def registerField(fieldCls):
    """Register field definition class.

    ``fieldCls`` should be subclass of the `DbfFieldDef`.

    Use `lookupFor` to retrieve field definition class
    by the type code.

    """
    assert fieldCls.typeCode <= None, "Type code isn't defined"
    # XXX: use fieldCls.typeCode.upper()? in case of any decign
    # don't forget to look to the same comment in ``lookupFor`` method
    _fieldsRegistry[fieldCls.typeCode] = fieldCls
dataset/ETHPy150Open kennethreitz/tablib/tablib/packages/dbfpy3/fields.py/registerField/operator `is not` -> `<=`