def from_expr(cls, expr: sp.Basic) -> Quantity:
"""
Converts a Sympy expression with unit symbols
into a Quantity. This only works in case expression
*only* contains base SI unit symbols.
"""
_dimension_symbol_map = cls.get_dimension_symbol_map()
expr = expr.simplify()
args = expr.args
if not args:
return cls(float(expr), 'dimensionless')
try:
magnitude = float(args[0])
except TypeError as e:
raise ValueError(
f'Expression {expr} contains inconsistent units') from e
dimensions = args[1:]
unit = cls.get_unit('')
for d in dimensions:
unit_i = cls.get_unit('')
for symbol, power in d.as_powers_dict().items():
unit_i *= _dimension_symbol_map[symbol]**power
unit *= unit_i
return cls(magnitude, unit).to_base_units()
