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The current non-test uses of PyMemoryView_FromBuffer() are in _io.BufferedReader.read(), _io.BufferedWriter.write(), PyUnicode_Decode().
It looks like they can each be made to leak a memoryview that references a deallocated buffer. (Maybe the answer is Don't Do That.)
import codecs, sys
def decode(buf):
 global view
 view = buf
 return codecs.latin_1_decode(buf)
def getregentry():
 return codecs.CodecInfo(name='foobar', decode=decode,
 encode=codecs.latin_1_encode)
@codecs.register
def search_function(encoding):
 if encoding == 'foobar':
 return codecs.CodecInfo(*getregentry())
b = b'hello'.upper()
b.decode('foobar')
print(view.tobytes()) # => b'HELLO'
del b
x = b'dump'.upper()
print(view.tobytes()) # => b'DUMP\x00'
import io, sys
class File(io.RawIOBase):
 def readinto(self, buf):
 global view
 view = buf
 n = len(buf)
 buf[:] = b'x'*n
 return n
 def readable(self):
 return True
f = io.BufferedReader(File())
f.read(1)
print(view[:5].tobytes()) # => b'xxxxx'
del f
print(view[:5].tobytes()) # => b'\xdd\xdd\xdd\xdd\xdd'