"Unboundlocalerror: Local Variable "Val" Referenced before Assignment" Error - python-2.7

Question

I have been trying to get my script to loop in such a way that it will load the outputs in 1 file, and then when it's done loading everything move the values to output file 2 , erase the values in output file 1 and start reloading them, then when those are down move the values into output two (overwriting the old ones) repeat.
I have been pretty successful so far and don't know what else to add to my script and am hoping someone here knows why I keep getting ""Unboundlocalerror: Local Variable "Val" Referenced before Assignment" error randomly midway through the loading process, when I have a very small input file, the script performs how I want.
Does anyone know how I can change my script to fix that error, I have tried to understand why it is happening but cannot.
I have tried to research it thoroughly but none of the suggestions I have found have worked (or I implemented them incorrectly, I have attached my script. Thanks!
import urllib2,re,urllib,urlparse,csv,sys,time,threading,codecs,shutil
from bs4 import BeautifulSoup
def extract(url):
try:
sys.stdout.write('0')
# global file
page = urllib2.urlopen(url).read()
soup = BeautifulSoup(page, 'html.parser')
product = soup.find("div", {"class": "js-product-price"})
price = product.findNext('div',{'class': 'js-price-display'}).getText().strip()
oos = product.findNext('p', attrs={'class': "price-oos"})
if oos is None:
oos = 'In Stock'
else:
oos = oos.getText()
val = url + "," + price + "," + oos + "," + time.ctime() + '\n'
# ifile.write(val)
sys.stdout.write('1')
except Exception as e:
print e
return val
while True:
ifile = open('output.csv', "w", 0)
inputs = csv.reader(open('input.csv'))
# inputs = csv.reader(codecs.open('input.csv', 'rU', 'utf-16'))
ifile.write('URL' + "," + 'Price' + "," + 'Stock' + "," + "Time" + '\n')
for i in inputs:
ifile.write(extract(i[0]))
ifile.close()
Update:
Thanks for the help guys! This is my new script:
import urllib2,re,urllib,urlparse,csv,sys,time,threading,codecs,shutil
from bs4 import BeautifulSoup
def extract(url):
try:
sys.stdout.write('0')
# global file
page = urllib2.urlopen(url).read()
soup = BeautifulSoup(page, 'html.parser')
product = soup.find("div", {"class": "js-product-price"})
price = product.findNext('div',{'class': 'js-price-display'}).getText().strip()
oos = product.findNext('p', attrs={'class': "price-oos"})
if oos is None:
oos = 'In Stock'
else:
oos = oos.getText()
val = url + "," + price + "," + oos + "," + time.ctime() + '\n'
# ifile.write(val)
sys.stdout.write('1')
except Exception as e:
print e
else:
return val
while True:
ifile = open('output.csv', "w", 0)
inputs = csv.reader(open('input.csv'))
# inputs = csv.reader(codecs.open('input.csv', 'rU', 'utf-16'))
ifile.write('URL' + "," + 'Price' + "," + 'Stock' + "," + "Time" + '\n')
for i in inputs:
val_to_write = extract(i[0])
if val_to_write:
ifile.write(val_to_write)
ifile.close()
shutil.copy('output.csv', 'output2.csv')
print("finished")
With the above script I am now getting the error: "ValueError: I/O operation on closed file". Thanks

Answer 1

Use try-except-else as you would only want to return val if no exception was raised (if an exception was raised then val wouldn't be assigned to when you try to return it). Another suggestion is not to use a "catch-em-all" except block.
def extract(url):
try:
sys.stdout.write('0')
# global file
page = urllib2.urlopen(url).read()
soup = BeautifulSoup(page, 'html.parser')
product = soup.find("div", {"class": "js-product-price"})
price = product.findNext('div',{'class': 'js-price-display'}).getText().strip()
oos = product.findNext('p', attrs={'class': "price-oos"})
if oos is None:
oos = 'In Stock'
else:
oos = oos.getText()
val = url + "," + price + "," + oos + "," + time.ctime() + '\n'
# ifile.write(val)
sys.stdout.write('1')
except Exception as e:
print e
else:
return val
But be warned: if an exception does occur then extract will return None and the calling code will have to take account for that, for example:
for i in inputs:
val_to_write = extract(i[0])
if val_to_write:
ifile.write(val_to_write)
ifile.close()