import sys

try:
    input_file_name = sys.argv[1]
except IndexError:
    print("Usage: python pdb_model_renum.py my_ensemble.pdb")
    sys.exit()

output_file = open(input_file_name.split(".")[0] + "_renum.pdb", "w")

model_n = 1

for line in open(input_file_name):
    if line.startswith("MODEL"):
        output_file.write("MODEL  " + str(model_n) + "\n")
        model_n += 1
    else:
        output_file.write(line)