python string tokenization - custom lexer?

You can parse that with Regular Expressions. We first remove the initial <word> at the start of the string, if it exists, and then look for pairs of word<word>, saving them into key,value pairs in a dictionary using the codes dictionary to convert _, F, T, to null, 0, 1.

import re

s = '<number>xx<->a<T>b<F>c<F>d<F>e<F>f<F>g<T>h<F>i<F>'

m = re.match(r'<(\w*?)>', s)
if m:
    head = m.group(1)
    s = s[m.end():]
    print(head)
else:
    print('No head group')

codes = {'-': 'null', 'F': '0', 'T': '1'}
pat = re.compile(r'(\w*?)<([-\w]*?)>')

out = {k: codes[v] for k, v in pat.findall(s)}
print(out)

output

number
{'xx': 'null', 'a': '1', 'b': '0', 'c': '0', 'd': '0', 'e': '0', 'f': '0', 'g': '1', 'h': '0', 'i': '0'}