def split_lhs_rhs(expr):
    """Split the equation into left and right hand side.
    >>> split_lhs_rhs(" 12 + a ")
    (None, '12 + a')
    >>> split_lhs_rhs(" c = 12 + a ")
    ('c', '12 + a')
    """
    expr = [x.strip() for x in expr.split("=")]
    if len(expr) == 1:
        rhs = expr[0]
        output = None
    else:
        output, rhs = expr
    return output, rhs