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CPython expects __builtins__ to be a dict, but it is interesting to be able to use another type. For example, my pysandbox project (sandbox to secure Python) requires a read-only mapping for __builtins__.
The PEP 416 was rejected, so there is no builtin frozendict type, but it looks like the dictproxy type will be exposed as a public type.
Attached patch uses PyDict_CheckExact() to check if __builtins__ is a dict and add a "slow-path" for other types. The overhead on runtime performance should be very low (near zero), PyDict_CheckExact() just dereference a pointer (to read the object type) and compare two pointers.
The patch depends on issue #14383 patch (identifier.patch) for the __build_class__ identifier. I can write a new patch without this dependency if needed.

See the issue #14386 which exposes dictproxy as a public type.

Example combining patches of #14385 and #14386 to run code with read-only __builtins__:
----------- test.py -------------
ns={'__builtins__': __builtins__.__dict__}
exec(compile("__builtins__['superglobal']=1; print(superglobal)", "test", "exec"), ns)
ns={'__builtins__': dictproxy(__builtins__.__dict__)}
exec(compile("__builtins__['superglobal']=2; print(superglobal)", "test", "exec"), ns)
----------- end of test.py -----
Output:
--------
$ ./python test.py
1
Traceback (most recent call last):
 File "x.py", line 4, in <module>
 exec(compile("__builtins__['superglobal']=1; print(superglobal)", "test", "exec"), ns)
 File "test", line 1, in <module>
TypeError: 'dictproxy' object does not support item assignment
--------
Note: this protection is not enough to secure Python, but it is an important part of a Python sandbox.

> Note: this protection is not enough to secure Python,
> but it is an important part of a Python sandbox.
Oh, and by the way, I workaround the lack of read-only mapping in pysandbox by removing dict methods: dict.__init__(), dict.clear(), dict.update(), etc. This is a problem because these methods are useful in Python.

With my patch, Python doesn't check __builtins__ type whereas ceval.c replaces any lookup error by a NameError. Example:
$ ./python 
Python 3.3.0a1+ (default:f8d01c8baf6a+, Mar 26 2012, 01:44:48) 
>>> code=compile("print('Hello World!')", "", "exec")
>>> exec(code,{'__builtins__': {'print': print}})
Hello World!
>>> exec(code,{'__builtins__': {}})
NameError: name 'print' is not defined
>>> exec(code,{'__builtins__': 1})
NameError: name 'print' is not defined
It should only replace the current exception by NameError if the current exception is a LookupError.
And my patch on LOAD_GLOBAL is not correct, it does still call PyDict_GetItem. I'm waiting until #14383 is done before writing a new patch.

New version:
 - if __build_class__ is missing, raise a NameError instead of surprising ImportError
 - add tests
 - if PyObject_GetItem on __builtins__ or globals fail, only raise NameError if the exception is a KeyError
Before my patch, a new dict was created for builtins if __builtins__ exists in global but is not a dict. With my patch, the __builtins__ is kept and the type is checked at runtime. If __builtins__ is not a mapping, an exception is raised on lookup in ceval.c.
We may check __builtins__ type in PyFrame_New() using:
PyDict_Check(builtins) || (PyMapping_Check(mapping) && !PyList_Check(mapping) && !PyTuple_Check(mapping))
(PyDict_Check(builtins) is checked first for performance)

Oops, patch version 2 was not correct: I forgot a { ... } in ceval.c.
New patch fixing this issue but leaves also the LOAD_GLOBAL code unchanged : keep the goto and don't try to factorize the 3 last instructions. LOAD_GLOBAL is really critical in performance.
With patch version 3, the overall overhead is +0.4% according to pybench.

- assert(!builtins || PyDict_Check(builtins));
+ assert(!builtins);
Oops, the assert must be replaced with assert(builtins != NULL) -> fixed in patch version 4.

This looks fine.

New changeset e3ab8aa0216c by Victor Stinner in branch 'default':
Issue #14385: Support other types than dict for __builtins__
http://hg.python.org/cpython/rev/e3ab8aa0216c 

I note that the documentation still states a dictionary is required for globals. Should that not be updated as well?
See http://docs.python.org/py3k/library/functions.html#exec 

Apologies, I meant to link to the dev docs:
 http://docs.python.org/dev/library/functions.html#exec 

The patch for this issue changed LOAD_GLOBAL to use PyObject_GetItem when globals() is a dict subclass, but LOAD_NAME, STORE_GLOBAL, and DELETE_GLOBAL weren't changed. (LOAD_NAME uses PyObject_GetItem for builtins now, but not for globals.)
This means that global lookup doesn't respect overridden __getitem__ inside a class statement (unless you explicitly declare the name global with a global statement, in which case LOAD_GLOBAL gets used instead of LOAD_NAME).
I don't have a strong opinion on whether STORE_GLOBAL or DELETE_GLOBAL should respect overridden __setitem__ or __delitem__, but the inconsistency between LOAD_GLOBAL and LOAD_NAME seems like a bug that should be fixed.
For reference, in the following code, the first 3 exec calls successfully print 5, and the last exec call fails, due to the LOAD_GLOBAL/LOAD_NAME inconsistency:
class Foo(dict):
 def __getitem__(self, index):
 return 5 if index == 'y' else super().__getitem__(index)
 
exec('print(y)', Foo())
exec('global y; print(y)', Foo())
exec('''
class UsesLOAD_NAME:
 global y
 print(y)''', Foo())
exec('''
class UsesLOAD_NAME:
 print(y)''', Foo())

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