Rename nature numbered files with fixed size number

rename_nature_with_fixed_size

import os 

def rename_nature_with_fixed_size(size=3, sep='_'):
    files = [f for f in os.listdir('.') if os.path.isfile(f)]
    for f in files:
        if sep in f and f[0] != sep:
            new_name = f.split(sep)
            if len(new_name[0]) < size:
                new_name[0] = '0' * (size - len(new_name[0])) + new_name[0]
            os.rename(f, sep.join(new_name))

if __name__ == "__main__":
    rename_nature_with_fixed_size()