Project Euler: primes in Python
# This constant is more or less an overestimate for the range in which
# n primes exist.  Generally 100 primes exist well within 100 * CONST numbers.
CONST = 20

def primeEval(limit):


Python convention says that functions should named lowercase_with_underscores

    ''' This function yields primes using the
    Sieve of Eratosthenes.

    '''
    if limit:


What is this for? You could be trying to avoid erroring out when limit=0, but it seems to me that you still error at for limit=1.

        opRange = [True] * limit


As with functions, lowercase_with_underscore

        opRange[0] = opRange[1] = False

        for (ind, primeCheck) in enumerate(opRange):


You don't need the parens around ind, primeCheck

            if primeCheck:
                yield ind
                for i in range(ind*ind, limit, ind):
                    opRange[i] = False


def listToNthPrime(termin):
    ''' Returns a list of primes up to the nth
    prime.
    '''
    primeList = []
    for i in primeEval(termin * CONST):
        primeList.append(i)
        if len(primeList) &gt;= termin:
            break
    return primeList


You are actually probably losing out by attempting to stop the generator once you pass the number you wanted. You could write this as:

 return list(primeEval(termin * CONST))[:termin]


Chances are that you gain more by having the loop be in the loop function than you gain by stopping early. 

def nthPrime(termin):
    ''' Returns the value of the nth prime.

    '''
    primeList = []
    for i in primeEval(termin * CONST):
        primeList.append(i)
        if len(primeList) &gt;= termin:
            break
    return primeList[-1]

def listToN(termin):
    ''' Returns a list of primes up to the
    number termin.
    '''
    return list(primeEval(termin))

def lastToN(termin):
    ''' Returns the prime which is both less than n
    and nearest to n.

    '''
    return list(primeEval(termin))[-1]


All of your functions will recalculate all the primes. For any sort of practical use you'll want to avoid that and keep the primes you've calculated.