On Sat, 7 Nov 2015 09:19 am, Thomas 'PointedEars' Lahn wrote: > It is certainly possible for attributes of (instances of) new-style > classes (starting with Python 3.2 at the latest) to be read-only by > declaring them a property that does not have a setter, or one that has a > setter that throws a specific exception (here: the former): > > #-------------------------- > class SafeMath(object): > def __init__ (self): > from math import pi > self._pi = pi > > @property > def pi (self): > return self._pi
The obvious problem with that is that it is trivially easy for the coder to set self._pi to change the value of self.pi. Here's a version that at first seems promising: py> def builder(): ... from math import pi as PI ... def pi(self): ... return PI ... return pi ... py> class MathLib(object): ... pi = property(builder()) ... py> mathlib = MathLib() py> mathlib.pi 3.141592653589793 py> mathlib.pi = 3.12 Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: can't set attribute There's no _pi accessible that somebody might change. If you dig into the property object itself, you eventually get to the cell object containing the value for pi: py> MathLib.pi.fget.__closure__[0] <cell at 0xb7bd5134: float object at 0xb7be5c30> but there doesn't appear to be any way to manipulate the cell internals to change the value. But you don't need to: py> MathLib.pi = property(lambda self: 4.2) py> mathlib.pi 4.2 A determined coder can change nearly anything done in pure Python code. *Personally*, I too would like Python to have some equivalent of (say) Pascal `const`, names which cannot be rebound once assigned to the first time. Not to prevent determined coders from changing things, but to avoid *accidental* changes done in error. -- Steven -- https://mail.python.org/mailman/listinfo/python-list
