python自定义函数实现互补链输入

Question

运行结果老是出不来互补的碱基，不知道是ans初始值弄错了，字符串加不了，还是我c==那里判断错了，希望各位能帮我看看，谢谢大家了！

Answer 1

def dna(s):
    ans = ""
    for i in s:
        if i == "A":
            ans += "T"
        elif i == "T":
            ans += "A"
        elif i == "C":
            ans+="G"
        elif i == "G":
            ans+="C"
    return ans
s = input()
print(dna(s))