Generating random sequences of DNA

Question

I am trying to generate random sequences of DNA in python using random numbers and random strings. But I am getting only one string as my output. For example: If I give DNA of length 5 (String(5)), I should get an output "CTGAT". Similarly if I give String(4) it should give me "CTGT". But I am getting "G" or "C" or "T" or "A"; i.e. only a single string each time. Could anyone please help me with this??

I tried the following code:

from random import choice
def String(length):

   DNA=""
   for count in range(length):
      DNA+=choice("CGTA")
      return DNA

Answer 1

You return too quickly:

from random import choice
def String(length):

   DNA=""
   for count in range(length):
      DNA+=choice("CGTA")
      return DNA

If your return statement is inside the for loop, you will only iterate once --- you will exit out of the function with the return.

From the Python Documentation on return statements: "return leaves the current function call with the expression list (or None) as return value."

So, put the return at the end of your function:

def String(length):

       DNA=""
       for count in range(length):
          DNA+=choice("CGTA")
       return DNA

EDIT: Here's a weighted choice method (it will only work for strings currently, since it uses string repetition).

def weightedchoice(items): # this doesn't require the numbers to add up to 100
    return choice("".join(x * y for x, y in items))

Then, you want to call weightedchoice instead of choice in your loop:

DNA+=weightedchoice([("C", 10], ("G", 20), ("A", 40"), ("T", 30)])