Pass-by-reference : Could a C#-like approach work in Python?

Peter Otten __peter__ at
Thu Sep 11 00:50:29 CEST 2003

Stephen Horne wrote:

> However, it would probably catch a very substantial portion of real
> accidental side-effect errors.

It may be a matter of taste, but I think it will cause a *very* substantial
portion of side effect errors. A simple example:

def nested(ref o):
    o = ReadOnly(o)

def fun(o):
    nested(ref o)
    # so glad we can do no harm to o...riginal

x = C()
x.attr = newValue # oops

Note that even in today's Python you can do weird stuff:

>>> class C: pass
>>> class D:
...     def __setattr__(self, n, v): raise Exception, "read-only"
>>> c = C()
>>> c.x = 1
>>> c.__class__ = D
>>> c.y = 2
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 2, in __setattr__
Exception: read-only
>>> c.x

This would at least propagate over the entire calling hierarchy.

To go back to your container example, if copying is costly and only
sometimes necessary, wrap the item into a copy-on-write proxy for every
logically distinct but physically identical instance. That decision should
be explicit rather than hidden as a side effect of a container class.


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