CODEWITHSUNDEEP
pythoncsvreverse
menchopez
menchopez

Reputation: 75

Best way to reverse lines in a csv file python

I want to know the best way to reverse the lines of a big csv file (+50000 lines) in python 2.7 and rewrite it, avoiding the first line.

input:
A;B;C
1;2;3
4;5;6

output
A;B;C
4;5;6
1;2;3

I need to know how to do it in a efficient way in python 2.7.

Thank you guys,

menchopez

Upvotes: 4

Views: 7336

Answers (3)

Jean-François Fabre
Jean-François Fabre

Reputation: 140276

read the csv file using csv module and open the output also using csv module. Now you're working with lists as rows.

Use next to write the title line as-is. Now that the first line is consumed, convert the rest of the data into a list to read it fully and apply writerows on the reversed list:

import csv

with open("in.csv") as fr, open("out.csv","wb") as fw:
    cr = csv.reader(fr,delimiter=";")
    cw = csv.writer(fw,delimiter=";")
    cw.writerow(next(cr))  # write title as-is
    cw.writerows(reversed(list(cr)))

writerows is the fastest way of doing it, because it involves no python loops.

Python 3 users have to open the output file using open("out.csv","w",newline="") instead.

Upvotes: 4

Isdj
Isdj

Reputation: 1856

Read as follows:

rows = []
first = True
for row in reader:
    if first:
        first = False
        first_row = row
        continue
    rows.append(row)

write as follows:

rows.append(first_row)
writer.writerows(rows[::-1])

Upvotes: 1

RHSmith159
RHSmith159

Reputation: 1592

If you can use external libraries, the pandas library is good for large files:

import pandas as pd

# load the csv and user row 0 as headers
df = pd.read_csv("filepath.csv", header = 0)

# reverse the data
df.iloc[::-1]

If you cannot use external libraries:

import csv

with open("filepath.csv") as csvFile:
    reader = csv.reader(csvFile)


    # get data
    data = [row for row in reader]
    # get headers and remove from data
    headers = data.pop(0)

# reverse the data
data_reversed = data[::-1]

# append the reversed data to the list of headers
output_data = headers.append(data_reversed)

Upvotes: 3

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