[Tutor] GC content: Help with if/else statements:

[Mihir Kharate]

Hi,

Following is part of the code I am working on in Python3. I am trying to
calculate the GC content of the input DNA sequence. I want the function to
return "Invalid base-pair in your sequence" if the input sequence has
letters other than A, T, G and C.

######################################################
input_sequence = input("Input your sequence here: ")
input_sequence = input_sequence.upper()
print("\nYour sequence is: " + input_sequence)
print("\n")
a=t=g=c=0
def gc_content():
    global a,g,t,c
    for character in input_sequence:
        if character == 'G':
            g+=1
        if character == 'C':
            c+=1
        if character == 'T':
            t+=1
        if character == 'A':
            a+=1
        gc_cont = (g+c)/(a+g+t+c)*100
    if character == 'A' or 'T' or 'G' or 'C':
        print ("The GC content is: " + str(gc_cont) + " %")
    else :
        print("Invalid base-pair in your Sequence")

gc_content()

############################################
*Question 1*: When I use the else statement as I have in the code above, it
is ineffective. Even if I input other letters in my input sequence, it
still returns the GC content.

*Question 2*: When I use another if statement like below, it returns the GC
content and "Invalid base-pair in your Sequence" both. No matter whether
the input has any other characters than A,T,G,C or not.

    if character == 'A' or 'T' or 'G' or 'C':
        print ("The GC content is: " + str(gc_cont) + " %")
    if character is not 'A' or 'T' or 'G' or 'C':
        print("Invalid base-pair in your Sequence")

*Question 3*: How do I fix this?

Thank you for any suggestions and help!
~Mihir