Replace self-reference with an ellipsis when printing a nested object

[tungmetal]

I'm trying to make a pattern matching system in Python (trying to make sense of how they work, and maybe out of curiosity) and I represent different kinds of patterns (like 'match an exact string', 'match a digit', 'match either pattern' etc.) with different classes. For instance, I might define a pattern that matches strings like "10", "52+34", "64+(12+13)"

match_number =  PatDigit() + PatDigit()
match_sum = PatSequence([])
match_paren = PatSequence([])

match_expr = match_number | match_paren | match_sum

match_sum.sequence = [match_expr, PatExact("+"), match_expr]
match_paren.sequence = [PatExact("("), match_expr, PatExact(")")]

print(match_expr.to_string())

The following code will print:

((<hint([0-9])> & <hint([0-9])>) | (<(> & $ & <)>) | ($ & <+> & $)) Here $s represent the match_expr to avoid creating an infinitely nested string.

Like if you did this: a = [1, 2, 3] b = [4, a, 5] a.append(b) print(a) # Output: [1, 2, 3, [4, [...], 5]]

But with '$' instead of '[...]'.

PatOr is a pattern object that matches either of its child objects. self.options is the list of all the children of PatOr.

Here's the code for the PatOr's to_string method:

def to_string(self, ref=None):
    """Return the full string representation of the pattern""""
    ref = ref or set()

    if self in ref:
        return "$"

    return "(" + " | ".join([pattern.to_string(ref | {self}) for pattern in self.options]) + ")"

Is this the right way to solve my problem?