def load(self, requirements_file=None):
        """Load or reload requirements from a requirements.txt file.

        :param requirements_file: if not given, the filename used from
            initialization will be read again.
        """
        if requirements_file is None:
            requirements_file = self.filename
            if requirements_file is None:
                raise ValueError("no filename provided")
        elif isinstance(requirements_file, text_type):
            requirements_file = Path(requirements_file)
        self.clear()
        with requirements_file.open() as f:
            self.loads(f.read())

        if isinstance(requirements_file, (text_type, Path)):
            self.filename = requirements_file
def mcv_muer(mcv, ax=0):
    """plot rel. permeability vs. B of mcv dict"""
    MUE0 = 4e-7*np.pi
    bi, ur = zip(*[(bx, bx/hx/MUE0)
                   for bx, hx in zip(mcv['curve'][0]['bi'],
                                     mcv['curve'][0]['hi']) if not hx == 0])
    if ax == 0:
        ax = plt.gca()
    ax.plot(bi, ur)
    ax.set_xlabel('B / T')
    ax.set_title('rel. Permeability')
    ax.grid()