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Why can I reassign dict.update but not dict.__setitem__

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1

I'm trying to modify a third-party dict class to make it immutable after a certain point.
With most classes, I can assign to method slots to modify behavior.
However, this doesn't seem possible with all methods in all classes. In particular for dict, I can reassign update, but not __setitem__.

Why? How are they different?

For example:

class Freezable(object):
 def _not_modifiable(self, *args, **kw):
 raise NotImplementedError()

 def freeze(self):
 """
 Disallow mutating methods from now on.
 """
 print "FREEZE"
 self.__setitem__ = self._not_modifiable
 self.update = self._not_modifiable
 # ... others
 return self

class MyDict(dict, Freezable):
 pass

d = MyDict()
d.freeze()
print d.__setitem__ # <bound method MyDict._not_modifiable of >

d[2] = 3 # no error -- this is incorrect.

d.update(4:5) # raise NotImplementedError

python

share|improve this question

asked Mar 26 at 1:08

user48956user48956

5,1951212 gold badges5151 silver badges8787 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  __setitem__ is a "special" method and is looked up using the class of self, not in the instance's __dict__.
  
  – martineau
  Mar 26 at 1:14
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Why isn’t update the same?
  
  – user48956
  Mar 26 at 1:15
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  update doesn't begin and end with double underscores (aka "dunder"), so no, it's not "special". Here's something about them in the documentation — notice there's no update — also see "special method" in the glossary.
  
  – martineau
  Mar 26 at 1:18
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You may be interested in this stackoverflow.com/questions/47915937/…
  
  – Jacques Gaudin
  Mar 26 at 1:30
  

  
  
  
  

add a comment | 

1

I'm trying to modify a third-party dict class to make it immutable after a certain point.
With most classes, I can assign to method slots to modify behavior.
However, this doesn't seem possible with all methods in all classes. In particular for dict, I can reassign update, but not __setitem__.

Why? How are they different?

For example:

class Freezable(object):
 def _not_modifiable(self, *args, **kw):
 raise NotImplementedError()

 def freeze(self):
 """
 Disallow mutating methods from now on.
 """
 print "FREEZE"
 self.__setitem__ = self._not_modifiable
 self.update = self._not_modifiable
 # ... others
 return self

class MyDict(dict, Freezable):
 pass

d = MyDict()
d.freeze()
print d.__setitem__ # <bound method MyDict._not_modifiable of >

d[2] = 3 # no error -- this is incorrect.

d.update(4:5) # raise NotImplementedError

python

share|improve this question

asked Mar 26 at 1:08

user48956user48956

5,1951212 gold badges5151 silver badges8787 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  __setitem__ is a "special" method and is looked up using the class of self, not in the instance's __dict__.
  
  – martineau
  Mar 26 at 1:14
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Why isn’t update the same?
  
  – user48956
  Mar 26 at 1:15
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  update doesn't begin and end with double underscores (aka "dunder"), so no, it's not "special". Here's something about them in the documentation — notice there's no update — also see "special method" in the glossary.
  
  – martineau
  Mar 26 at 1:18
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You may be interested in this stackoverflow.com/questions/47915937/…
  
  – Jacques Gaudin
  Mar 26 at 1:30
  

  
  
  
  

add a comment | 

I'm trying to modify a third-party dict class to make it immutable after a certain point.
With most classes, I can assign to method slots to modify behavior.
However, this doesn't seem possible with all methods in all classes. In particular for dict, I can reassign update, but not __setitem__.

Why? How are they different?

For example:

class Freezable(object):
 def _not_modifiable(self, *args, **kw):
 raise NotImplementedError()

 def freeze(self):
 """
 Disallow mutating methods from now on.
 """
 print "FREEZE"
 self.__setitem__ = self._not_modifiable
 self.update = self._not_modifiable
 # ... others
 return self

class MyDict(dict, Freezable):
 pass

d = MyDict()
d.freeze()
print d.__setitem__ # <bound method MyDict._not_modifiable of >

d[2] = 3 # no error -- this is incorrect.

d.update(4:5) # raise NotImplementedError

python

share|improve this question

asked Mar 26 at 1:08

user48956user48956

5,1951212 gold badges5151 silver badges8787 bronze badges

class Freezable(object):
 def _not_modifiable(self, *args, **kw):
 raise NotImplementedError()

 def freeze(self):
 """
 Disallow mutating methods from now on.
 """
 print "FREEZE"
 self.__setitem__ = self._not_modifiable
 self.update = self._not_modifiable
 # ... others
 return self

class MyDict(dict, Freezable):
 pass

d = MyDict()
d.freeze()
print d.__setitem__ # <bound method MyDict._not_modifiable of >

d[2] = 3 # no error -- this is incorrect.

