[Patches] [ python-Patches-1402289 ] Fix dictionary subclass semantics when used as global dicts.

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Patches item #1402289, was opened at 2006-01-11 01:24
Message generated for change (Comment added) made by loewis
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Category: Core (C code)
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Submitted By: crutcher (crutcher_gmail)
Assigned to: Raymond Hettinger (rhettinger)
Summary: Fix dictionary subclass semantics when used as global dicts.

Initial Comment:
There is an inconsistancy in the way that dictionary
subclasses behave
when they are used as as namespaces in execs.

Basically, while python 2.4 permits the usage of
dictionary subclasses
for local environments, it still bypasses the subclass
functions and
uses the C API for global environments. The attached
patch (and
unittest!) addresses this issue.

I'm pretty sure we keep the fast path in this.

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>Comment By: Martin v. Löwis (loewis)
Date: 2006-01-12 07:11

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I think the history is this: it originates from
python.org/sf/215126, which requested that you can pass dict
subtypes to eval. Armin noted that eval will always produce
LOAD/STORE_NAME, so just modifying these opcodes is
sufficient to fulfil the feature request.

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Comment By: Raymond Hettinger (rhettinger)
Date: 2006-01-12 02:21

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Okay, I will take it under consideration.  Also, I'll try to
remember the reason it wasn't done the first time around.

No promises though -- by itself consistency is a weak
motivation.

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Comment By: crutcher (crutcher_gmail)
Date: 2006-01-12 00:11

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Here's a more interesting example. This works fine, unless
the variables fall through to the global dictionary, or some
code marks them global.

import code

class ManagedVariable:
  def get(self):
    return None

  def set(self, value):
    pass

  def delete(self):
    # Return false to stop the delete.
    return True

class ManagedEnvironment(dict):
 def __setitem__(self, key, value):
   if self.has_key(key):
     if isinstance(dict.__getitem__(self, key),
ManagedVariable):
       dict.__getitem__(self, key).set(value)
       return
   dict.__setitem__(self, key, value)

 def __getitem__(self, key):
   if self.has_key(key):
     if isinstance(dict.__getitem__(self, key),
ManagedVariable):
       return dict.__getitem__(self, key).get()
   return dict.__getitem__(self, key)

 def __delitem__(self, key):
   if self.has_key(key):
     if isinstance(dict.__getitem__(self, key),
ManagedVariable):
       if not dict.__getitem__(self, key).delete():
         return
   dict.__delitem__(self, key)


class RangedInt(ManagedVariable):
  def __init__(self, value, (low, high)):
    self.value = value
    self.low = low
    self.high = high

  def get(self):
    return self.value

  def set(self, value):
    if value < self.low:
      value = self.low
    if value > self.high:
      value = self.high
    self.value = value


class FunctionValue(ManagedVariable):
  def __init__(self, get_func = None, set_func = None,
del_func = None):
    self.get_func = get_func
    self.set_func = set_func
    self.del_func = del_func

  def get(self):
    if self.get_func:
      return self.get_func()
    return None

  def set(self, value):
    if self.set_func:
      self.set_func(value)

  def delete(self):
    if self.del_func:
      return self.del_func()
    return True

class Constant(ManagedVariable):
  def __init__(self, value):
    self.value = value

  def get(self):
    return self.value

  def delete(self):
    return False


import time
d = ManagedEnvironment()
d['ranged'] = RangedInt(1, (0, 100))
d['time'] = FunctionValue(lambda: time.time())
d['constant'] = Constant(42)

code.interact(local=d)


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Comment By: crutcher (crutcher_gmail)
Date: 2006-01-11 19:12

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Here's an example of a little typed environment. It's not
the most robust, but it gets you thinking.

import code
class TypedDictionary(dict):
 def __setitem__(self, key, value):
   if self.has_key(key):
     t = type(self[key])
     if t != type(value):
       try:
         value = t(value)
       except Exception:
         raise TypeError, \
             "illegal assignment to '%s':" \
             " %s cannot be coerced to %s" \
               % (key, type(value), t)
   dict.__setitem__(self, key, value)
code.interact(local=TypedDictionary())

----------------------------------------------------------------------

Comment By: crutcher (crutcher_gmail)
Date: 2006-01-11 03:16

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With the ability to use dictionary subclasses for local and
global environments, it becomes very easy to implement
execution environments with extended variable semantics for
special purposes.

This includes driving interactive processes, config file
processors, type checking, lazy object construction, and
read-only variables.

As it currently stands, the semantics are inconsistent. The
local case should not have been changed if the global case
was not going to be changed.

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Comment By: Raymond Hettinger (rhettinger)
Date: 2006-01-11 03:03

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Do you have any use cases?  AFAICT, there were no unmet
needs with locals option.

Also, there was a reason that globals weren't included but I
remember what it was.

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