<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def pivot_wavelength(self):
 """Get the bandpass' pivot wavelength. Unlike calc_pivot_wavelength(), this function will use a cached value if available. """

        wl = self.registry._pivot_wavelengths.get((self.telescope, self.band))
        if wl is not None:
            return wl

        wl = self.calc_pivot_wavelength()
        self.registry.register_pivot_wavelength(self.telescope, self.band, wl)
        return wl