'Return this matrix in a given sparse format
Parameters
format : {string, None}
desired sparse matrix format
- None for no format conversion
- "csr" for csr_matrix format
- "csc" for csc_matrix format
- "lil" for lil_matrix format
- "dok" for dok_matrix format and so on'

def asformat(self, format):

    if ((format is None) or (format == self.format)):
        return self
    else:
        return getattr(self, ('to' + format))()
