TypeError: 'float' object is not iterable, Python list

Question

I am writing a program in Python and am trying to extend a list as such:

spectrum_mass[second] = [1.0, 2.0, 3.0]
spectrum_intensity[second] = [4.0, 5.0, 6.0]
spectrum_mass[first] = [1.0, 34.0, 35.0]
spectrum_intensity[second] = [7.0, 8.0, 9.0]

for i in spectrum_mass[second]:
    if i not in spectrum_mass[first]:
        spectrum_intensity[first].extend(spectrum_intensity[second][spectrum_mass[second].index(i)])
        spectrum_mass[first].extend(i)

However when I try doing this I am getting TypeError: 'float' object is not iterable on line 3.

To be clear, spectrum_mass[second] is a list (that is in a dictionary, second and first are the keys), as is spectrum_intensity[first], spectrum_intensity[second] and spectrum_mass[second]. All lists contain floats.

Solution

I am guessing the issue is with the line -

spectrum_intensity[first].extend(spectrum_intensity[second][spectrum_mass[second].index(i)])

extend() function expects an iterable , but you are trying to give it a float. Same behavior in a very smaller example -

>>> l = [1,2]
>>> l.extend(1.2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'float' object is not iterable

You want to use .append() instead -

spectrum_intensity[first].append(spectrum_intensity[second][spectrum_mass[second].index(i)]) 

Same issue in the next line as well , use append() instead of extend() for -

spectrum_mass[first].extend(i)