[Tutor] process and modify a list of strings, in place

[John Martinetti]

Steve -

Dave pointed out earlier in the thread the primary mistake I made...which
was that when I assembled the record to be store in the list "openPOs", I
did so and made it a tuple, instead of a list.  That was a mistake, I meant
to make that a list and after reviewing my code, over and over again, I
missed the fact that I made it a tuple.

I like the idea of using a for-loop, I'm going to incorporate that...thanks
for the suggestions.
In addition to that, I like your suggestion for detecting the blank buyerID
field, I plan on incorporating that as well.

It's great to have a list like this to get another set of eyes reviewing the
code, very very helpful.
Thanks again Steve for your response, I really appreciate it.
I'll report back once I get the changes implemented.



On Thu, Feb 10, 2011 at 7:31 PM, Steven D'Aprano <steve at pearwood.info>wrote:

> John Martinetti wrote:
>
>> Hello -
>>
>> I'm a novice programmer, not even amateur level and I need some help with
>> developing an algorithm to process a list of strings.
>> I hope this list is tolerant of n00bs, if not, please let me know and I'll
>> take this elsewhere.
>>
>
> Hi John! This list is specifically for n00bs, so don't be shy.
>
>
>
>  #! /usr/bin/python
>> import sys, re
>> txtreport=open("open_pos.txt",'r')
>>
>> openPOs=[]
>>
>> while 1:
>>
>>   record = txtreport.readline() # - reads in a line of data from the
>> report
>> file
>>
>>   vendornum = record[:6]       # - breaks out each column of the report
>> into fields
>>
>
> [snip slicing and dicing of the line]
>
>
>    # - create a record from all the fields
>>   linedata = (vendornum, vendorname, ordernum, ordersuffix, orderdate,
>> buyer, partnum, qty, comment)
>>   # - append the record to the list of records representing the CQ report
>>   openPOs.append(linedata)
>>
>>   # if not the end of the file, do it again
>>   if not record:
>>      break
>>
>
> The first thing that comes to mind is that you can simplify the line
> processing by using a for loop instead of an infinite while loop:
>
> for line in txtreport:
>    # walks the txtreport file, yielding each line in turn
>    # and stopping at the end of the file
>    vendornum = line[:6]
>    # etc.
>    record = (vendornum, vendorname, ordernum, ordersuffix,
>
>              orderdate, buyer, partnum, qty, comment)
>    openPOs.append(record)
>
> Notice I've swapped the names around -- what you called "record" I'm
> calling "line", and what you called "linedata" I'm calling a record.
>
>
>
>  The part I'm having a problem with is once I've detected that a record has
>> a
>> blank buyerID field, I can't seem to figure out how to change it, in
>> place.
>> I've tried finding the index of the openPOs using this:
>> openPOs.index(line)
>> and then trying to change the item like this:
>> openPOs[openPOs.index(line),5] = "NOBUYER"
>> but I'm getting a strange error:  "TypeError: 'tuple' object does not
>> support item assignment"
>>
>
> Correct. Out of the box, Python supports two main built-in sequence types,
> the tuple and the list. A tuple is designed for fixed-size records, and is
> designed to be immutable -- once created, you can't modify it.
>
> Instead of writing to a tuple in place, you would create a new tuple and
> replace it:
>
> for i in range(len(openPOs)):  # i = 0, 1, 2, ... N
>    record = openPOs[i]
>    if record[5] == '      ':
>        record = record[:5] + ('NOBUYER',) + record[6:]
>        openPOs[i] = record
>
>
> The only part that's a bit tricky is that you need a one-item tuple for the
> NOBUYER element. To do that, you need to know a trick for creating a
> one-element tuple: take note of the comma inside the brackets.
>
> The alternative is to use a list instead of a tuple. Lists are designed for
> variable length data structures, and so lists can shrink and grow as needed,
> and elements can be changed in place. To use a list, change the line in the
> earlier for-loop to use square brackets instead of round:
>
>    record = [vendornum, vendorname, ordernum, ordersuffix,
>              orderdate, buyer, partnum, qty, comment]
>
>
> and then replace NOBUYER fields in place:
>
> for i in range(len(openPOs)):
>    record = openPOs[i]
>    if record[5] == '      ':
>        record[5] = 'NOBUYER'
>
>
> One final improvement... whether you use a list or a tuple, the code that
> checks for blank buyers can be simplified. Both versions start off with the
> same two lines of boilerplate code:
>
> for i in range(len(openPOs)):
>    record = openPOs[i]
>    # ...
>
> We can simplify this to a single line that gets the record number i and the
> record itself together:
>
> for i,record in enumerate(openPOs):
>    # ...
>
>
>
> Hope this helps.
>
>
>
> --
> Steven
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