import re, urllib, decimal

def molarmass(compound):

    '''Parses formulas and calculates molar mass using atomic weights at http://www.chem.qmul.ac.uk/iupac/AtWt/'''

    elementfinder=re.compile('([()]?)([A-Z]?[a-z]*)([()]?)(\d*)([()]?)'*len(compound))

    element=re.search(elementfinder, compound)

    elementlist=[]

    for a in range(1, len(compound)*5+1):

        try:

            elementlist.append(int(element.group(a)))

        except ValueError:

            if element.group(a) != '':

                elementlist.append(element.group(a))

​

    var(' '.join(str(x) for x in elementlist))

​

    #Adds multiplication for subscripts

    for index, i in enumerate(elementlist):

        try:

            eval(i)

            if isinstance(elementlist[index+1], int):

                elementlist.insert(index+1, '*')

        except:

            pass

​

    #Adds multiplication after brackets

    for index, a in enumerate(elementlist):

        if a==')' and isinstance(elementlist[index+1], int):

            elementlist.insert(index+1, '*')

    #Adds addition before brackets

    for index, a in enumerate(elementlist):

        try:

            if isinstance(a, int) and elementlist[index+1]=='(':

                elementlist.insert(index+1, '+')

        except IndexError:

            break

    #Adds addition

    for index, a in enumerate(elementlist):

        try:

            eval(a); eval(elementlist[index+1])

            elementlist.insert(index+1, '+')

        except:

            pass

    for index, a in enumerate(elementlist):

        try:

            eval(elementlist[index+1])

            if isinstance(a, int):

                elementlist.insert(index+1, '+')

        except:

            pass

    #Convert to ints

    for index, a in enumerate(elementlist):

        try:

            int(a) 

            elementlist[index]='int(%s)' %a

        except: pass

​

    elementpage=urllib.urlopen('http://www.chem.qmul.ac.uk/iupac/AtWt/').read()

​

    for index, atom in enumerate(elementlist):

        try:

            massfinder=re.compile('%s</td><td>\w+\s*</td><td>\[?(\d*\.?\d*)[](]' %atom)

            found=re.search(massfinder, elementpage)

            elementlist[index]='decimal.Decimal("%s")' %found.group(1)

        except: pass

    print 'Calculation:',

    for a in elementlist:

        try: print str(eval(a)),

        except: print a,

    elementlist=''.join(str(x) for x in elementlist)

​

    print '\n\n', 'Molar mass:', eval(elementlist), 'g/mol'

​

@interact

def calc(compound=input_box(label='Compound', default='H2O', type=str)):

    molarmass(compound)