def unit_from_expression(expr):
    """Takes a unit string like ``'1. * volt'`` and returns the BRIAN2 unit."""
    if expr == '1':
        return get_unit_fast(1)
    elif isinstance(expr, str):
        mod = ast.parse(expr, mode='eval')
        expr = mod.body
        return unit_from_expression(expr)
    elif expr.__class__ is ast.Name:
        return ALLUNITS[expr.id]
    elif expr.__class__ is ast.Num:
        return expr.n
    elif expr.__class__ is ast.UnaryOp:
        op = expr.op.__class__.__name__
        operand = unit_from_expression(expr.operand)
        if op=='USub':
            return -operand
        else:
            raise SyntaxError("Unsupported operation "+op)
    elif expr.__class__ is ast.BinOp:
        op = expr.op.__class__.__name__
        left = unit_from_expression(expr.left)
        right = unit_from_expression(expr.right)
        if op=='Add':
            u = left+right
        elif op=='Sub':
            u = left-right
        elif op=='Mult':
            u = left*right
        elif op=='Div':
            u = left/right
        elif op=='Pow':
            n = unit_from_expression(expr.right)
            u = left**n
        elif op=='Mod':
            u = left % right
        else:
            raise SyntaxError("Unsupported operation "+op)
        return u
    else:
        raise RuntimeError('You shall not pass')
Takes a unit string like ``'1. * volt'`` and returns the BRIAN2 unit.