Read a small random sample from a big CSV file into a Python data frame

Question

The CSV file that I want to read does not fit into main memory. How can I read a few (~10K) random lines of it and do some simple statistics on the selected data frame?

Answer 1

Assuming no header in the CSV file:

import pandas
import random

n = 1000000 #number of records in file
s = 10000 #desired sample size
filename = "data.txt"
skip = sorted(random.sample(range(n),n-s))
df = pandas.read_csv(filename, skiprows=skip)

would be better if read_csv had a keeprows, or if skiprows took a callback func instead of a list.

With header and unknown file length:

import pandas
import random

filename = "data.txt"
n = sum(1 for line in open(filename)) - 1 #number of records in file (excludes header)
s = 10000 #desired sample size
skip = sorted(random.sample(range(1,n+1),n-s)) #the 0-indexed header will not be included in the skip list
df = pandas.read_csv(filename, skiprows=skip)