How to insert character in the beginning of all items in a list, while excluding specific items, based on another list?

Question

I have a list1 (which can be much longer than this one) :

data_into_list = ["a", "good", "with", "amor", "and", "friand"]

and I want to add a hashtag (‘#’) only to certain items, excluding some, based on another list2 : if a word from list2 is in list1, no hashtag is added to it.

Like :

excluded_words = ["a", "with", "and"]

(I tried several solutions, but since I’m rather a beginner in Python, I can’t find a correct way to do it. I tried to add a hashtag to all the words in the list, convert it to string, and then to do a substitution, via a loop and .replace. I also tried to use a dictionnary, and use re.sub. But in both cases, it doesn’t match the exact character (it removes the # not only to the "a" item, but also on all items that begin with "a", like "amor" in my list). And it seems that a dictionnary can’t use regex, to match the exact character, as far as I understand…)

In any case, it seems more logical to do a list comparison, and exclude some items based on a second list, but I can’t manage to find how…

Thx in advance

Edit, here are one of my failed solutions :

import re

# opening the file in read mode
my_file = open("LastPrompt.txt", "r")
  
# reading the file
data = my_file.read()
  
# split each word into a list
data_into_list = data.replace('n', ' ').split()

string = '#'

#add hashtags on every words
addhashtag = [string + x for x in data_into_list]


#convert list to string -> needed to be saved in .txt
hashtags = ' '.join(map(str, addhashtag))

#replace every undesired word with # by the same word without #
for r in (("#A", "A"), ("#is", "is"), ("#a", "a"), ("#with", "with")):
     hashtag = hashtag.replace(*r)

Asked By: Aleph

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Source

Answer 1

You can convert the second list into a set and look up each string from the first list in that set.

This will do the trick:

data_into_list = ["a", "good", "with", "amor", "and", "friand"]
excluded_words = ["a", "with", "and"]

excluded_set = set(excluded_words)
new_list = [
    item if item in excluded_set else "#" + item
    for item in data_into_list
]
print(new_list)

Output: ['a', '#good', 'with', '#amor', 'and', '#friand']

EDIT: updated to use the exclude list from your updated question.

Answered By: joanis

How to insert character in the beginning of all items in a list, while excluding specific items, based on another list?

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