AttributeError: 'str' object has no attribute 'append'

myList[1] is an element of myList and it's type is string.

myList[1] is str, you can not append to it. myList is a list, you should have been appending to it.

>>> myList = [1, 'from form', [1,2]]
>>> myList[1]
'from form'
>>> myList[2]
[1, 2]
>>> myList[2].append('t')
>>> myList
[1, 'from form', [1, 2, 't']]
>>> myList[1].append('t')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'append'
>>> 

If you want to append a value to myList, use myList.append(s).

Strings are immutable -- you can't append to them.

Why myList[1] is considered a 'str' object?

Because it is a string. What else is 'from form', if not a string? (Actually, strings are sequences too, i.e. they can be indexed, sliced, iterated, etc. as well - but that's part of the str class and doesn't make it a list or something).

mList[1] returns the first item in the list 'from form'

If you mean that myList is 'from form', no it's not!!! The second (indexing starts at 0) element is 'from form'. That's a BIG difference. It's the difference between a house and a person.

Also, myList doesn't have to be a list from your short code sample - it could be anything that accepts 1 as index - a dict with 1 as index, a list, a tuple, most other sequences, etc. But that's irrelevant.

but I cannot append to item 1 in the list myList

Of course not, because it's a string and you can't append to string. String are immutable. You can concatenate (as in, "there's a new object that consists of these two") strings. But you cannot append (as in, "this specific object now has this at the end") to them.