为列表项分配“无”时出错

Question

I'm getting a strange error in my program which I can't seem to replicate in a simplified example. I'm trying to assign the value of None to an item in a list, but for some reason it throws an error. I don't understand the problem, since I'm not even using tuples.

#Execute SQL query returning five columns.
cursor.execute(query)

#Iterate the rows.
for row in cursor:
    #Get one row, and copy it to a new list variable.
    my_list         = row[:5]

    #If index 3 is zero, convert it to None.
    if (my_list[3] == 0):
        print type(my_list[3])
        ##Prints <type 'int'>

        my_list[3]  = None
        ##TypeError: 'tuple' object does not support item assignment

Answer 1

For some reason, MySQLdb returns rows as tuples instead of lists. This is getting copied to my_list, in spite of the row[:5] syntax.

print type(my_list)
##Prints <type 'tuple'>

Since tuples are immutable, they must be first converted to lists, after which individual elements can be reassigned.

my_list = list(row[:5])