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I have a program that reads lines from a textfile, each line contains a word, such as price or product name. The program reads through all lines and saves each word in a list. Each product in the file contains of three lines, which are: product name, price and weight. So in the list will look like the following: ['Mjölk', '15 kr', '1 liter', 'Köttfärs', '80 kr', '1 kg', 'Morötter', '20 kr', '1 kg']

Now to the problem, I have the class:

class Produkt:
def __init__(self, produkt, pris, vikt):
    self.produkt = produkt
    self.pris = pris
    self.vikt = vikt

I want to make Produkt objects from the items from the list, but I don't know how to loop through the items in the list and then save them as objects. I was thinking I'd make a list for the objects, and then like: for item in listofprodukts: objectlits.append(Produkt(item, item,item))

but this does obviously not work, anyone have an idea how to do this?

That1Guy
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Jullix993
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5 Answers5

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for i in range(len(listofprodukts)//3): #use xrange if it's python2
                                        #// -> floor division. Will silently chop off excess if len(listofprodukts) isn't a multiple of 3
    #print(listofprodukts[3*x:3*(x+1)]) # this expression makes chunks of three. Uncomment me to see :P
    objectlits.append(Produkt(*listofprodukts[3*x:3*(x+1)])) 
                      #       ^This * is for argument unpacking

I'm using list slicing to bundle your list in groups of three. And because I'm lazy, I'm using argument unpacking to feed them into your object creator.

Admendum: I agree with TextGeek in that if you're going to do this type of thing, having some validation would be useful.

I'd personally put it in the model, but that's just me.

class Produkt:
    def __init__(self, produkt, pris, vikt):
        if not pris.endswith('kr'):
             raise ValueError("Invalid price {}. Prices should be in kr".format(pris)) 
            #raise ValueError, "Prices ..etc" for python 2
        self.produkt = produkt
        self.pris = pris
        self.vikt = vikt
Community
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NightShadeQueen
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  • Is this a good way to accomplish what I'm trying to do btw? Reading line by line like that, or is there a better way of doing it? Like using keywords for produkts and such. – Jullix993 Jul 24 '15 at 17:15
  • And would you mind explaining what 3*x:3*(x+1) does? – Jullix993 Jul 24 '15 at 17:16
  • it creates the xth slice of three from listofprodukts. for the first iteration: `x=0` `[3*0: 3*(0+1)] = [0:3]` then for `x=1`: `[3*1: 3*(1+1)] = [3:6]` and so on... – Aaron Jul 24 '15 at 17:18
  • IT's list slicing: grab me the list from index `3*x` to index `3*(x+1)-1`. See [here](http://stackoverflow.com/questions/509211/explain-pythons-slice-notation) – NightShadeQueen Jul 24 '15 at 17:18
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assuming that the three attributes are consistently in order, you could simply iterate every third index and slice the list:

class Produkt:
    def __init__(self, produkt, pris, vikt):
        self.produkt = produkt
        self.pris = pris
        self.vikt = vikt

lst =  ['Mjölk', '15 kr', '1 liter', 'Köttfärs', '80 kr', '1 kg', 'Morötter', '20 kr', '1 kg']

produkts = []
for i in xrange(0,len(lst),3):  #in python 3.x use range() instead
    produkts.append(Produkt(lst[i], lst[i+1], lst[i+2]))
Aaron
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  • Stepping by three seems clearer than stepping by 1 and then multiplying. But in either case, you may want to add "if not lst[i+1].endswith('kr'): raise ValueError()" or a similar test(s), because sooner or later, the data will turn out to be broken (say, someone was editing it to update something, and accidentally inserted or dropped one line), and you'll want to know right away. – TextGeek Jul 24 '15 at 17:14
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If you want to loop through try

For x in range(0 , 3):

You can change the three for amount of iterations you want. The example will do 3 iterations.

feltersnach
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  • Thanks, this will however not help me with what I want to do. I need the first three values in the first loop, how can I do that? Maybe like for x in range(0,10): Produkt(list[x], list[x+1], list[x+2]) this would work for the first object, but then it would mess up. – Jullix993 Jul 24 '15 at 17:02
  • [usage](https://docs.python.org/2/library/functions.html#range) for `range()` and `xrange()` is `range(stop)` or `range(start, stop [step])` – Aaron Jul 24 '15 at 17:06
  • this would simply give the first product set. – Aaron Jul 24 '15 at 17:08
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ProduktList =[]
for i in xrange(0,len(ListOfAllProuctsStuff),3)
  ProduktList.append(Produkt(ListOfAllProuctsStuff[i],ListOfAllProuctsStuff[i+1],ListOfAllProuctsStuff[i+2])
Kurt
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0

Assuming the attribute order is consistent.

text_list = ['Mjolk', '15 kr', '1 liter', 'Kottfers', '80 kr', '1 kg', 'Morotter', '20 kr', '1 kg']

class Produkt: def init(self, produkt, pris, vikt): self.produkt = produkt self.pris = pris self.vikt = vikt

##make a representation of the Produkt object so we can verify we're getting what we want
def __repr__(self):
    return_string = "|{}, {}, {}|".format(self.produkt, self.pris, self.vikt)
    return(return_string)

object_list = [] ##create an empty object list index = 0 ##create an index variable and set it to 0

Loop the list that you ripped from the text file

while index < len(text_list):

##Use the index to get this Produkt's produkt, pris, and vikt
produkt = text_list[index]
pris = text_list[index+1]
vikt = text_list[index+2]

##Create the new Produkt object
new_product = Produkt(produkt, pris, vikt)

##add the new product to the object list
object_list.append(new_product)

##Set the index to the next produkt in the list
index += 3

print(object_list) ##print the list objects to make sure we have what we want

Kevin McLaughlin
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