com­plete, much less Ex­cel-­com­pat­i­ble (see Ex­cel for­mu­la parsers are hell in this same blog.

So, let's start with a toy lan­guage, sup­port­ing the fol­low­ing:

• As­sign­­ment to a var­i­able

• Clas­sic 4-op arith­met­ic­s.

• Func­­tion calls

• Cell ranges

That's enough for a toy spread­sheet, and it should be easy to ex­tend.

Here's a de­scrip­tion of the gram­mar for such a lan­guage, writ­ten us­ing Ape­ri­ot [1]:

# This is a simple language for arithmetic expressions

numbers
     number

operators
     plus   "+"
     times  "*"
     minus  "-"
     div    "/"
     equal  "="
     colon ":"
     comma ","
     semicolon ";"

brackets
     lpar  "("
     rpar  ")"


identifiers
     label

start
     LIST

rules

LIST             -> ASSIGNMENT                : "[$1]"
                  | ASSIGNMENT semicolon LIST : "[$1]+$3"
                  | ASSIGNMENT semicolon : "[$1]"

ASSIGNMENT       -> label equal EXPR : "($1,$3)"

ARGLIST          -> ARG comma ARGLIST : "[$1]+$3"
                  | ARG          : "[$1]"

ARG              -> RANGE       : "$1"
                  | EXPR        : "$1"
                  | label       : "$1"

EXPR             -> TERM              : "$1"
                  | TERM plus EXPR    : "(\'+\',$1,$3)"
                  | TERM minus EXPR   : "(\'-\',$1,$3)"

TERM             -> FACTOR               : "$1"
                  | FACTOR times TERM    : "(\'*\',$1,$3)"
                  | FACTOR div TERM      : "(\'/\',$1,$3)"


FACTOR           -> number           : "$1.val()"
                  | lpar EXPR rpar  : "(\'group\',$2)"
                  | FUNCALL     : "$1"
                  | label               : "$1"
                  | minus FACTOR    : "-$2"

FUNCALL          ->  label lpar ARGLIST rpar : "(\'funcall\',$1,$3)"

RANGE            -> label colon label   : "(\'range\',$1,$3)"

This trans­forms this:

A1=SUM(A1:A7)*2;
A3=2+2;

In­to this:

[(<aperiot.lexer.Identifier instance at 0xb7af10ac>,
  ('*',
   ('funcall',
    <aperiot.lexer.Identifier instance at 0xb7af142c>,
    [('range',
      <aperiot.lexer.Identifier instance at 0xb7af15cc>,
      <aperiot.lexer.Identifier instance at 0xb7af144c>)]),
   2)),
 (<aperiot.lexer.Identifier instance at 0xb7b4c72c>, ('+', 2, 2))]

Which is sort of a tree with all the ex­pres­sions in pre­fix no­ta­tion in them.

Now, here is the twist: I will "com­pile" this tree in­to.... python code. So I can use eval to do the eval­u­a­tion, just like in the orig­i­nal python spread­sheet recipe.

So this is sort of a pre­pro­ces­sor:

• The us­er writes ex­cel-­­like for­­mu­las.

• The spread­­sheet stores python code ob­­tained through com­pi­la­­tion.

• The spread­­sheet evals the python code.

Of course we have the same prob­lem as usu­al: cell de­pen­den­cies, which is the rea­son why I start­ed play­ing with Py­Cells in the first place!

But... well, here's an­oth­er trick: since I am com­pil­ing, I know when­ev­er there is a vari­able ref­er­enced in the code. And I can re­mem­ber them :-)

So, I can turn this:

A1=SUM(A1:A3)*2;
A3=2+2;

In­to this:

[['A1=SUM(a1,a2,a3)*2;', set(['a1', 'a3', 'a2'])],
 ['A3=2+2;', set([])]]

The "com­piled" python code and a de­pen­den­cy set. And voila, this spread­sheet will prop­a­gate cor­rect­ly.

Here's the com­pil­er... in about 60 lines of python [2]. And since the whole point of this lan­guage is to track de­pen­den­cies... let's call it Trax­ter.

Of course, this is a toy right now. But it's a toy with po­ten­tial!

from pprint import pprint
from aperiot.parsergen import build_parser
import aperiot
import cellutils
import sys

dependencies=set()

def addOp(*args):
        return '+'.join([compile_token(a) for a in args])
def mulOp(*args):
        return '*'.join([compile_token(a) for a in args])
def subOp(*args):
        return '-'.join([compile_token(a) for a in args])
def divOp(*args):
        return '/'.join([compile_token(a) for a in args])

def groupOp(*args):
        return '(%s)'%compile_token(args[0])

def funcOp(*args):
        return '%s(%s)'%(args[0].symbolic_name,
                         ','.join([compile_token(a) for a in args[1]]))

def rangeOp(*args):
        c1=args[0].symbolic_name
        c2=args[1].symbolic_name
        return ','.join([compile_token(a) for a in cellutils.cellrange(c1,c2)])

operators={'+':addOp,
           '-':subOp,
           '*':mulOp,
           '/':divOp,
           'group':groupOp,
           'funcall':funcOp,
           'range':rangeOp
           }


