[Tutor] calculate the sum of a variable - python

[Wayne Werner]

On Sun, Mar 6, 2011 at 9:31 PM, nookasree ponamala <nookasree at yahoo.com>wrote:

> Hi :
>
> I'm a Senior SAS Analyst. I'm trying to learn Python. I would appreciate if
> anybody could help me with this. It works fine if I give input  instead of
> reading a text file. I don't understand where I'm going wrong.
>
> I'm trying to read a text file and find out the following:
> 1. Sum of amt for each id
> 2. Count of id
> 3. minimum of date1
> 4. maximum of date1
>
> Here is the sample text file:
>
> test.txt file:
>
> bin1    cd1     date1   amt     cd    id cd2
> 452  2       2010-02-20      $23.26  0    8100059542        06107
> 452  2       2010-02-20      $20.78  0          8100059542        06107
> 452  2       2010-02-24      $5.99   2          8100839745        20151
> 452  2       2010-02-12      $114.25 7          8100839745        98101
> 452  2       2010-02-06      $28.00  0          8101142362        06032
> 452  2       2010-02-09      $15.01  0          8100274453        06040
> 452  18      2010-02-13      $113.24 0          8100274453        06040
> 452  2       2010-02-13      $31.80  0          8100274453        06040
>
>
> Here is the code I've tried out to calculate sum of amt by id:
>
> import sys
> from itertools import groupby
> from operator import itemgetter
> t = ()
> tot = []
> for line in open ('test.txt','r'):
>        aline = line.rstrip().split()
>        a = aline[5]
>        b = (aline[3].strip('$'))
>        t = (a,b)
>        t1 = str(t)
>        tot.append(t1)
>        print tot
> def summary(data, key=itemgetter(0), value=itemgetter(1)):
>        for k, group in groupby(data, key):
>                yield (k, sum(value(row) for row in group))
>
> if __name__ == "__main__":
>        for id, tot_spend in summary(tot, key=itemgetter(0),
> value=itemgetter(1)):
>            print id, tot_spend
>
>
> Error:
> Traceback (most recent call last):
>  File "<stdin>", line 2, in <module>
>  File "<stdin>", line 3, in summary
> TypeError: unsupported operand type(s) for +: 'int' and 'str'
>

Of course I first have to commend you for including the full traceback with
the code because it makes this entirely easy to answer.

In general, the traceback tells you the most important stuff last, so I'll
start with this line:

> TypeError: unsupported operand type(s) for +: 'int' and 'str'

That tells us that the problem is you are trying to use + (addition) on an
integer and a string - which you can't do because of the type mismatch
(TypeError).

The next line

> File "<stdin>", line 3, in summary

tells us that the error occurred on line3 in summary:

1 | def summary(data, key=itemgetter(0), value=itemgetter(1)):
2 |        for k, group in groupby(data, key):
3 |                yield (k, sum(value(row) for row in group))

Well, there's no '+', but you do have 'sum', which uses addition under the
hood. So how do you go about fixing it? Well, you change the value getting
passed to sum to an integer (or other number):

sum(int(value(row)) for row in group)

Should either fix your problem, or throw a differen error if you try to
convert a string like 'Hello' to an integer. (Alternatively, use float if
you're interested in decimals)

HTH,
Wayne
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