'int' object not iterable - Python Lists

Question

For context, I am trying to create a program that calculates the molar mass of a compound given its chemical formula. For each capital letter in the user's input (which is converted into a list), I plan on adding the string "+".

However, when I assign a list to the variable 'position' I run into the following error:

Traceback (most recent call last):
  File "/Users/robertg21/PycharmProjects/MolecularMassCalculator/MolecularMassCalculator.py", line 42, in <module>
    new_formula.insert(return_index(), "+")
  File "/Users/robertg21/PycharmProjects/MolecularMassCalculator/MolecularMassCalculator.py", line 35, in return_index
    position = list(formula.index(element))
TypeError: 'int' object is not iterable

Here is my code:

# This program calculates the molar mass of a compound given its chemical formula and number of atoms.

# Dictionary containing relative atomic mass of elements
relative_atomic_mass = {'H': 1.00794, 'He': 4.002602, 'Li': 6.941, 'Be': 9.012182, 'B': 10.811, 'C': 12.0107, 'N': 14.0067,
              'O': 15.9994, 'F': 18.9984032, 'Ne': 20.1797, 'Na': 22.98976928, 'Mg': 24.305, 'Al': 26.9815386,
              'Si': 28.0855, 'P': 30.973762, 'S': 32.065, 'Cl': 35.453, 'Ar': 39.948, 'K': 39.0983, 'Ca': 40.078,
              'Sc': 44.955912, 'Ti': 47.867, 'V': 50.9415, 'Cr': 51.9961, 'Mn': 54.938045,
              'Fe': 55.845, 'Co': 58.933195, 'Ni': 58.6934, 'Cu': 63.546, 'Zn': 65.409, 'Ga': 69.723, 'Ge': 72.64,
              'As': 74.9216, 'Se': 78.96, 'Br': 79.904, 'Kr': 83.798, 'Rb': 85.4678, 'Sr': 87.62, 'Y': 88.90585,
              'Zr': 91.224, 'Nb': 92.90638, 'Mo': 95.94, 'Tc': 98.9063, 'Ru': 101.07, 'Rh': 102.9055, 'Pd': 106.42,
              'Ag': 107.8682, 'Cd': 112.411, 'In': 114.818, 'Sn': 118.71, 'Sb': 121.760, 'Te': 127.6,
              'I': 126.90447, 'Xe': 131.293, 'Cs': 132.9054519, 'Ba': 137.327, 'La': 138.90547, 'Ce': 140.116,
              'Pr': 140.90465, 'Nd': 144.242, 'Pm': 146.9151, 'Sm': 150.36, 'Eu': 151.964, 'Gd': 157.25,
              'Tb': 158.92535, 'Dy': 162.5, 'Ho': 164.93032, 'Er': 167.259, 'Tm': 168.93421, 'Yb': 173.04,
              'Lu': 174.967, 'Hf': 178.49, 'Ta': 180.9479, 'W': 183.84, 'Re': 186.207, 'Os': 190.23, 'Ir': 192.217,
              'Pt': 195.084, 'Au': 196.966569, 'Hg': 200.59, 'Tl': 204.3833, 'Pb': 207.2, 'Bi': 208.9804,
              'Po': 208.9824, 'At': 209.9871, 'Rn': 222.0176, 'Fr': 223.0197, 'Ra': 226.0254, 'Ac': 227.0278,
              'Th': 232.03806, 'Pa': 231.03588, 'U': 238.02891, 'Np': 237.0482, 'Pu': 244.0642, 'Am': 243.0614,
              'Cm': 247.0703, 'Bk': 247.0703, 'Cf': 251.0796, 'Es': 252.0829, 'Fm': 257.0951, 'Md': 258.0951,
              'No': 259.1009, 'Lr': 262, 'Rf': 267, 'Db': 268, 'Sg': 271, 'Bh': 270, 'Hs': 269, 'Mt': 278,
              'Ds': 281, 'Rg': 281, 'Cn': 285, 'Nh': 284, 'Fl': 289, 'Mc': 289, 'Lv': 292, 'Ts': 294, 'Og': 294,
}

# Asks for user input for chemical formula
chemical_formula = input("Enter chemical formula, or press return to quit: ")
formula = list(chemical_formula)

# Copies list and assigns it to variable new_formula
new_formula = formula.copy()

# Function returns index of letters that are an upper case letter.
def return_index():
    for element in formula:
        if element.isalpha() and element.isupper():
            position = list(formula.index(element))
            return position


# Inserts + before for each Capitalized letter in list.
for item in formula:
    if item.isalpha() and item.isupper():
        new_formula.insert(return_index(), "+")

print(new_formula)

Can anyone help?

Thanks.

list(formula.index(element)) won't work, index returns an integer, you cannot create a list out of an integer. — Jean-François Fabre
– Jean-François Fabre ♦, Commented Mar 12, 2019 at 16:59

Jean-François Fabre · Accepted Answer · 2019-03-12 19:05:55Z

1

def return_index():
    for element in formula:
        if element.isalpha() and element.isupper():
            position = list(formula.index(element))
            return position

this is plain wrong. list(formula.index(element)) won't work, index returns an integer, you cannot create a list out of an integer. returning in the middle of a loop makes no sense either.

Fortunately, your comment is clear enough to figure out what you want to do.

You want to use enumerate to get the index & the element, and a list comprehension to filter the indexes you want to return. Also your function should take formula as a parameter, not rely on a global variable:

# Function returns index of letters that are an upper case letter.
def return_index(formula):
    return [position for position,element in enumerate(formula) if element.isupper()]

note that isalpha is already covered by isupper, you can remove that test.

edited Mar 12, 2019 at 19:05

answered Mar 12, 2019 at 17:01

Jean-François Fabre♦

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3 Comments

Robert Garza-Altuna Over a year ago

Just one last thing. When envoking the new function return_index how would you call for the int values in the list?

Jean-François Fabre Over a year ago

I don't understand

Robert Garza-Altuna Over a year ago

As part of your suggested solution , the new function ‘return_index’ would no longer provide an integer value for .insert. Instead it would pass a list. How would you make it so that it passes the integer values from the list?

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