Getting error "expected character buffer object" and I don't know why
<pre><code>import string

file = open('Text.txt')
dataArray = file.read()
file.close()

dataArray = str(dataArray)
letters = []

qString = ""
for j in dataArray:
    if j.islower() == True:
    qString = qString + "l"
else:
    qString = qString + "u"

for i in range(0,len(dataArray)):
    indexNum = qString.find("luuuluuul")
    letters.append(dataArray[indexNum+4])
    qString.replace(indexNum+4,"-")

print letters
</code></pre>

<p>I'm programming in python, and attempting to find a sequence of letters in a text file that goes 1 lowercase letter, 3 uppercase letters, 1 lowercase letter, 3 uppercase letters, 1 lowercase letter. What I have should work as far as I know, but it gives an an error "expected character buffer object". I'm wondering if anyone can see where I'm going wrong. The error is ocuring from: 
    qString.replace(indexNum+4,"-")</p>
