在子目录中随机选择 x 个文件

Question

I need to take exactly 10 files (images) in a dataset randomly, but this dataset is hierarchically structured.

So I need that for each subdirectory that contains images hold just 10 of them randomly. Is there an easy way to do that or I should do it manually?

def getListOfFiles(dirName):
    ### create a list of file and sub directories 
    ### names in the given directory 
    listOfFile = os.listdir(dirName)
    allFiles = list()
    ### Iterate over all the entries
    for entry in listOfFile:

        ### Create full path
        fullPath = os.path.join(dirName, entry)
        ### If entry is a directory then get the list of files in this directory 
        if os.path.isdir(fullPath):
            allFiles = allFiles + getListOfFiles(fullPath)
        else:
            allFiles.append(random.sample(fullPath, 10))
    return allFiles

dirName = 'C:/Users/bla/bla'

### Get the list of all files in directory tree at given path
listOfFiles = getListOfFiles(dirName)

with open("elements.txt", mode='x') as f:
    for elem in listOfFiles:
        f.write(elem + '\n')

Answer 1

Good approach to sample from unknown size directory listing is to use Reservoir Sampling . With this approach, you don't have to run upfront and list all files in the directory. Read it one-by-one and sample. It even works when you have to sample fixed number of files across multiple directories.

It would be good to use generator-based directory scanning code, which picks one file at a time, thus you don't use gobs of memory upfront to hold all file names.

Along the lines (NB! undested code!)

import numpy as np
import os

def ResSampleFiles(dirname, N):
    """pick N files from directory"""

    sampled_files = list()
    k = 0
    for item in scandir(dirname):
        if item.is_dir():
            continue
        full_path = os.path.join(dirname, item.name)
        if k < N:
            sampled_files.append(full_path)
        else:
            idx = np.random.randint(0, k+1)
            if (idx < N):
                sampled_files[idx] = full_path
        k += 1

    return sampled_files