Question

I want to increment a variable and – if a particular condition is fulfilled – I want to assign the next element of an iterator to it. In both cases the result should be appended to a list.

Problem is, that the function only recognizes the values already in the iterator.

The input-data is a nested list.

import datetime as dt

dates_prices = [[dt.datetime(2008, 6, 3, 0, 0), 48.54],
                [dt.datetime(2008, 6, 6, 0, 0), 47.99]]

def fillDates(dates_prices):
    filled = []
    iter_data = iter(dates_prices)
    item = iter_data.next()
    filled.append(item)
    while True:
        item[0] += dt.timedelta(1)
        try:
            if item in dates_prices:
                item = iter_data.next()
            filled.append(item)
        except StopIteration:
            return filled

a = fillDates(dates_prices)
print a

The function should check, which dates are missing in the original nested list. It should add all missing dates together with the last known price-point, so the output shoud be this:

a =
[[dt.datetime(2008, 6, 3, 0, 0), 48.54], 
[dt.datetime(2008, 6, 4, 0, 0), 48.54], 
[dt.datetime(2008, 6, 5, 0, 0), 48.54], 
[dt.datetime(2008, 6, 6, 0, 0), 47.99]]

What did I miss?

EDIT:

I altered the function that it is working now by creating a seperate list of dates from the nested list “dates_prices” and applying the suggestion by Sevenforce.

However, I still don’t know why my first solution didn’t work. I guess something with the variable assignment was wrong. But I don’t know what.

This is the new function:

import datetime as dt

dates_prices = [[dt.datetime(2008, 6, 3, 0, 0), 48.54], [dt.datetime(2008, 6, 6, 0, 0), 47.99]]

def fillDates(dates_prices):
    filled = []
    dates = [x[0] for x in dates_prices] #added this list
    iter_data = iter(dates_prices)
    item = iter_data.next()
    filled.append(item[:])

    while item[0] < dates[-1]:
        item[0] += dt.timedelta(1)
        if item[0] in dates: #using the new list here
            item = iter_data.next()
        filled.append(item[:]) #added colon here
    return filled


a = fillDates(dates_prices)
print a

Answer 1

I am suspecting dates_prices is a nested list.

You possibly want to add a copy of item to filter and not the same object. To do this, change the line filled.append(item) to filled.append(item[:]). This will prevent item[0] += dt.timedelta(1) from changing already appended values in filled.

---

To answer your edit:

• Another [:] was missing: iter_data = iter(dates_prices[:]) prevents changes in input dates_prices itself (by item[0] += dt.timedelta(1), btw this is still happening in your updated code). This led to if item in dates_prices always evaluate to True.

• With above change if item in dates_prices will be always False since [dt.datetime(2008, 6, 6, 0, 0), 48.54] != datetime.datetime(2008, 6, 6, 0, 0), 47.99] and therefore leads in an endless loop.

Another working version (edited):

import datetime as dt
import copy

dates_prices = [[dt.datetime(2008, 6, 3, 0, 0), 48.54],
    [dt.datetime(2008, 6, 6, 0, 0), 47.99]]

def fillDates(dates_prices):
    filled = []
    iter_data = iter(copy.deepcopy(dates_prices))  #to copy the datetime objects
    item = iter_data.next()
    filled.append(item[:])
    dates_idx = 1
    while dates_idx < len(dates_prices):
        item[0] += dt.timedelta(1)
        if item[0] == dates_prices[dates_idx][0]:
            item = iter_data.next()
            dates_idx += 1
        filled.append(item[:])
    return filled

a = fillDates(dates_prices)
print a

But there is still room for improvement, like using a dictionary for dates_prices.

@jsbueno:
You are right. The thing to learn here is to use something like

new_item = [item[0] + dt.timedelta(1), item[1]]

I think.