[Tutor] I've run into a jam on the exercise on file I/O

[Danny Yoo]

On Mon, 1 Aug 2005, Nathan Pinno wrote:

> I've seem to run into a jam while working on the exercise on file I/O.
> Here's the error:
> Filename to save: university.txt
> Traceback (most recent call last):
>   File "D:\Python22\grades.py", line 99, in ?
>     save_grades(students,filename)
>   File "D:\Python22\grades.py", line 51, in save_grades
>     out_file.write(x+","+max_points[x]+"\n")
> TypeError: sequence index must be integer


Hi Nathan,


I'll try to capture what I think of when I see error messages like this,
to better illustrate how to take something like this, and puzzle it
through.


Let's take a close look at the error message, first:

> TypeError: sequence index must be integer

Python doesn't like some place where we're doing an indexing operation.
Where would that be?


Let's look at the line that Python pointed out, around line 99.

>     out_file.write(x+","+max_points[x]+"\n")

The only indexing operation I can see here is the subexpression:

    max_points[x]

and the error makes sense if if 'x' isn't a number.  Let's take a look at
where 'x' is being assigned, and see if we're assigning it to a
non-number.


> def save_grades(students,filename):
>     out_file = open(filename, "w")
>     for x in students.keys():
>         out_file.write(x+","+max_points[x]+"\n")
>     out_file.close

Ok, 'x' comes from the keys of our 'students' dictionary.  Now our problem
turns into: are the keys of the students dictionary numbers, or are they
something else?


At a high level, we're trying to determine: is our use of max_points[x] in
the save_grades() function the thing that's broken, or is the value 'x'
that's broken?

For the moment, we'll follow 'x', and if that turns out ok, then we should
go back to save_grades() and see what it's trying to write out.  (We can
do things in the other order, too, of course.  Flip a coin.  *grin*)


Let's look at how 'students' is being constructed, since the values of 'x'
comes from the keys of the 'students' dictionary.

######
> max_points = [25,25,50,25,100]
> assignments = ['hw ch 1','hw ch 2','quiz   ','hw ch 3','test']
> students = {'#Max':max_points}
######

Ok.  We certainly see that one of the keys in the 'students' dictionary
isn't a number, so at least we can confirm the error message.  '#Max', for
example, is certainly not a number.


What is 'students' supposed to represent?  That is, can we try describing
what the keys are supposed to be, and what the values are supposed to be?

I will guess that you're thinking of it more like a table, with:

        #Max | [25, 25, 50, 25, 100]
    ---------+-----------------------
       Danny | [20, 20, 40, 20, 90]
       Nick  | [25, 25, 49, 24, 99]


where the keys are student names, and the values are their respective test
scores.  It might be good to rename 'students' to 'student_grades' then,
to make that relationship more clear.


Anyway, in that case, the 'students' dictionary actually looks fine: it
contains the right thing.  Ok, let's go back to the definition of
save_grades():

> def save_grades(students,filename):
>     out_file = open(filename, "w")
>     for x in students.keys():
>         out_file.write(x+","+max_points[x]+"\n")
>     out_file.close

and, for the moment, let's completely ignore the body of the function, and
just imagine: what do we imagine should happen when we save the grades to
disk?  It should read off the students (the "student to grades")
dictionary, and write it to disk.

Let's look back at the body of save_grades() now.  How does max_points
relate to writing out all of the students and their grades to disk?  Does
it really matter, or did we mean to use a different value?



Please feel free to ask questions on any point of this.  Good luck!