def compile_token(token):
        if isinstance (token,aperiot.lexer.Identifier):
                v=token.symbolic_name.lower()
                dependencies.add(v)
                return v
        if isinstance(token,list) or isinstance(token,tuple):
            return apply(operators[token[0]],token[1:])
        return str(token)

def compile_assignment(tokens):
        target=tokens[0].symbolic_name
        compiled=compile_token(tokens[1])
        return '%s=%s;'%(target,compiled)


myparser = build_parser('traxter')
t='A1=SUM(A1:A7)*2;A3=2+2;'
assign_list=myparser.parse(t)
pprint (assign_list)

compiled=[]
for assignment in assign_list:
        dependencies=set()
        c=compile_assignment(assignment)
        compiled.append([c,dependencies])

print compiled

In 2004 I saw a recipe about how to make a "spread­sheet" in python in 10 lines of code:

class SpreadSheet:
    _cells = {}
    tools = {}
    def __setitem__(self, key, formula):
        self._cells[key] = formula
    def getformula(self, key):
        return self._cells[key]
    def __getitem__(self, key ):
        return eval(self._cells[key], SpreadSheet.tools, self)

It's shock­ing. And it work­s, too:

>>> from math import sin, pi
>>> SpreadSheet.tools.update(sin=sin, pi=pi, len=len)
>>> ss = SpreadSheet()
>>> ss['a1'] = '5'
>>> ss['a2'] = 'a1*6'
>>> ss['a3'] = 'a2*7'
>>> ss['a3']
210
>>> ss['b1'] = 'sin(pi/4)'
>>> ss['b1']
0.70710678118654746
>>> ss.getformula('b1')
'sin(pi/4)'

I was so awed, I wrote a PyQt ver­sion . Of course there is a catch in that code: it sucks if you are try­ing to write a spread­sheet with it.

Why? Be­cause it does­n't store re­sult­s, but on­ly for­mu­las.

For ex­am­ple:

A1=2
A2=A1*2

If you ask for the val­ue of A2, you get 4. If you set A1 to 7, what's the val­ue of A2?

Well, it's noth­ing yet, be­cause it's on­ly cal­cu­lat­ed when you ask for it. But sup­pose you are try­ing to dis­play that sheet... you need to know A2's val­ue changed when you set A1!

That's cell de­pen­den­cies, and while that sim­ple code han­dles them in a way, it to­tal­ly sucks in an­oth­er.

So, I went ahead and cod­ed around it suc­cess­ful­ly. Of course the code was not so pret­ty any­more (although a large part of the ug­ly­ness is just for mak­ing it work with Python 2.3 and rel­a­tive cell­s).

Then yes­ter­day, while look­ing at the ex­cel for­mu­la pars­er mad­ness I saw a ref­er­ence to Py­Cells, a python port of Cells from CLOS.

Here is a blog com­ment­ing on Py­cells:

It ba­si­cal­ly takes the con­cept of a cell in a spread­sheet that get up­dat­ed au­to­mat­i­cal­ly to pro­gram­ming where there are a lot of in­ter­nal da­ta states that are de­pen­dent on one an­oth­er in a chain, or a com­plex graph of de­pen­den­cies. Like, the col­or of a but­ton de­pends on whether you se­lect­ed a ra­dio but­ton or not. Or, shut down the mo­tor if the sen­sor reads above 100 de­grees (ex­am­ple giv­en in tex­t).

Al­most ev­ery­one us­es that anal­o­gy... how­ev­er, no mat­ter how hard I looked, I could­n't find any­one who had ac­tu­al­ly tried writ­ing a spread­sheet us­ing Py­Cell­s! Not even as an ex­am­ple!

So here it is:

import cells

class Cell(cells.Model):
    formula=cells.makecell(value='')
    @cells.fun2cell()
    def value(self,prev):
        print "eval ",self.formula
        return eval(self.formula, {}, self.ss)
    def __init__(self, ss, *args, **kwargs):
        self.ss=ss
        cells.Model.__init__(self, *args, **kwargs)


class ssDict:
        def __init__(self):
                self.ss={}

        def __getitem__(self,key):
                return self.ss[key].value

        def __setitem__(self,key,v):
                if not self.ss.has_key(key):
                        c=Cell(self)
                        c.formula=v
                        self.ss[key]=c
                else:
                        self.ss[key].formula=v

if __name__ == "__main__":
        ss=ssDict()
        ss['a1'] ='5'

        ss['a2']='2*a1'
        print "a1: ", ss['a1']
        print "a2: ", ss['a2']

        ss['a1'] = '7'
        print "a1: ", ss['a1']
        print "a2: ", ss['a2']

And here you can see it run­ning:

[ralsina@monty cells]$ python ctest.py
a1:  eval  5
5
a2:  eval  2*a1
10
eval  7
eval  2*a1
a1:  7
a2:  14

See how when I set a1 to 7 I get "e­val 7" and "e­val 2*a1"? That's be­cause it's prop­a­gat­ing changes the right way. And that's why this would work as a ba­sis for a spread­sheet.