def _is_aromatic(atom_symbol: str) -> bool:
"""Checks whether a SMILES atom symbol is an aromatic SMILES atom symbol.
An aromatic SMILES atom symbol is indicated by an element substring
that is not capitalized.
:param atom_symbol: a SMILES atom symbol.
:return: True, if ``atom_symbol`` is an aromatic atom symbol,
and False otherwise.
"""
s, e = find_element(atom_symbol)
if e == len(atom_symbol): # optimization to prevent string copying
element = atom_symbol
else:
element = atom_symbol[s:e]
if element[0].isupper(): # check if element is capitalized
return False
if element not in _aromatic_valences:
raise ValueError(
"unrecognized aromatic symbol '{}'".format(atom_symbol))
return True
def _capitalize(atom_symbol: str) -> str:
"""Capitalizes the element portion of an aromatic SMILES atom symbol,
converting it into a standard SMILES atom symbol.
:param atom_symbol: an aromatic SMILES atom symbol.
:return: the capitalized ``atom_symbol``.
"""
s, _ = find_element(atom_symbol)
return atom_symbol[:s] + atom_symbol[s].upper() + atom_symbol[s + 1:]