[Python] manipuler un string

chiffres= ('1','2','3','4','5','6','7','8','9')

chaine="jsldkgh\rm.qeigj534qshfsgn\n"

for caractere in chaine:
        if not caractere in chiffre:
                 chaine=chaine.split(caractere)
                 chaine="".join(chaine)

def getxnylist(listemere):
       xlist = [n for n in listemere if listemere.index(n) % 2 == 0]
       ylist = [n for n in listemere if listemere.index(n) % 2 != 0]
       return xlist, ylist

def getxlist(firstlist):
        lx = []
        if firstlist:
                lx = [firstlist[0]] + getxlist(firstlist[2:])         
        return lx               

def getylist(firstlist):
        ly = []
        if firstlist:
                ly = [firstlist[1]] + getylist(firstlist[2:])
        return ly
        
def getxnylist(firstlist):
        """Creates two lists. Each list has one out of two elements of the first list.
        
        The first list will represent x-values, the second  y-values."""
        lx = getxlist(firstlist)
        ly = getylist(firstlist)
        return lx, ly    

import sys

def main():
        """Will calculate a simple linear regression."""
        try:
                filename = raw_input("Which file to import? ")
                f = open(filename, "r")
                try:
                        stringdata = f.read()
                        f.close()
                        okdata = convert(stringdata)
                        xlist, ylist = getxnylist(okdata)
                        T = len(xlist)
                        Ex = sum(xlist)
                        Ey = sum(ylist)
                        Exsq = listsqsum(xlist)
                        Exy = listxysum(okdata)
                        b2 = (T*Exy - Ex*Ey)/(T*Exsq - Ex**2)
                        b1 = (Ey/T) - b2*(Ex/T)
                        print "yt = " + str(b1) + " + (" + str(b2) + ")Xt"                 
                except ValueError:
                        print "Invalid values in there, sorry, only take numbers.."
                        sys.exit()                
        except IOError:
                print "File does not exist!"
                sys.exit()
                
def convert(ustring):
        """Formats input text file into a list of floating numbers.
        
        Admits anything as separators between numbers but '.'. Will take all nu-
        bers in consecutively, ignoring anything in between. Make sure your x-values
        are first."""
        accepted = ('.', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9')
        
        for char in ustring:
                if not char in accepted:
                        ustring = ustring.split(char)#knock off anything not in
                        ustring = " ".join(ustring)#accepted, replace by whitespace
                        
        ustring = ustring.split()#then split on whitespace
                       
        ustring = [float(num) for num in ustring]#make sure numbers are decimals
        
        return ustring

def alternativeconvert(ustring):
        """Formats through regular expression."""
        pat = re.compile('[^0-9.]')#import re..
        ustring = re.split(pat, ' ', ustring)
        ustring = ustring.split()
        ustring = [decimal.Decimal(num) for num in ustring]#import decimal..
        return ustring
        
def getxlist(firstlist):
        lx = []
        if firstlist:
                lx = [firstlist[0]] + getxlist(firstlist[2:])         
        return lx               

def getylist(firstlist):
        ly = []
        if firstlist:
                ly = [firstlist[1]] + getylist(firstlist[2:])
        return ly
        
def getxnylist(firstlist):
        """Creates two lists. Each list has one out of two elements of the first list.
        
        The first list will represent x-values, the second  y-values."""
        lx = getxlist(firstlist)
        ly = getylist(firstlist)
        if len(lx) != len(ly):
            print "Missing coordinates.. Make sure you have pairs."
            sys.exit()
        else:
            pass
        return lx, ly                  
        
def listsqsum(l):
        """Sums the squares of the elements of a list."""
        total = 0
        if l:
                total = (l[0])**2 + listsqsum(l[1:])
        return total
        
def listxysum(l):
        """Sums the product of the l-list elements two by two."""
        total = 0
        if l:
                total = l[0]*l[1] + listxysum(l[2:])
        return total
        
if __name__ == "__main__":
        main()

        
#TODO:
#code a general summation function --> summation(expression, indexstart, indexstop)
#as yet one can only calculate y=b1+b2x --> allow for different transformations on x
#expand simple linear regression to multiple regression
#add standard errors, R**2 and other interesting values

#def summation(expr, i=0, n):