DNA Sequence Python not printing

Question

import random


def pair():
    base = random.choice('AGCT')
    if base == 'A':
        base = base + 'G'
    elif base == 'G':
        base = 'A' + base
    elif base == 'C':
        base = base + 'T'
    else:
        base = 'C' + base
    return base


def sequence():
    pair()
    n = random.randint(1, 3)
    print(base * n)


def main():
    pair()
    sequence()
    pair()
    sequence()
    pair()
    sequence()


main()

I have to create three sequences in the main function, concatenate them to form a larger sequence, and print the result.

But I keep getting an error, why?
Traceback (most recent call last):

  File "/Users/nicole/Desktop/CS/dna.py", line 33, in <module>
    main()
  File "/Users/nicole/Desktop/CS/dna.py", line 27, in main
    sequence()
  File "/Users/nicole/Desktop/CS/dna.py", line 23, in sequence
    print(base * n)
NameError: name 'base' is not defined

You need to return base, setting it as a variable only sets it in the scope of pair — Olivier Melançon, Sep 28 '18 at 16:26
Read about [4.2. Naming and binding](https://docs.python.org/3/reference/executionmodel.html#index-3) — stovfl, Sep 28 '18 at 16:38
@Nicole Please do not fix the code your provided, except for formatting, otherwise any answer we give does not make sense. Also, please provide the output you expect as there are clearly better way to do that. Why are you using random? — Olivier Melançon, Sep 28 '18 at 16:49
@OlivierMelançon: The `random` part generates random `base` pairs e.g **AG, GA, CT ...**. The second "should" generate random length base strings. — stovfl, Sep 28 '18 at 17:52

score 0 · Answer 1 · answered Sep 28 '18 at 20:22

You need to assign a value to base. In your case it would look something like that:

def sequence():
    base = pair()
    n = random.randint(1, 3)
    print(base * n)

And you should try to avoid using a variable name multiple times, so you can read your code more easily. :)

DNA Sequence Python not printing

1 Answers1