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

TL;DR

If you know the size of the sample you want, but not the size of the input file, you can efficiently load a random sample out of it with the following pandas code:

import pandas as pd
import numpy as np

filename = "data.csv"
sample_size = 10000
batch_size = 200

rng = np.random.default_rng()

sample_reader = pd.read_csv(filename, dtype=str, chunksize=batch_size)

sample = sample_reader.get_chunk(sample_size)

for chunk in sample_reader:
    chunk.index = rng.integers(sample_size, size=len(chunk))
    sample.loc[chunk.index] = chunk

Explanation

It's not always trivial to know the size of the input CSV file.

If there are embedded line breaks, tools like wc or shuf will give you the wrong answer or just make a mess out of your data.

So, based on desktable's answer, we can treat the first sample_size lines of the file as the initial sample and then, for each subsequent line in the file, randomly replace a line in the initial sample.

To do that efficiently, we load the CSV file using a TextFileReader by passing the chunksize= parameter:

sample_reader = pd.read_csv(filename, dtype=str, chunksize=batch_size)

First, we get the initial sample:

sample = sample_reader.get_chunk(sample_size)

Then, we iterate over the remaining chunks of the file, replacing the index of each chunk with a sequence of random integers as long as size of the chunk, but where each integer is in the range of the index of the initial sample (which happens to be the same as range(sample_size)):

for chunk in sample_reader:
    chunk.index = rng.integers(sample_size, size=len(chunk))

And use this reindexed chunk to replace (some of the) lines in the sample:

sample.loc[chunk.index] = chunk

After the for loop, you'll have a dataframe at most sample_size rows long, but with random lines selected from the big CSV file.

To make the loop more efficient, you can make batch_size as large as your memory allows (and yes, even larger than sample_size if you can).

Notice that, while creating the new chunk index with np.random.default_rng().integers(), we use len(chunk) as the new chunk index size instead of simply batch_size because the last chunk in the loop could be smaller.

On the other hand, we use sample_size instead of len(sample) as the "range" of the random integers, even though there could be less lines in the file than sample_size. This is because there won't be any chunks left to loop over in this case so that will never be a problem.