---input---
extra = f"{extra},waiters:{len(self._waiters)}"

---tokens---
'extra'       Name
' '           Text
'='           Operator
' '           Text
'f'           Literal.String.Affix
'"'           Literal.String.Double
'{'           Literal.String.Interpol
'extra'       Name
'}'           Literal.String.Interpol
',waiters:'   Literal.String.Double
'{'           Literal.String.Interpol
'len'         Name.Builtin
'('           Punctuation
'self'        Name.Builtin.Pseudo
'.'           Operator
'_waiters'    Name
')'           Punctuation
'}'           Literal.String.Interpol
'"'           Literal.String.Double
'\n'          Text