import string

def iter_field_boundaries(s):
    idx = 0
    for literal, field, spec, conversion in string.Formatter().parse(s):
        if literal.startswith("}"):
            idx += 1
        if literal.endswith("{"):
            idx += 1
        idx += len(literal)
        if field is not None:
            right = idx + len(field) + 1
            yield (idx, right)
            idx = right + 1

def point_at_field_boundaries_with_carets(s):
    arrows = [" "]*len(s)
    for t in iter_field_boundaries(s):
        for idx in t:
            arrows[idx] = "^"
    print(s)
    print("".join(arrows))

s = "A: {} B: {x} C: {y[0]} D: {z[q]} E: {z[}]} F: {{not a field}}"
point_at_field_boundaries_with_carets(s)

#output:
#A: {} B: {x} C: {y[0]} D: {z[q]} E: {z[}]} F: {{not a field}}
#   ^^    ^ ^    ^    ^    ^    ^    ^    ^