How to save multiple values in CSV file from loop in python?

You didn't show how you write it but there can be different methods to do it.

First. There is standard rule: if you use for-loop then use list to keep all results.

And this way you can create empty list before loop, append result to this list inside loop, and write all results after loop.

To make all versions similar I will NOT use with open(..) as fh - but you could try to use it.

# --- before loop ---

all_results = []  # <-- list for all results

# --- loop ---

for threshold in np.arange(1,10):
    threshold_p = calculate_th(n, m, threshold)
    
    for root, dirs, files in os.walk(dir_path, topdown=False):
        for name in tqdm.tqdm(files, desc="files progress"):
            filename = os.path.join(root, name)
            output_g = np.load(filename)
            filtered = np.sum(output_g > threshold_p)
            result = [filename, filtered, threshold_p, threshold]
     
            all_results.append( result )  # <-- keep result           

# --- after loop ---

fh = open('output.csv', 'w')
cvs_writer = cvs.writer(fh)

# write one row with headers (using `writerow` without `s` at the end)
cvs_writer.writerow(["filename", "filtered", "threshold_p", "threshold"]    cvs_writer.writerow(["filename", "filtered", "threshold_p", "threshold"]

# write many rows with results (using `writerows` with `s` at the end)
cvs_writer.writerows(all_results)
                                                                                                
fh.close()

Second. You should open file before loop, write row inside loop, and close file after loop.

# --- before loop ---

fh = open('output.csv', 'w')
cvs_writer = cvs.writer(fh)

# write one row with headers (using `writerow` without `s` at the end)
cvs_writer.writerow(["filename", "filtered", "threshold_p", "threshold"]    cvs_writer.writerow(["filename", "filtered", "threshold_p", "threshold"]

# --- loop ---

for threshold in np.arange(1,10):
    threshold_p = calculate_th(n, m, threshold)
    
    for root, dirs, files in os.walk(dir_path, topdown=False):
        for name in tqdm.tqdm(files, desc="files progress"):
            filename = os.path.join(root, name)
            output_g = np.load(filename)
            filtered = np.sum(output_g > threshold_p)
            result = [filename, filtered, threshold_p, threshold]
     
            # write row row with result (using `writerow` without `s` at the end)
            cvs_writer.writerow(result)

# --- after loop ---
                                                                                                
fh.close()

Third. Before loop create file only with headers - and close it. Inside loop open file in append mode to add new row at the end of file.

This method will keep all results if code crash.

# --- before loop ---

fh = open('output.csv', 'w')
cvs_writer = cvs.writer(fh)

# write one row with headers (using `writerow` without `s` at the end)
cvs_writer.writerow(["filename", "filtered", "threshold_p", "threshold"]    cvs_writer.writerow(["filename", "filtered", "threshold_p", "threshold"]
                                                                                                
fh.close()
                                                                                                
# --- loop ---

for threshold in np.arange(1,10):
    threshold_p = calculate_th(n, m, threshold)
    
    for root, dirs, files in os.walk(dir_path, topdown=False):
        for name in tqdm.tqdm(files, desc="files progress"):
            filename = os.path.join(root, name)
            output_g = np.load(filename)
            filtered = np.sum(output_g > threshold_p)
            result = [filename, filtered, threshold_p, threshold]

            fh = open('output.csv', 'a')  # `a` for `append mode`
            cvs_writer = cvs.writer(fh)
     
            # write row row with result (using `writerow` without `s` at the end)
            cvs_writer.writerow(result)

            fh.close()

# --- after loop ---

# nothing