Luke Paireepinart wrote:
On Mon, Oct 26, 2009 at 2:08 PM, Eduardo Vieira <eduardo.su...@gmail.com>wrote:
Hello, I need your help in designing a function. I have created this
script to check out if certain column in some csv files has value "0":
import csv
def zerofound(csvfile, outputfile, lastcolumn ):
"""Finds columns with zero prices. Column have 0 index"""
final = csv.writer(open(outputfile, 'wb'), dialect='excel')
reader = csv.reader(open(csvfile, 'rb'), dialect='excel')
for row in reader:
if '0' in row[:lastcolumn]:
final.writerow(row)
if __name__ == '__main__':
zerofound('pricesfrombv.csv', 'noprices_in_BV.csv', 4)
zerofound('excelbv.csv', 'noprices_in_ExcelBV.csv', 6)
My question is. Is it OK to create functions with no "returns"?
Basically what I did resembles a subroutine, right? How could I
redesign this to use "return"?
Yes, the general "rule of thumb" in Computer Science is that you try to have
functions that either have side-effects or return values, but not both. In
practice people sometimes have both (eg. try to do some side-effects, if
they work return a 0 and if they don't return an error code) but try to
avoid it if possible.
For example, string.strip() has a return value but no side-effects, because
it does not modify the original string, it returns a new copy of the string
that is stripped. Whereas list.sort() has no return value (or it returns
None, depending on how you look at it) because it modifies the _ORIGINAL
LIST_ which is a side-effect.
As for your function, try to define the side-effects and change them into a
return value.
An example of a side-effect is printing to the screen or writing to a file,
not just modifying global scoped or input variables.
*************SPOILER*****************
Your side-effect is that you're writing rows to "final". So you should get
rid of all your "final" code and just return the list of rows. I would also
suggest renaming the function to find_zeros() and obviously you would pass
just the input filename. Then you would have another function
write_rows(outfile, rows) and it would output the rows to outfile. I feel
that would be a much cleaner design. write_rows probably won't have a
return value since you're writing to a function (side-effect!)
*****************************************
Hope that helps!
-Luke
Thanks for your input,
Eduardo
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I agree with Luke's comments. But I'd like to point out an apparent bug
(I haven't tried the code, this is just by inspection).
You use the test
if '0' in row[.....]
that's not going to check for a zero value, it's going to check for a
zero digit character somewhere in the value. So 504 would also be
recognized, as well as 540.
Normally, if it's a numeric int field, you want to convert it to int,
and check its value, rather than do string manipulations directly, since
a value 05 might really be intended to be the same as 5.
DaveA
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