• File cannot be opened,
• Input parameters are out of range,
• Object does not have the requested property,
• SMARTS string cannot be parsed,
• Bond cannot be added (eg, dt_mod_is_on == FALSE),
• Canonical SMILES cannot be generated.

• You must explicitly ignore exceptions,
• Limit problems from spreading,
• Out-of-band error messages are hard to check,
• Code can be half the size as similar code with explicit checks.

  try:
    mol = Smiles.smilin("COO")
    atoms = mol.atoms
    atoms[0].addbond(atoms[2])  # Won't work; mod is off!
    mol.cansmiles()
  except DaylightError, msg:
    print "Cannot do that:", msg