locks in threads

[Aahz Maruch]

In article <slrn9089tk.l4j.bwinton at tor.dhs.org>,
Blake Winton <bwinton at iname.com> wrote:
>On 3 Nov 2000 13:29:11 -0800, Aahz Maruch wrote:
>>
>>   [when to set a lock]
>>* you need consistant data across multiple variables
>
>Note that you don't need a lock in this case, as long as you store all
>the data in a Wrapper object, and have a method to get a copy of the
>object, and another method to set the object.

I believe you're wrong.  Consider the following trivial example:

class Foo:
  def set (self, x, y):
    self.x = x
    self.y = y
  def get (self):
    return (x, y)

If thread 1 calls Foo.set() and thread 2 calls Foo.get(), it is possible
for the return to be executed after setting self.x but before setting
self.y.  Now, one could in theory do this

class Foo:
  def set (self, x, y):
    (self.x, self.y) = (x, y)
  def get (self):
    return (x, y)

but I'm not a fan of relying implicitly on a coding convention in
multi-threaded programs.  Far better to just use a lock in case the
program changes later.
-- 
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