Python function returning none sometimes

gouche · 14-01-2015 4:28pm #1

Hi

I've got the following function which reads in a csv file and searches for the provided argument in row 1 and returns the value in row2.

def generate_bic(sort_code):
	for row in fooreader:
		value = row[0]
		if value == sort_code:
			bic = row[1]
			return bic

The csv file is just a list of bank sort codes in row 1 with the corresponding BIC in row 2. So when the function is called with a sort code it returns the corresponding BIC as a string.
Anybody any ideas on why it's returning none sometimes?

14-01-2015 5:00pm

Are you sure there's a value there? Some sort codes don't have a corresponding BIC

oscarBravo · 14-01-2015 8:05pm

Yup - if the sort code provided isn't in the file, the function will return None, which is the default return value from a Python function if you don't explicitly return something.

Splinter · 15-01-2015 10:19am

maybe try edit it to add an else?

def generate_bic(sort_code):
	for row in fooreader:
		value = row[0]
		if value == sort_code:
			bic = row[1]
			return bic
                else:
                        ##do something else

you could also use "try" and "except" instead which may do the same

oscarBravo · 15-01-2015 10:40am

Mmm, that won't work - it will "do something else" if the first line of the file doesn't match.

The "do something else" just has to follow the "for" loop - if that loop completes, the sort code wasn't in the file.

gouche · 15-01-2015 2:20pm

Thanks for the replies.

There is definitely a corresponding BIC.
The file only includes sort codes that are reachable by SEPA.
I've also manually checked!

So this is the full code. I've added a flag that will print True if the code thinks the sort code isn't in the file.

import sys
import csv

fooreader = csv.reader(open('nsc_list.csv', 'rb'), delimiter = ',', quotechar = "\'")

def generate_bic(sort_code):
	flag = True
	for row in fooreader:
		value = row[0]
		if value == sort_code:
			bic = row[1]
			print bic
			print type(bic)
			flag = False
			return bic
	print flag
			
generate_bic('990639')
generate_bic('990658')
generate_bic('990639')
generate_bic('990637')
generate_bic('906603')
generate_bic('933341')

And this is the output:

IPBSIE2DXXX
<type 'str'>
IPBSIE2DXXX
<type 'str'>
True
True
True
True

What's interesting here is that for sort code 990639 is correctly resolved when first called but when called again is not.

I may have gone wrong with my logic so feel free to correct!

oscarBravo · 15-01-2015 2:27pm

Ah. The problem is that your csv.reader is an iterable, which means that you don't start from the top of the file each time you call the function; you resume after the last line that was read before the function last returned.

You're going to have to open the csv.reader each time you use it (and close it again!), or find a way to rewind it to the start of the file. If I get a chance later I'll have a look at the docs for it and give you more detail.

oscarBravo · 15-01-2015 2:34pm

Thinking some more about it: better than re-opening the file each time would be to cache its contents in a list:

import sys
import csv

fooreader = csv.reader(open('nsc_list.csv', 'rb'), delimiter = ',', quotechar = "\'")
foolist = [row for row in fooreader]

def generate_bic(sort_code):
	for row in foolist:
		value = row[0]
		if value == sort_code:
			bic = row[1]
			return bic

gouche · 15-01-2015 2:44pm

Thanks OscarBravo!

If I move opening the file inside the function it works. But if I understand correctly this opens the file and keeps it in memory each time function is called?

I had a quick look and there doesn't seem to be a close option for csv.reader.

Looks like your solution to store contents of the file in a list works well though.

Thanks for your help!

oscarBravo · 15-01-2015 2:59pm

An even more efficient approach would be to build a dictionary from the csv file, and drop the function:

foodict = {}
fooreader = csv.reader(open('nsc_list.csv', 'rb'), delimiter=',', quotechar="\'")
for row in fooreader:
    foodict[row[0]] = row[1]

Then just use foodict[sort_code] instead of calling a generate_bic() function.

Donnelly117 · 19-01-2015 1:22pm

Something like

fooreader.seek(0)

should reset back to the start of the CSV no?

Splinter · 19-01-2015 2:42pm

Donnelly117 wrote: »

Something like

fooreader.seek(0)

should reset back to the start of the CSV no?

wouldn't this just keep resetting on the first entry it finds though?

oscarBravo · 19-01-2015 3:10pm

Donnelly117 wrote: »

Something like

fooreader.seek(0)

should reset back to the start of the CSV no?

fooreader is a csv.reader instance; it's an iterator, not a file-like object, and it doesn't provide a seek() method.

Python function returning none sometimes

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