d.update(4:5) # raise NotImplementedError

share|improve this question

asked Mar 26 at 1:08

user48956user48956

5,1951212 gold badges5151 silver badges8787 bronze badges

share|improve this question

asked Mar 26 at 1:08

user48956user48956

5,1951212 gold badges5151 silver badges8787 bronze badges

asked Mar 26 at 1:08

user48956user48956

5,1951212 gold badges5151 silver badges8787 bronze badges

asked Mar 26 at 1:08

user48956user48956

5,1951212 gold badges5151 silver badges8787 bronze badges

1 Answer
1

active

oldest

votes

2

Note that you can define the class __setitem__, e.g.:

def __setitem__(self, key, value):
 if self.update is Freezable._not_modifiable:
 raise TypeError(' has been frozen'.format(id(self)))
 dict.__setitem__(self, key, value)

(This method is a bit clumsy; there are other options. But it's one way to make it work even though Python calls the class's __setitem__ directly.)

share|improve this answer

answered Mar 26 at 1:35

torektorek

214k2121 gold badges276276 silver badges361361 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thank you -- seems like a good solution. I was hoping to replace the method because the original has a particularly efficient machine code implementation -- gains that are lost with wrapping the method. If there's another solution, I'd prefer that.
  
  – user48956
  Mar 26 at 20:08
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Consider: def __setitem__(self, key, value): self.setitem(self, key, value) which means you can then def setitem in the usual way and use your existing trick, just as you do for update.
  
  – torek
  Mar 26 at 20:18
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I think that still leaves x[..] = y with the original behavior.
  
  – user48956
  Mar 26 at 20:20
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Try it out. Define __setitem__ in the class to call some function f. Have f print that it's called and call dict.__setitem__ as above. Then set self.f = self._not_modifiable as you do above. Then invoke x[key] = y.
  
  – torek
  Mar 26 at 20:28
  

  
  
  
  

add a comment | 

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2

Note that you can define the class __setitem__, e.g.:

def __setitem__(self, key, value):
 if self.update is Freezable._not_modifiable:
 raise TypeError(' has been frozen'.format(id(self)))
 dict.__setitem__(self, key, value)

(This method is a bit clumsy; there are other options. But it's one way to make it work even though Python calls the class's __setitem__ directly.)

share|improve this answer

answered Mar 26 at 1:35

torektorek

214k2121 gold badges276276 silver badges361361 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thank you -- seems like a good solution. I was hoping to replace the method because the original has a particularly efficient machine code implementation -- gains that are lost with wrapping the method. If there's another solution, I'd prefer that.
  
  – user48956
  Mar 26 at 20:08
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Consider: def __setitem__(self, key, value): self.setitem(self, key, value) which means you can then def setitem in the usual way and use your existing trick, just as you do for update.
  
  – torek
  Mar 26 at 20:18
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I think that still leaves x[..] = y with the original behavior.
  
  – user48956
  Mar 26 at 20:20
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Try it out. Define __setitem__ in the class to call some function f. Have f print that it's called and call dict.__setitem__ as above. Then set self.f = self._not_modifiable as you do above. Then invoke x[key] = y.
  
  – torek
  Mar 26 at 20:28
  

  
  
  
  

add a comment | 

2

Note that you can define the class __setitem__, e.g.:

def __setitem__(self, key, value):
 if self.update is Freezable._not_modifiable:
 raise TypeError(' has been frozen'.format(id(self)))
 dict.__setitem__(self, key, value)

(This method is a bit clumsy; there are other options. But it's one way to make it work even though Python calls the class's __setitem__ directly.)

share|improve this answer

answered Mar 26 at 1:35

torektorek

214k2121 gold badges276276 silver badges361361 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thank you -- seems like a good solution. I was hoping to replace the method because the original has a particularly efficient machine code implementation -- gains that are lost with wrapping the method. If there's another solution, I'd prefer that.
  
  – user48956
  Mar 26 at 20:08
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Consider: def __setitem__(self, key, value): self.setitem(self, key, value) which means you can then def setitem in the usual way and use your existing trick, just as you do for update.
  
  – torek
  Mar 26 at 20:18
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I think that still leaves x[..] = y with the original behavior.
  
  – user48956
  Mar 26 at 20:20
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Try it out. Define __setitem__ in the class to call some function f. Have f print that it's called and call dict.__setitem__ as above. Then set self.f = self._not_modifiable as you do above. Then invoke x[key] = y.
  
  – torek
  Mar 26 at 20:28
  

  
  
  
  

add a comment | 

Note that you can define the class __setitem__, e.g.:

def __setitem__(self, key, value):
 if self.update is Freezable._not_modifiable:
 raise TypeError(' has been frozen'.format(id(self)))
 dict.__setitem__(self, key, value)

(This method is a bit clumsy; there are other options. But it's one way to make it work even though Python calls the class's __setitem__ directly.)

share|improve this answer

answered Mar 26 at 1:35

torektorek

214k2121 gold badges276276 silver badges361361 bronze badges

def __setitem__(self, key, value):
 if self.update is Freezable._not_modifiable:
 raise TypeError(' has been frozen'.format(id(self)))
 dict.__setitem__(self, key, value)

share|improve this answer

answered Mar 26 at 1:35

torektorek

214k2121 gold badges276276 silver badges361361 bronze badges

answered Mar 26 at 1:35

torektorek

214k2121 gold badges276276 silver badges361361 bronze badges

answered Mar 26 at 1:35

torektorek

214k2121 gold badges276276 silver badges361361 bronze badges