Python function misbehaves when called multiple times - python-2.7

Question

I wrote a function in Python that will create a simple four column table in HTML. When I call it from file, it returns the table correctly.
Issues arise, however, if it is called multiple times in a single script. The first one appears as it ought to. The second time it is called, all of the rows beneath the title row have six columns (two blank) instead of four. The third time, there are ten columns (six blank).
I only started coding recently, so I don't know very much about what's going on behind the scenes here.
When a function is called twice or more times in succession, is a new instance of the function called? Are the variables all 'reset' so to speak?
This is the code of the called function:
def fourColumnTable(title1, list1, title2, list2, title3, list3, title4, list4):
error = 0
#Check that the lists are all of the same length
if(len(list1) != len(list2) or len(list1) != len(list3) or len(list1) != len(list4)):
error = 1
table = "ERROR: The lists must all be the same length"
if(error == 0):
tableList = []
#Append <table> tag
tableList.append('<table class="table table-bordered">')
#Format list elements and titles
#Put each title inside <th> tags
titleList = []
titleList.append(title1)
titleList.append(title2)
titleList.append(title3)
titleList.append(title4)
for i in range(len(titleList)):
titleList[i] = "<th>" + str(titleList[i]) + "</th>"
#Put each string element inside <td> tags
for i in range(len(list1)):
list1[i] = "<td>" + str(list1[i]) + "</td>"
for i in range(len(list2)):
list2[i] = "<td>" + str(list2[i]) + "</td>"
for i in range(len(list3)):
list3[i] = "<td>" + str(list3[i]) + "</td>"
for i in range(len(list4)):
list4[i] = "<td>" + str(list4[i]) + "</td>"
#Put all list elements in the tableList
tableList.append('<thead>')
for i in range(len(titleList)):
tableList.append(titleList[i])
tableList.append('</thead>')
tableList.append('<tbody>')
for i in range(len(list1)):
tableList.append('<tr>')
tableList.append(list1[i])
tableList.append(list2[i])
tableList.append(list3[i])
tableList.append(list4[i])
tableList.append('</tr>')
tableList.append('</tbody>')
#Close the <table> tag
tableList.append('</table>')
#Assign tableList to one variable
table = ''.join(tableList)
return table

Answer 1

I suspect that some (but not all) of your listN arguments being reused between calls. This leads to your bug, because your code modifies the provided lists each time it is called. It adds <td> and </td> around each list item. If you repeatedly do this on the same list, the items will end up with multple nested tags, e.g. <td><td><td>...</td></td></td>.
This then gets rendered as extra empty columns, as your browser fills in the missing closing tags between the repeated <td> opening tags and ignores the extra closing tags at the end.
A first quick fix would be to create a new list with the modified items, rather than modifying the provided list (here using a list comprehension):
list1 = ["<td>" + item + "</td>" for item in list1]
A further improvement would be to use a library to create your table, rather than creating it by string manipulation yourself. There are a variety of XML templating libraries you could use, but I don't have enough experience with any of them to make a strong suggestion. This page might be a good place to start browsing.