I have a webscraper to download all the stocks from TradingView.com that meet a certain criteria, then stores the names of these stocks in a list. I then have a script to save the names of the stocks along with the date they were added, into a CSV file.
My question is why do I need to loop through a list multiple times to remove the irrelevant data? I've tried using two lists to avoid editing a list you are currently looping through.
Here's the code :
# Removes irrelevant, old stocks
for i in range(5):
to_be_added = old_stocks
for stock in old_stocks:
if stock[0] not in new_stocks:
print(f"Removed irrelevant stock : {stock}")
del to_be_added[old_stocks.index(stock)]
If I remove the for i in range at the top, It only removes two irrelevant stocks.
to_be_added and old_stocks are both the same list, I use them to avoid editing a list I'm currently looping through as stated above. new_stocks is a list of stock names from the webscraper. old_stocks is a list (imported CSV file) containing nested lists containing the stock names and dates added to the file.
# An example of new_stocks
['NASDAQ:CMBG', 'NASDAQ:GLG', 'NASDAQ:KTCC']
# An example of old_stocks
[['NASDAQ:CBMG', 'November 07, 2020'], ['NASDAQ:GLG', 'November 08, 2020'], ['NASDAQ:KTCC', 'November 08, 2020'], ['NASDAQ:ATHE', 'November 08, 2020']]
Iterators over lists are just wrappers over looping with the index, and deleting an element shifts the entire list. Let's say you have the list [1, 2, 3], and you start iterating over it:
[1, 2, 3]
^ iterator here
Then, you delete the element and the iterator advances to slot 2:
[2, 3]
^ iterator here, 2 got skipped
If you meant for to_be_added to be a copy of old_stocks, then you have to use to_be_added = old_stocks.copy(). Since python copies by reference. Alternatively, just use list comprehension filtering:
to_be_added = [stock for stock in old_stocks if stock[0] not in new_stocks]
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