[BUG (??)] Terminal modifications are kept regardless of slicing

[jspaezp]

Hello!

I was wondering whether this is a desired behavior. I noticed that when slicing a proforma object, it always keeps the terminal modifications. This feels odd to me, since I would expect terminal modifications to be removed when "slicing away" that side of the modification.

Example:

>>> from pyteomics import proforma
>>> pep = proforma.ProForma.parse("[U-1]-MPEP/2")
>>> pep
ProForma([('M', None), ('P', None), ('E', None), ('P', None)], {'n_term': [GenericModification('U-1', None, None)], 'c_term': None, 'unlocalized_modifications': [], 'labile_modifications': [], 'fixed_modifications': [], 'intervals': [], 'isotopes': [], 'group_ids': [], 'charge_state': ChargeState(2, [])})
>>> pep[:2]
ProForma([('M', None), ('P', None)], {'n_term': [GenericModification('U-1', None, None)], 'c_term': None, 'unlocalized_modifications': [], 'labile_modifications': [], 'fixed_modifications': [], 'intervals': [], 'isotopes': [], 'group_ids': [], 'charge_state': ChargeState(2, [])})
>>> pep[2:] # Here I would expect the modification to be gone
ProForma([('E', None), ('P', None)], {'n_term': [GenericModification('U-1', None, None)], 'c_term': None, 'unlocalized_modifications': [], 'labile_modifications': [], 'fixed_modifications': [], 'intervals': [], 'isotopes':  [], 'group_ids': [], 'charge_state': ChargeState(2, [])})

LMK what you think Best, Sebastian

[jspaezp]

Noticed the fix, thanks a lot! amazing work yall!

[mobiusklein]

Thank you for reporting these issues. Without your engagement we wouldn't know what was missing.