random_select_500

import random
from Bio import SeqIO

input_file = 'add_path_to_your_fasta_file'
output_file = 'add_path_to_desired_place'

with open(output_file, "w") as out_handle:
    
    for record in SeqIO.parse(input_file, "fasta"):
        sequence = str(record.seq)
        print("length:", len(sequence)) 
     
        if len(sequence) <= 500:
            out_handle.write(">" + record.id + "\n")
            out_handle.write(sequence + "\n")
        else:
            start = random.randint(0, len(sequence) - 500)
            print("Startindex:", start) 
            end = start + 500
            if end > len(sequence):  
                end = len(sequence)
                start = end - 500
            print("Endindex:", end)  
            random_sequence = sequence[start:end] 
            out_handle.write(">" + record.id + "\n")
            out_handle.write(random_sequence + "\n")