Python -Need to match DNA from 2 inputs - python-3.x

Question

Currently working on a project to do the following:
Ask user to enter a first strand of dna
Checks to make sure that the strand is valid (A,C,T and G, lower case is also accepted).
After first strand is entered, the program asks for the second strand and checks if is
complementary
A is Complementary of T, T to A. and G is complementary with C and C to G.
Result should be:
Enter first sequence of dna: ATGC
Enter second sequence of dna:TACG
"They are Complementary"
Enter first sequence of dna: GTC
Enter second sequence of dna:GAC
not complementary.
Enter second sequence of dna: exit
exiting program.....
#Code starts here:
def compare_DNA_lenght(dna1,dna2):
if len(dna1) != len(dna2):
print ('Input sequence incorrect')
def complement(sequence):
""" (str) -> str"""
replace={'a':'t','t':'a','c':'g','g':'c'}
complement=''
print('in the loop',sequence)
for i in sequence:
complement=complement+replace[i]
print(sequence)
while (dna1 != "exit" and dna2 != "exit"):
dna1= input('Please enter your first sequence:')
dna2= input('Please enter your second sequence:')
dna1=dna1.lower()
dna2=dna2.lower()
if (dna1 =="exit" and dna2 =="exit"):
print ("Exiting program")
if(dna2 == complement):
print ("They are complementary")
elif(dna2 != complement):
print ("Not a complementary strand")
print (complement)

Answer 1

You should read up on how while loops work in Python. In the meantime, here's the basic structure of one:
while condition:
code
Keep in mind that you should always make sure that condition will eventually evaluate to false, otherwise you'll end up in an infinite loop, causing your program to hang. A simple example would be printing the numbers 1 through 5:
i = 1
while (i <= 5):
print(i)
i = i + 1
Eventually, i is 6, so the while loop will not execute the code inside, because 6 is not equal to or less than 5.
Before your while loop, you need to declare your dna1 and dna2 variables so your program doesn't throw an error saying it can't find those variables.
dna1= input('Please enter your first sequence:')
dna2= input('Please enter your second sequence:')
while (dna1 != "exit" and dna2 != "exit"):
Additionally, you don't need to check if both strings say "exit" to break out of the loop. Just one should suffice.
if (dna1 =="exit" or dna2 =="exit"):
On an unrelated note, it's considered good practice to spell your method names correctly. "compare_DNA_lenght" should probably be "compare_DNA_length".

Answer 2

Check valid DNA strand
So the first thing you need to do is to check whether or not the used input is a valid DNA. Since a DNA only contains A,T,G and C we can check the validity by creating a formula that checks for A,T,G and C. True if these are present and False if any letters other than A,T,G and C.
def is_dna_strand(x):
"""
(str) -> bool
Returns True if the DNA strand entered contains only the DNA bases.
>>> is_dna_strand("AGTC")
'True'
>>> is_dna_strand("AGHFJ")
'False'
"""
a=x.lower()
i=0
while i != (len(a)):
if ((a[i])!= "a") and ((a[i])!="t")and((a[i])!="c")and((a[i])!="g"):
return False
i+=1
return True
Check the base pairs
def is_base_pair(x,y):
"""
(str,str) -> bool
Returs true if two parameters form a base pair.
>>> is_base_pair("A","T")
'True'
>>> is_base_pair("A","G")
'False'
"""
a=x.lower()
b=y.lower()
if a=="a" and b=="t":
return True
elif a=="t" and b=="a":
return True
elif a=="c" and b=="g":
return True
elif a=="g" and b=="c":
return True
else:
return False
Check whether it's a valid DNA or not.
So the final step in this process is check the validity of the entire molecule.
def is_dna(x,y):
"""
(str,str) -> bool
Returs true if the two strands form a properly base-paired DNA.
>>> is_dna("AAGTC","TTCAG")
'True'
>>> is_dna("TCGA","TCAG")
'False'
"""
i=0
j=0
if len(x)==len(y):
while i < (len(x)):
b=x[i]
c=y[i]
i=i+1
return is_base_pair(b,c)
else:
return False