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I am trying to create a database to store the information generated by my code in the form of a 1 x 21 vector. I have called this prices, and would like to store each element of this vector in a new line of a text file. Then, the second time I run the program, I wish to append the new entries of the vector prices onto each respective line in the text file. The purpose of this is so that once a large set of results has been gathered, it is easy to produce a plot to see how each of these 21 elements changed over time.

I tried to copy (with a bit of modification with the condition in the if loop, as well as the overall for loop) the method shown in this answer, but for some reason, I get a blank text file when I run the code. I changed one thing, the w to w+, but it doesn't work with the w either. What am I doing wrong?

prices = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1] # this is just a sample output
for exp in range(21):
    with open("price_database.txt", 'r') as f:
        lines = f.readlines()

    with open("price_database.txt", 'w+') as f:
        for x, line in enumerate(lines):
            if x == exp:
                f.write(str(prices[exp]))
            f.write(line)

Edit 1:

prices = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1]
for exp in range(21):
    with open("price_database.txt", 'r') as f:
        lines = f.readlines()
    with open("price_database.txt", 'a') as f:
        for x, line in enumerate(lines):
            if x == exp:
                f.write(str(prices[exp]))
John Doe
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2 Answers2

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You need to close the file every time you open to read or write results with f.close() but i cant undertand why you use txt file to do this job you should really use csv or even mysql it will be much better

Edited: Open the file with append mode so you can write at the end:

with open('something.txt', 'a') as f:
    f.write('text to be appended')
    f.close()
Ευάγγελος Γρηγορόπουλος
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  • I am quite new to python and asked a friend who did something similar with a text file. I don't really know how to do anything with a csv file, but I will look into this perhaps when I look to improve this. I just tried to add f.close onto the end of each of the `with` loops, but the text file is still empty. If this is much easier with a csv file, and isnt too complicated, would you mind pointing out how I can do it that way? – John Doe Aug 25 '19 at 13:30
  • Edited and for the future look at csv module so you can structure your data – Ευάγγελος Γρηγορόπουλος Aug 25 '19 at 13:37
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    If you open the file with "with" operator, you don't need to close explicitly. – Debendra Aug 25 '19 at 13:40
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    You don't need close the file when you use the `with` statement, because it does it by itself, you can also write a file using `print()` as well, you just need inform the argument `file`. – Tuna Aug 25 '19 at 13:50
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If you want to be a little less careful, then do it this way, it is slightly faster because you don’t have to keep closing.

app = 'append text'
with open('something.txt', 'a') as f:
    f.write(app)
f.close()
Number File
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