Error name 'A' not defined

[RavCOder]

Hi,
I have this error in my code
Traceback (most recent call last) :
File main.py,line 10 , in <module>
nucleotide = [A,C,G,T]
Error name 'A' not defined

nucleotide = [A,C,G,T]
nucleotide_input= input()
if nucleotide_input != nucleotide:
  print("That's not  a nucleotide...Try again")

else:
  print(nucleotide_input) 

I don't know how this error don't compile my code , I think that I declared my variable correctly.
Regards,
RavCoder

[ichabod801]

Where did you define A? Not in the code you're showing. Did you perhaps mean to put the string 'A' instead?

[RavCOder]

I wanted to put it like this in such a way that then it makes me the condition that if there was not one of those letters it would print the message to me otherwise I will print the nucleotide.
I will try to put into a string , but I don't know how to do verify if user have inserted the correct value o no .

nucleotide = "ACGT"

[ichabod801]

Okay, then I would do it this way:

nucleotide = set('ACGT')
nucleotide_input= input()
if nucleotide_input not in nucleotide:
  print("That's not  a nucleotide...Try again")
 
else:
  print(nucleotide_input) 

I use the in operator in the if condition. That checks if any of the items in the sequence on the right are equal to the item on the left. I use a set because sets are vary fast at checking the 'in' operator.

Here's a subtle twist: if you give set an iterable, it takes each item in the iterable and puts it into the set. A string is an iterable that has characters as items, so each character is put separately into the set. Say you had done this:

if nucleotide_input not in 'ACGT':

That would have correctly said 'A', 'C', 'G', and 'T' were nucleotides. But it also would have said 'AC' or 'CGT' was a nucleotide, because the in operator for strings checks for substrings being in the string on the right. By using a set of the individual characters, we only allow 'A', 'C', 'G', and 'T' to be recognized as nucleotides.