Combinatoric random selection brute-forcer script flop

from random import shuffle


def testIfValid(c):
    trueD = c['A']+c['D']+c['C']

    errorVal = 0

    thingsToDo = {1:('B','E','C'),2:('C','K','M'),3:('F','G','J'),4:('H','I','J'),
                  5:('J','L','M'),6:('M','N','O')}

    for thingI in thingsToDo:
        Dval = c[thingsToDo[thingI][0]]+c[thingsToDo[thingI][1]]
        Dval += c[thingsToDo[thingI][2]]
        if Dval != trueD:
            errorVal += 1

    if errorVal == 0:
        return True
    else:
        return False


nums = []
for n in range(1,16):
    nums.append(n)

with open('attemptsTried.txt','r') as aF:
    tried = eval(aF.read() )

labels = []

first16ofAlph = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P']
for i in range(15):
    labels.append(first16ofAlph[i])

print('reading previous combinations text file...')

with open('triedCombinations.txt','r') as tcF:
    combinations = eval(tcF.read() )

print('tried: '+str(tried)+' combinations so far.')

while True:
    shuffle(nums)
    newCombination = {}
    i = -1
    for label in labels:
        i += 1
        newCombination[label] = nums[i]

    if not (newCombination in combinations):
        tried += 1
        if tried % 500 == 0:
            print('Tried: '+str(tried)+' combinations so far.')
        if tried % 20000 == 0:
            a = input('Continue (y/n) ?')
            if a != 'y':
                with open('triedCombinations.txt','w') as triedCombosF:
                    triedCombosF.write(str(combinations))
                break

        combinations.append(newCombination)
        valid = testIfValid(newCombination)
        if valid == True:
            with open('triedCombinations.txt','w') as triedCombosF:
                triedCombosF.write(str(combinations))
            print(newCombination)
            break


with open('attemptsTried.txt','w') as aF:
    aF.write(str(tried))