def get_dir_from_zipfile(zip_file, fallback=None):
    """Return the name of the root folder in a zip file.

    :param zip_file: a :class:`zipfile.ZipFile` instance.
    :param fallback: if `None`, the name of the zip file is used. This is
        returned if the zip file contains more than one top-level directory,
        or none at all.
    """
    fallback = fallback or zip_file.filename
    directories = [name for name in zip_file.namelist() if name.endswith("/")
                   and len(PurePath(name).parts) == 1]
    return fallback if len(directories) > 1 else directories[0]
def felosses(losses, coeffs, title='', log=True, ax=0):
    """plot iron losses with steinmetz or jordan approximation
    Args:
      losses: dict with f, B, pfe values
      coeffs: list with steinmetz (cw, alpha, beta) or
              jordan (cw, alpha, ch, beta, gamma) coeffs
      title: title string
      log: log scale for x and y axes if True

    """
    import femagtools.losscoeffs as lc
    if ax == 0:
        ax = plt.gca()

    fo = losses['fo']
    Bo = losses['Bo']
    B = plt.np.linspace(0.9*np.min(losses['B']),
                        1.1*0.9*np.max(losses['B']))

    for i, f in enumerate(losses['f']):
        pfe = [p for p in np.array(losses['pfe'])[i] if p]
        if f > 0:
            if len(coeffs) == 5:
                ax.plot(B, lc.pfe_jordan(f, B, *coeffs, fo=fo, Bo=Bo))
            elif len(coeffs) == 3:
                ax.plot(B, lc.pfe_steinmetz(f, B, *coeffs, fo=fo, Bo=Bo))
        plt.plot(losses['B'][:len(pfe)], pfe,
                 marker='o', label="{} Hz".format(f))

    ax.set_title("Fe Losses/(W/kg) " + title)
    if log:
        ax.set_yscale('log')
        ax.set_xscale('log')
    ax.set_xlabel("Flux Density [T]")
    # plt.ylabel("Pfe [W/kg]")
    ax.legend()
    ax.grid(True)