how to write a python script to count number of residues in a 'pdb' file

[iromi]

#!/usr/bin/python
import sys
import os
f1=open('1tfz.pdb','r')
residue=[]
for line in f1:
if line.startswith('ATOM') or line.startswith('HETATM'):
res_number=line[22-26]
if res_number not in residue and line[17-20]!='HOH' and line[17-20]!='WAT':
residue.append(res_number)
else:
continue
else:
continue
f1.close()
res_total=len(residue)
if res_total>=50 and res_total<=1000:
B=res_total
else:
B='NO'

# please can anyone explain me what's wrong with this codes. as the output i want to know number of residues in this pdb file

[dwblas]

Your question is really to vague to answer; i.e we do not know what the input is and if you really are counting "residues" or something else in the file. Also, there are no code tags (see How to Ask a Question at https://bytes.com/faq.php?faq=postin...ask_a_question) so it would be a waste of time to try and guess as to the indentations. So the only helpful information is to add some print statements to see what is happening

res_number=line[22-26]
print("res_number =", res_number)
if res_number not in residue and line[17-20]!='HOH' and line[17-20]!='WAT':
    print("res_number not in")
else:
    print("res_number else")
##  etc. 

[iromi]

thank you very much for your kind explanation. I will proceed with this.