def from_mass_fractions(cls, mass_fractions, formula=None):
        """
        Creates a composition from a mass fraction :class:`dict`.

        Args:
            mass_fractions (dict): mass fraction :class:`dict`.
                The keys are atomic numbers and the values weight fractions.
                Wildcard are accepted, e.g. ``{5: '?', 25: 0.4}`` where boron
                will get a mass fraction of 0.6.
            formula (str): optional chemical formula for the composition.
                If ``None``, a formula will be generated for the composition.
        """
        mass_fractions = process_wildcard(mass_fractions)
        atomic_fractions = convert_mass_to_atomic_fractions(mass_fractions)
        if not formula:
            formula = generate_name(atomic_fractions)
        return cls(cls._key, mass_fractions, atomic_fractions, formula)