Question

I know it is probably a stupid question, but I am new to OOP in Python and if I declare a function def myFunction( b) and pass an instance of an object to it, I get TypeError: expected string or buffer.

To be more specific, I have a following code that I use to parse a summary molecular formula and make an object out of it.

class SummaryFormula:
    def __init__( self, summaryFormula):
        self.atoms = {}
        for atom in re.finditer( "([A-Z][a-z]{0,2})(\d*)", summaryFormula):
          symbol = atom.group(1)
          count = atom.group(2)

    def extend( self, b):
        # these are the two dictionaries of both molecules
        originalFormula = self.atoms.copy()
        self.atoms.clear()
        addAtoms = SummaryFormula( b)

        # and here both dictionaries are merged
        for atom in addAtoms.atoms.keys():
          if atom in originalFormula.keys():
            self.atoms[ atom] = originalFormula[ atom]
            self.atoms[ atom] += addAtoms.atoms[ atom]
          else:
            pass
        for atom in originalFormula.keys():
          if atom not in self.atoms.keys():
            self.atoms[ atom] = originalFormula[ atom]

 #this is what works now
 test = SummaryFormula( "H2CFe2")
 test.extend("H5C5") #result is a molecule H7C6Fe2

 #this is what I want instead
 test = SummaryFormula( "H2CFe2")
 toExtend = SummaryFormula( "H5C5")
 test.extend( toExtend)

Thank you, Tomas

Answer 1

Richard Cook is correct. There is another problem, however: In extend, you say:

addAtoms = SummaryFormula( b)

Thus, a SummaryFormula instance is passed into the __init__ method of SummaryFormula. Here (modulo the typo mentioned before), this object is given to re.finditer:

for atom in re.finditer( "([A-Z][a-z]{0,2})(\d*)", summaryFormula)

The function re.finditer expects a string; it doesn’t know what to do with a SummaryFormula instance.

There are a few ways to fix this. The immediately simplest is to check whether you already have a SummaryFormula instance before trying to create one:

if isinstance(b, SummaryFormula):
    addAtoms = b
else if isinstance(b, str):
    addAtoms = SummaryFormula(b)
else:
    raise TypeError("Expected a SummaryFormula or equivalent string.")