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

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