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[Python-Dev] Why are class dictionaries not accessible?

Random832 random832 at fastmail.com
Wed Jun 22 10:17:27 EDT 2016 
The documentation states: """Objects such as modules and instances have
an updateable __dict__ attribute; however, other objects may have write
restrictions on their __dict__ attributes (for example, classes use a
dictproxy to prevent direct dictionary updates)."""

However, it's not clear from that *why* direct dictionary updates are
undesirable. This not only prevents you from getting a reference to the
real class dict (which is the apparent goal), but is also the
fundamental reason why you can't use a metaclass to put, say, an
OrderedDict in its place - because the type constructor has to copy the
dict that was used in class preparation into a new dict rather than
using the one that was actually returned by __prepare__.

[Also, the name of the type used for this is mappingproxy, not
dictproxy]
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