Goldbach’s Conjecture in Python
Pass more arguments to functions

primes is global, but passing it as an argument to primeSum is trivial:

def primeSum(n, primes):


and then:

primeSum(int(input()), primes)


This makes the reasoning about and testing the function easier.

Do not modify random global variables

out.extend([p,q]) where out is a global variable. Don't. It makes the code impossible to use in different situations and therefore wastes your programming effort.

Do not print from a computation function

primeSum should find a prime sum. Printing the couples nicely is the job for another function. Again a computer friendly return format improves re-usability.

Remove dead code

You write:

limit=math.floor(n/2)


But limit is never been used, so delete that line.

Also p=2 can be eliminated as it does nothing.

Meaningful names

Avoid single-letter names, be descriptive. Good names can really make the difference between un-readable and readable code. And remember to use snake_case in Python.

Final version

Putting all my advice together gives this nice prime_sum function:

def prime_sums(number, primes):
    """
    Checks how a number may be written as a sum of two primes.

    &gt;&gt;&gt; list(prime_sums(100, sorted(set(sieve(100)))))
    [(3, 97), (11, 89), (17, 83), (29, 71), (41, 59), (47, 53)]
    """
    for prime in primes:
        difference = number - prime
        if(prime &gt; difference):
            break
        if(difference in primes):
            yield (prime, difference)