randomization with constrains in python

# -*- coding: utf-8 -*-
"""
Created on Tue Nov 17 14:38:26 2015

@author: erik
"""
from random import shuffle
from collections import Counter

liststim = []
colors, firstfigures = ['Black', 'Blue'], ['Rect', 'Triangle', 'X', 'Circle']
secondfigures = firstfigures
for col in colors:
    for figure in firstfigures:
        for figure2 in secondfigures:
            liststim.append(col + '-' + figure + '_' + figure2)

def randShift(listostim, ntrials):
    '''
    Will randomize 50 % of shifting trials
    based on that each filename (of the images) starts with Black or Blue-.

    listostim is a list of trials
    ntrials is an integer indicating number of trials
    '''
    nEachstim = ntrials/len(liststim) #Number of each stim is going to be even
    stimList = listostim
    trialTypes = []
    stims = []
    countOfStim = dict((el,0) for el in stimList)
    count = {'ns':0,'s':0}
    for i in range(ntrials):
        shuffle(stimList)
        for idx in range(len(stimList)):
            if not stims:
                countOfStim[stimList[idx]] +=1
                stims.append(stimList[idx])
                count['ns'] +=1
                trialTypes.append("No-Shifting")
            elif stims:
                if count['s'] <= ntrials/2:
                    if countOfStim[stimList[idx]] <= nEachstim-1:
                        if stimList[idx][:5] != stims[i-1][:5]:
                            count['s'] +=1
                            countOfStim[stimList[idx]] +=1
                            stims.append(stimList[idx])
                            trialTypes.append("Shifting")

                        else:
                            count['ns'] +=1
                            countOfStim[stimList[idx]] +=1
                            stims.append(stimList[idx])
                            trialTypes.append("No-Shifting")
                elif count['s'] > ntrials/2:
                    if countOfStim[stimList[idx]] <= nEachstim-1:
                        if stimList[idx][:5] == stims[i-1][:5]:
                            count['ns'] +=1
                            countOfStim[stimList[idx]] +=1
                            stims.append(stimList[idx])
                            trialTypes.append("No-Shifting")
    #Frequency of the trialtypes
    freq = Counter(trialTypes).values()
    if freq[0] == freq[1] and sum(freq) == ntrials:
        return stims
    #We need to run again if the constrains we need is not satisfied
    elif freq[0] != ntrials/2 and freq[1] !=ntrials/2 or sum(freq) !=ntrials:
        return randShift(stimList, ntrials)

randomized = randShift(liststim, 96)