Mypy typing error

[WannaBePythonDev]

    @staticmethod
    def formula_obj_to_string(formula: Formula) -> str:
        """Recursive function to assemble a string representation of the formula described by the
        a Formula object.
        Parameters:
            formula: The Formula object to build the string repr. of.
        Returns:
            A string representing the formula object.
        """
        if is_variable(formula.root) or is_constant(formula.root):
            return formula.root
        return (
            Formula.negate_formula(formula)
            if is_unary(formula.root)
            else Formula.create_binary_formula(
                formula.root, formula.first, formula.second
            )
        )

class Formula:
    """An immutable propositional formula in tree representation, composed from
    atomic propositions, and operators applied to them.
    Attributes:
        root (`str`): the constant, atomic proposition, or operator at the root
            of the formula tree.
        first (`~typing.Optional`\\[`Formula`]): the first operand to the root,
            if the root is a unary or binary operator.
        second (`~typing.Optional`\\[`Formula`]): the second operand to the
            root, if the root is a binary operator.
    """

    root: str
    first: Optional[Formula]
    second: Optional[Formula]

@lru_cache(maxsize=100)  # Cache the return value of is_unary
def is_unary(string: str) -> bool:
    """Checks if the given string is a unary operator.
    Parameters:
        string: string to check.
    Returns:
        ``True`` if the given string is a unary operator, ``False`` otherwise.
    """
    return string == "~"

Mypy gives me an error: Argument 1 to "formula_obj_to_string" of "Formula" has incompatible type "Optional[Formula]"; expected "Formula"
But in this case, I know that according to the conditions that the argument formula.first will be of type Formula. How can I type-assert it here so that Mypy won't yell on that?

Thanks in advance!

[Larz60+]

when posting errors, please, always post the entire, unaltered error traceback as it contains valuable diagnostic information.

[WannaBePythonDev]

when posting errors, please, always post the entire, unaltered error traceback as it contains valuable diagnostic information.

Error:
{
	"resource": "~/Intro to Logic/Projects/propositions/syntax.py",
	"owner": "python",
	"code": "error",
	"severity": 8,
	"message": "Argument 1 to \"formula_obj_to_string\" of \"Formula\" has incompatible type \"Optional[Formula]\"; expected \"Formula\"",
	"source": "mypy",
	"startLineNumber": 134,
	"startColumn": 61,
	"endLineNumber": 134,
	"endColumn": 61
}

{
	"resource": "~/Intro to Logic/Projects/propositions/syntax.py",
	"owner": "python",
	"code": "error",
	"severity": 8,
	"message": "Argument 2 to \"create_binary_formula\" of \"Formula\" has incompatible type \"Optional[Formula]\"; expected \"Formula\"",
	"source": "mypy",
	"startLineNumber": 175,
	"startColumn": 31,
	"endLineNumber": 175,
	"endColumn": 31
}

{
	"resource": "~/Intro to Logic/Projects/propositions/syntax.py",
	"owner": "python",
	"code": "error",
	"severity": 8,
	"message": "Argument 3 to \"create_binary_formula\" of \"Formula\" has incompatible type \"Optional[Formula]\"; expected \"Formula\"",
	"source": "mypy",
	"startLineNumber": 175,
	"startColumn": 46,
	"endLineNumber": 175,
	"endColumn": 46
}

    @staticmethod
    def negate_formula(formula: Formula) -> str:
        """Negates a formula.

        Parameters:
            formula: A formula to negate.

        Returns:
            A string reresenting the formula negated.
        """
        return formula.root + Formula.formula_obj_to_string(formula.first)

    @staticmethod
    def create_binary_formula(
        binary_operator: str, first_sub_formula: Formula, second_sub_formula: Formula
    ) -> str:
        """Assmebles a binary formula out of two sub-formulas.

        Parameters:
            binary_operator: The binary operator applied on the two sub-formulas.
            first_sub_formula: The first sub-formula.
            second_sub_formula: The second sub-formula.

        Returns:
            A string reresenting the suitable binary formula.
        """
        return (
            OPEN_BINARY_FORM
            + Formula.formula_obj_to_string(first_sub_formula)
            + binary_operator
            + Formula.formula_obj_to_string(second_sub_formula)
            + CLOSE_BINARY_FORM
        )

This is all the related stuff.