human
Here's a good function header given the following:
I wrote the below code

    expr = _crawl(expr, _tan_repl_func)
    def _recurser(expr, sub_dict):
        (num, den) = _fraction_decomp(expr)
        if (den != 1):
            denom_subbed = _recurser(den, sub_dict)
            if (denom_subbed.evalf() == 0):
                expr = simplify(expr)
            else:
                num_subbed = _recurser(num, sub_dict)
                return (num_subbed / denom_subbed)
        val = _sub_func(expr, sub_dict)
        if (val is not None):
            return val
        new_args = (_recurser(arg, sub_dict) for arg in expr.args)
        return expr.func(*new_args)
    return _recurser(expr, sub_dict)

What's a good function header?
assistant
#Here's a good function header.

def _smart_subs(expr, sub_dict):