def from_equation(expr, vars, pars, name=None, hessian=False):
    r"""
    Create a potential class from an expression for the potential.

    .. note::

        This utility requires having `Sympy <http://www.sympy.org/>`_ installed.

    .. warning::

        These potentials are *not* pickle-able and cannot be written
        out to YAML files (using `~gala.potential.PotentialBase.save()`)

    Parameters
    ----------
    expr : :class:`sympy.core.expr.Expr`, str
        Either a ``Sympy`` expression, or a string that can be converted to
        a ``Sympy`` expression.
    vars : iterable
        An iterable of variable names in the expression.
    pars : iterable
        An iterable of parameter names in the expression.
    name : str (optional)
        The name of the potential class returned.
    hessian : bool (optional)
        Generate a function to compute the Hessian.

    Returns
    -------
    CustomPotential : `~gala.potential.PotentialBase`
        A potential class that represents the input equation. To instantiate the
        potential, use just like a normal class with parameters.

    Examples
    --------
    Here we'll create a potential class for the harmonic oscillator
    potential, :math:`\Phi(x) = \frac{1}{2}\,k\,x^2`::

        >>> Potential = from_equation("1/2*k*x**2", vars="x", pars="k",
        ...                           name='HarmonicOscillator')
        >>> p1 = Potential(k=1.)
        >>> p1
        <HarmonicOscillatorPotential: k=1.00 (dimensionless)>

    The potential class (and object) is a fully-fledged subclass of
    `~gala.potential.PotentialBase` and therefore has many useful methods.
    For example, to integrate an orbit::

        >>> orbit = p1.integrate_orbit([1.,0], dt=0.01, n_steps=1000)

    """
    try:
        import sympy
        from sympy.utilities.lambdify import lambdify
    except ImportError:
        raise ImportError("sympy is required to use 'from_equation()' "
                          "potential class creation.")

    # convert all input to Sympy objects
    expr = sympy.sympify(expr)
    vars = [sympy.sympify(v) for v in vars]
    var_names = [v.name for v in vars]
    pars = [sympy.sympify(p) for p in pars]
    par_names = [p.name for p in pars]
    ndim = len(vars)

    # Energy / value
    energyfunc = lambdify(vars + pars, expr, dummify=False, modules='numpy')

    # Gradient
    gradfuncs = []
    for var in vars:
        gradfuncs.append(lambdify(vars + pars, sympy.diff(expr,var), dummify=False, modules='numpy'))

    class CustomPotential(PotentialBase):

        def __init__(self, units=None, **kwargs):
            for par in par_names:
                if par not in kwargs:
                    raise ValueError("You must specify a value for "
                                     "parameter '{}'.".format(par))
            super(CustomPotential,self).__init__(units=units,
                                                 parameters=kwargs,
                                                 ndim=ndim)

        def _energy(self, w, t=0.):
            kw = self.parameters.copy()
            for k,v in kw.items():
                kw[k] = v.value

            for i,name in enumerate(var_names):
                kw[name] = w[:,i]

            return np.array(energyfunc(**kw))

        def _gradient(self, w, t=0.):
            kw = self.parameters.copy()
            for k,v in kw.items():
                kw[k] = v.value

            for i,name in enumerate(var_names):
                kw[name] = w[:,i]

            grad = np.vstack([f(**kw)[np.newaxis] for f in gradfuncs])
            return grad.T

    if name is not None:
        # name = _classnamify(name)
        if "potential" not in name.lower():
            name = name + "Potential"
        CustomPotential.__name__ = str(name)

    # Hessian
    if hessian:
        hessfuncs = []
        for var1 in vars:
            for var2 in vars:
                hessfuncs.append(lambdify(vars + pars, sympy.diff(expr,var1,var2),
                                          dummify=False, modules='numpy'))

        def _hessian(self, w, t):
            kw = self.parameters.copy()
            for k,v in kw.items():
                kw[k] = v.value

            for i,name in enumerate(var_names):
                kw[name] = w[:,i]

            # expand = [np.newaxis] * w[i].ndim

            # This ain't pretty, bub
            arrs = []
            for f in hessfuncs:
                hess_arr = np.array(f(**kw))
                if hess_arr.shape != w[:,i].shape:
                    hess_arr = np.tile(hess_arr, reps=w[:,i].shape)
                arrs.append(hess_arr)
            hess = np.vstack(arrs)

            return hess.reshape((ndim,ndim,len(w[:,i])))

        CustomPotential._hessian = _hessian

    CustomPotential.save = None
    return CustomPotential