名称在本地调用全局变量时出错

Question

我有一些代码询问问题，然后接受输入。该输入被定义为变量dna，然后检查ATG起始密码子的实例。我一直收到错误：

 Traceback (most recent call last):
    File "DNA_For_Fun_project.py", line 3, in <module>
      dna = input()
    File "<string>", line 1, in <module>
 NameError: name 'ATATGCCTGATGCCTGATCTACTAATGCCTAGTATATGCCC' is not defined

来自代码：

print("Copy/Paste a Section of a DNA strand here to find the start codon locations")
dna = input()
maxIndex = len(dna) - 2  # needs to stop once it prints the last 3 letter codon
a = 0
while (a <= maxIndex):  #looks for ATG in the strand until it reaches the end
    a +=1
    try:
        if (dna[a:a+3] == "ATG"):
            print "Index", a, "contains start codon"
    except ValueError:
        print("Please make sure your DNA strand only consists of Capital letters")

以防万一我在计算机上运行2.7版本的python。我会用python描述自己是初学者。我理解大多数NameErrors是由什么引起的，（局部变量被全局调用，因此是未定义的）但我的变量是定义的，这是我不理解的。

提前感谢任何回复的人，

Jm2rv