float change from python 3.0.1 to 3.1.2

Question

I'm trying to learn python. I am using 3.1.2 and the o'reilly book is using 3.0.1 here is my code:

import urllib.request

price = (99.99)

while price > 4.74:
    page = urllib.request.urlopen ("http://www.beans-r-us.biz/prices-loyalty.html")
    text = page.read().decode("utf8")

    where = text.find('>$')

    start_of_price = where + 2
    end_of_price = start_of_price + 6

    price = float(text[start_of_price:end_of_price])

print ("Buy!")

Here is my error:

Traceback (most recent call last):
  File "/Users/odin/Desktop/Coffe.py", line 14, in <module>
    price = float(text[start_of_price:end_of_price])
ValueError: invalid literal for float(): 4.59</
>>> 

What is wrong?

Answer 1

The problem is that you have extra characters at the end of your float, probably because the content of the page changed since the code was written (the number appears to change every fifteen minute). You could try changing the following line to make the code slightly more robust:

end_of_price = text.find('<', start_of_price)

For an even better solution you should use something like BeautifulSoup to parse the HTML.

Answer 2

You should never use float for money. Read accepted answer: http://stackoverflow.com/questions/61872/use-float-or-decimal-for-accounting-application-dollar-amount

Answer 3

You are calculating end_of_price as start_of_price + 6. Actually your price value seems to be only 4 characters long, so that you also include the two characters following in the string you want to convert to a float. Python then complains that 4.59</ is not a number.

If you instead set end_of_price to start_of_price + 4 it should work.