Handling errors while working with code snippets

Question

I am running a bunch of code all at once in python by copying it from my editor and pasting it into python. This code includes nested for loops. I am doing some web scraping and the program quits at different times. I suspect that this is because it doesn't have time to load. I get the following error (once again - the program scrapes different amounts of text each time):

Traceback (most recent call last):
  File "<stdin>", line 35, in <module>
IndexError: list index out of range

First, what does line 35 refer to? Is this the place in the relevant inner for-loop?

Second, I think that the error might be caused by a line of code using selenium like this:

driver.find_elements_by_class_name("button")[j-1].click()

In this case, how can handle this error? What is some example code with either explicit waits or exception handling that would address the issue?

Normally, for anything more than very small snippets, you would save your Python script into a file and then run it through the `python` command line command. This will be far more sustainable than pasting into a Python REPL. The benefit, in your current situation, is that the line number reported by Python will refer to an actual line number in your file. — Greg Hewgill, Nov 19 '14 at 22:35
Is there any advantage to using the `python` method vs. `execfile(filename)` in the Python REPL? — bill999, Nov 19 '14 at 22:46
I suppose you could use `execfile()`, that would work equally well. — Greg Hewgill, Nov 19 '14 at 23:02
It was the 35th line pasted. Pasting a script can fail if one defined function calls another later in the posted file. But you get an error about a name not being defined. Why not just write the script to a file and test that? — tdelaney, Nov 19 '14 at 23:04

score 2 · Answer 1 · answered Nov 19 '14 at 22:40

2

It means that [j-1] doesn't exist for a given value of j, possibly if j-1 exceeds the max number of elements in the list

answered Nov 19 '14 at 22:40

AtAFork

376
3
9

Not sure who down voted, but this is a good explanation of the error. – tdelaney Nov 19 '14 at 23:02

score 1 · Accepted Answer · answered Nov 20 '14 at 03:18

You can try your code and catch an IndexError exception like this:

try:
    # your code here
except IndexError:
    # handle the error here

An IndexError happens when you try to access an index of a list that does not exist. For example:

>>> a = [1, 2, 3]
>>> print(a[10])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

It's difficult to say how you should handle the error without more detail.

score 1 · Answer 3 · answered Nov 20 '14 at 04:11

When working with code snippets, it's convenient to have them open in a text editor and either

only copy-paste into a console the part you're currently working on so that all the relevant variables are in the local namespace that you can explore from the console, or
copy-paste a moderate-to-large chunk as a whole while having enabled automatic post-mortem debugger calling, e.g. with Automatically start the debugger on an exception Activestate recipe or IPython's %pdb magic, or
run a script as a whole under debugger e.g with -m pdb, IPython's %run or by using an IDE.

Handling errors while working with code snippets

3 Answers3