gistfile1.txt

from concurrent.futures import ThreadPoolExecutor, ProcessPoolExecutor
import pandas as pd
from pathlib import Path
import sys

base_dir = sys.argv[1]
output_csv = sys.argv[2]
files = Path(base_dir).rglob('*.csv')

def read_file(file):
    return pd.read_csv(file)

with ProcessPoolExecutor(12) as pool:
    df = pd.concat(pool.map(read_file, files))
    df.to_csv(output_csv, index=False)