[Tutor] Debugging a sort error.

[Cameron Simpson]

Discussion inline below.

On 13Jan2019 13:16, mhysnm1964 at gmail.com <mhysnm1964 at gmail.com> wrote:
>I am hoping someone can help with the below error using Python3.5 in 
>the Windows 10 bash environment. I found the below link which I am not sure if
>this is related to the issue or not. As I don't fully understand the answer.
>
>https://github.com/SethMMorton/natsort/issues/7

I'm not sure that URL is very relevant, except to the extent that it 
points out that Python 3 issues an error when comparing incomparable 
types. In Python 2 this problem could go unnoticed, and that just leads 
to problems later, much farther from the source of the issue.

>Issue, following error is generated after trying to sort a list of strings.
>
>description.sort()
>TypeError: unorderable types: float() < str()
>
>Each list items (elements) contain a mixture of alpha chars, numbers,
>punctuation chars like / and are in a string type. Below is an example
>extract of the data from the list.
>
>['Description', 'EFTPOS WOOLWORTHS      1294     ", "withdrawal Hudson
>street 3219"]

The error message says that some of these values are not strings. One at 
least is a float.

My expectation is that the openpyxl module is reading a floating point 
value into your description array. This might be openpxyl being too 
clever, or it might be (more likely IMO) be Excel turning something that 
looked like a float into a float. Spreadsheets can be ... helpful like 
that.

>There is over 2000 such entries. This used to work and now doesn't.  

You'll need to examine the values. But I see that you're trying to do 
this. I've snipped the data loading phase. Here:

>description = data['Description']
>for i in description:
>  if not str(i):
>    print "not a string")

This is not a valid check that "i" is a string. That expression:

  str(i)

tries to convert "i" into a string (via its __str__ method). Most 
objects have such a method, and str() of a float is the textual 
representation of the float. So the if statement doesn't test what you 
want to test. Try this:

  if not isinstance(i, str):
    print("not a string:", i, type(i))

>description.sort()
>I am suspecting it is something to do with the data but cannot track 
>down the cause. Any suggestions on how to debug this?

Your exception is in here, but as you expect you want to inspect the 
description types first.

If the description column does contain a float in the original data then 
you could convert it to a string first! Note that this may not match 
visually what was in the spreadsheet. (BTW, your cited code never fills 
out the description list, not it cannot be current.)

But first, fine out what's wrong. Try the type test I suggest and see 
how far you get.

Cheers,
Cameron Simpson <cs at cskk.id.au>