python 列表中的 IndexError - “列表索引超出范围”

Question

I'm trying to get certain data in columns from my MySQL database but I get this error when trying to index a list in a for loop. Everything works well until I'm trying to print out data from 1 column.

Here's the list. I do believe this is a tuple but that shouldn't be the problem. (I think)

(1, 'Router', '192.168.1.1', '80')

Here's my code:

myresult = mycursor.fetchall()
            for x in myresult:
                print(x)
                time.sleep(0.2)
            ip = myresult[2]
            print(ip)

This is the IndexError:

.....
File "directory..", line 172, in huvudmeny2
    ip = myresult[2]
IndexError: list index out of range

Process finished with exit code 1

How can it be out of range when it has 4 items?

Answer 1

The fetchall method returns a list of the remaining rows, each a tuple. It does not return the tuple.

Hence there's a good chance you got less than three rows.

If you want the IP address of the first row from the query (for example), that would be:

myresult[0][2]

The following transcript shows this in action (after dummying up some data):

>>> myresult = [(1, 'Router', '192.168.1.1', '80')]

>>> print(myresult)
[(1, 'Router', '192.168.1.1', '80')]

>>> print(myresult[2])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

>>> print(myresult[0][2])
192.168.1.1

---

An alternative, if you know there will only be one row of useful data, is to use fetchone instead of fetchall . This actually returns a single tuple rather than a list of tuples, so you can use your original indexing method. You just need to check for None first since that's what you'll get if there were no rows returned.

But you already have that problem since you're not checking for an empty list in your original code, which is what fetchall gives you when there are no rows.

Answer 2

fetchall() returns a list, so you are printing index 2 of this list.

maybe you want something like:

for x in myresult:
  print(x)
  time.sleep(0.2)
ip = myresult[0][2]
print(ip)

So you will access the 1st element of the list of tuples, then access the element you want.

You also can put the last 2 lines in the for loop to make sure they exist, like so:

for x in myresult:
  print(x)
  time.sleep(0.2)
  ip = myresult[2]
  print(ip)