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I have .csv file looking like this: https://gyazo.com/746719669f079b0b0c22965b63e52910, I have to open this file and proccess data few million times, my code was working with with open() as f: but then I realized it might be super slow since it opens file every time, so I tried to put that data into variable and just use it later, problem is data disappears after first read, here is my code:

import csv

TEST = open("C:/Users/Krzysztof/Documents/SO.csv")
CCSSVV = csv.DictReader(TEST, delimiter=";")

def printdate(FILE):
    print("start")
    for LINE in FILE:
        print(LINE["MATCH DATE"])
    print("end")

printdate(CCSSVV)
printdate(CCSSVV)
printdate(CCSSVV)

I wanted output to look like:

start
25.05.2018
25.05.2018
25.05.2018
end
start
25.05.2018
25.05.2018
25.05.2018
end
start
25.05.2018
25.05.2018
25.05.2018
end

but instead it's

start
25.05.2018
25.05.2018
25.05.2018
end
start
end
start
end
Improve this question
3
  • A file object is an iterator of lines, not a sequence of lines—once you iterate it, it's empty. And, since a csv.DictReader just wraps that file object, it's also an iterator of dicts, not a sequence. Commented Jun 12, 2018 at 23:39
  • If you want to make this work, options include: (a) close and reopen the file each time. (b) call TEST.seek(0), then re-create just the DictReader. (c) TEST.seek(0), then next(TEST) to skip the header line, then the existing DictReader will continue to work. (d) read the whole iterator of dicts into a sequence in memory, like CCSSVV = list(CCSSVV), so you can loop over it repeatedly. (e) rewrite your code so it can do everything in a single pass instead of needing three passes. Commented Jun 12, 2018 at 23:41
  • thank you so much, CCSSVV = list(CCSSVV) worked perfectly, much love! Commented Jun 12, 2018 at 23:42
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