creating class instances from a text file in pytho

Question

I have a polynomial class:

class Polynomial:
    def __init__(self, *termpairs):
        self.termdict = dict(termpairs)

To add an instance to this class, you enter the following:

P = Polynomial((3,2), (4,5), (9,8))

Now I'm trying to create a function that reads information from a text file, and from that text file, it creates an instance of the Polynomial class. The information is stored in the text file like this:

4 6
2 3
9 8
3 5
0 42

So far, the function looks like this:

def read_file(polyfilename):
    polyfilename = open("polyfilename.txt", "r")
    poly1 = polyfilename.read()
    polyfilename.close()
    poly1 = [line.split() for line in poly1.split("\n")]
    poly1 = [[int(y) for y in x] for x in poly1]
    for item in poly1:
        return Polynomial(item)

It only creates a Polynomial instance for the first pair in the text file, how can I make a Polynomial instance for all of the pairings in the text file?

EDIT: I now have this as my function:

def read_file(polyfilename):
    polyfilename = open("polyfilename.txt", "r")
    poly = polyfilename.read()
    polyfilename.close()
    poly = [line.split() for line in poly.split("\n")]
    poly = [[int(y) for y in x] for x in poly]
    return Polynomial(poly[0], poly[1], poly[2], poly[3], poly[4])

It gives me the answer I'm looking for, however, the length of these text files can vary, so typing poly[1], poly[2] won't work. Is there a way I can go through each index no matter what the length is?

Answer 1

All you need is

return Polynomial(*poly)

instead of

return Polynomial(poly[0], poly[1], poly[2], poly[3], poly[4])

Docs here ... read the paragraph starting with """If the syntax *expression appears in the function call"""

Bonus: Your function doesn't use its arg, reuses the name poly like crazy, and isn't idiomatic, so I rewrote the whole thing:

def read_file(polyfilename):
    with open(polyfilename) as f:
        return Polynomial(*[[int(x) for x in line.split()] for line in f])

HTH

Answer 2

At a quick glance, it looks like this is returning after the first one is created, so you only get one back. Try replace the return statement with a yield then:

for p in read_file('somefile'):
  # p is a polynomial object, do what you will with it

In case it helps the first response here is a good summary of yield. Again this is just from a quick look at what you've written.

    def read_file(polyfilename):
        polyfilename = open(polyfilename, "r")
        poly1 = polyfilename.read()
        polyfilename.close()
        poly1 = [line.split() for line in poly1.split("\n")]
        poly1 = [[int(y) for y in x] for x in poly1]
        for item in poly1:
            yield Polynomial(item)

    for p in read_file('sample.txt'):
        print p

Produces

<__main__.Polynomial instance at 0x39f3f0>
<__main__.Polynomial instance at 0x39f418>
<__main__.Polynomial instance at 0x39f3f0>
<__main__.Polynomial instance at 0x39f418>
<__main__.Polynomial instance at 0x39f3f0>

Answer 3

return always exits the function immediately upon calling and can only return one item. So if you want multiple items (in a sense) to be returned, you must pack them in a list, then return that.

In place of the for loop:

polys = []
for item in poly1:
    polys.append(Polynomial(item))
return polys

You will of course have to deal with this new result as a list.