Getting Error: "not enough values to unpack" when returning two outputs using list comprehension with Python

Question

The objective is to create a list comprehension that outputted two values.

The for loops look like below

paper_href_scopus = []
paper_title = []
for litag in all_td.find_all('a', {'class': 'ddmDocTitle'}):
    paper_href_scopus.append(litag['href'])
    paper_title.append(litag.text)

While creating list comprehension as below work without any error

[(paper_href_scopus.append(litag['href']), paper_title.append(litag.text)) \
   for litag in all_td.find_all('a', {'class': 'ddmDocTitle'})]

Modifying the list comprehension as below:

paper_href_scopus, paper_title = [(litag['href'], litag.text) \
    for litag in all_td.find_all('a', {'class': 'ddmDocTitle'})]

resulting an error of :

ValueError: not enough values to unpack (expected 2, got 1)

May I know how such an error can be avoided?

Answer 1

You can just use zip like,

>>> paper_href_scopus, paper_title = zip(*[(litag['href'], litag.text) for litag in all_td.find_all('a', {'class': 'ddmDocTitle'})])

Also, no need to create the list inside, you can use a genexpr, but i don't think it will matter here. Neverthless:

>>> paper_href_scopus, paper_title = zip(*((litag['href'], litag.text) for litag in all_td.find_all('a', {'class': 'ddmDocTitle'})))

Note:

As suggested by balandongiv, here's a link to genexpr usage and another link to official docs