'''equation solver using attributes and introspection'''
from __future__ import division
class Solver(object):
'''takes a function, named arg value (opt.) and returns a Solver object'''
def __init__(self,f,**args):
self._f=f
self._args={}
# see important note on order of operations in __setattr__ below.
for arg in f.func_code.co_varnames[0:f.func_code.co_argcount]:
self._args[arg]=None
self._setargs(**args)
def __repr__(self):
argstring=','.join(['%s=%s' % (arg,str(value)) for (arg,value) in
self._args.items()])
if argstring:
return 'Solver(%s,%s)' % (self._f.func_code.co_name, argstring)
else:
return 'Solver(%s)' % self._f.func_code.co_name
def __getattr__(self,name):
'''used to extract function argument values'''
self._args[name]
return self._solve_for(name)
def __setattr__(self,name,value):
'''sets function argument values'''
# Note - once self._args is created, no new attributes can
# be added to self.__dict__. This is a good thing as it throws
# an exception if you try to assign to an arg which is inappropriate
# for the function in the solver.
if self.__dict__.has_key('_args'):
if name in self._args:
self._args[name]=value
else:
raise KeyError, name
else:
object.__setattr__(self,name,value)
def _setargs(self,**args):
'''sets values of function arguments'''
for arg in args:
self._args[arg] # raise exception if arg not in _args
setattr(self,arg,args[arg])
def _solve_for(self,arg):
'''Newton's method solver'''
TOL=0.0000001 # tolerance
ITERLIMIT=1000 # iteration limit
CLOSE_RUNS=10 # after getting close, do more passes
args=self._args
if self._args[arg]:
x0=self._args[arg]
else:
x0=1
if x0==0:
x1=1
else:
x1=x0*1.1
def f(x):
'''function to solve'''
args[arg]=x
return self._f(**args)
fx0=f(x0)
n=0
while 1: # Newton's method loop here
fx1 = f(x1)
if fx1==0 or x1==x0: # managed to nail it exactly
break
if abs(fx1-fx0)<TOL: # very close
close_flag=True
if CLOSE_RUNS==0: # been close several times
break
else:
CLOSE_RUNS-=1 # try some more
else:
close_flag=False
if n>ITERLIMIT:
print "Failed to converge; exceeded iteration limit"
break
slope=(fx1-fx0)/(x1-x0)
if slope==0:
if close_flag: # we're close but have zero slope, finish
break
else:
print 'Zero slope and not close enough to solution'
break
x2=x0-fx0/slope # New 'x1'
fx0 = fx1
x0=x1
x1=x2
n+=1
self._args[arg]=x1
return x1
def tvm(pv,fv,pmt,n,i):
'''equation for time value of money'''
i=i/100
tmp=(1+i)**n
return pv*tmp+pmt/i*(tmp-1)-fv
