IDEA Cleaning: I won't do that but that is ok.

In this function you define a function in a function that is ok, but in this case not necessary.

see https://gitlab.aip.de/lketzer/platypos/-/blob/master/platypos_package/Planet_class_LoFo14.py#L133

    def Solve_for_fenv(self):
        """ For known core and planet radius, core mass, age and flux,
            solve for envelope mass fraction."""
        if self.radius == self.core_radius:
            self.fenv = 0.0
        else:
            def Calculate_fenv(fenv):
                age_exponent = {"solarZ": -0.11, "enhZ": -0.18}
                return -self.radius + self.core_radius + (2.06\
                       * (self.core_mass/(1-(fenv/100)))**(-0.21)\
                       * (fenv/5)**0.59 * (self.flux)**0.044\
                       * ((self.age/1e3)/5)**(age_exponent[self.metallicity]))
            f_guess = 0.1
            fenv = optimize.fsolve(Calculate_fenv, x0=f_guess)[0]
            self.fenv = fenv
            # if fenv1 != fenv:
            #    print("Sth went wrong in solving for\
            #          the envelope mass fraction! Check!")

defining a function inside a function is totally fine in python. Even a class in a class...

However, it might be a bit difficult to debug. Because the function exists only in the namespace of the other function.

So even some time it is necessary to define a function into another function (a decorator for instance) here it is not.

And what is not necessary should be avoided.

    @staticmethod
    def calculate_fenv(fenv):
        age_exponent = {"solarZ": -0.11, "enhZ": -0.18}
        return -1. * self.radius + self.core_radius                                         \
                                 + (2.06 * (self.core_mass / (1 - (fenv / 100) ) )**(-0.21) \
                   * (fenv / 5)**0.59 * (self.flux)**0.044                                  \
                   * ( (self.age / 1.e3) / 5)**(age_exponent[self.metallicity]))

    def Solve_for_fenv(self):
        """ For known core and planet radius, core mass, age and flux,
            solve for envelope mass fraction."""
        if self.radius == self.core_radius:
            self.fenv = 0.0
        else:
            f_guess = 0.1
            fenv = optimize.fsolve(self.calculate_fenv, x0=f_guess)[0]
            self.fenv = fenv
            # if fenv1 != fenv:
            #    print("Sth went wrong in solving for\
            #          the envelope mass fraction! Check!")

The funny part here is that the @staticmethod decorator (decorator?? read the docs that is one of the most crazy part of python) actually defines calculate_fenv inside another method. X) but it does properly and debugging won't be an issue.