if os.path.exists('plik.pdb'):
with open('plik.pdb') as f:
for line in f:
if line[0:4]=='ATOM' and line[13:15]=='CA':
# print line
if os.path.exists('plik.pdb'):
with open('plik.pdb') as f:
for line in f:
if line[0:4]=='ATOM' and line[13:15]=='CA':
# print line