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238 lines
8.2 KiB
Python
238 lines
8.2 KiB
Python
import abc
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import collections.abc
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from notmuch2 import _capi as capi
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from notmuch2 import _errors as errors
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__all__ = ['NotmuchObject', 'BinString']
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class NotmuchObject(metaclass=abc.ABCMeta):
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"""Base notmuch object syntax.
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This base class exists to define the memory management handling
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required to use the notmuch library. It is meant as an interface
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definition rather than a base class, though you can use it as a
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base class to ensure you don't forget part of the interface. It
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only concerns you if you are implementing this package itself
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rather then using it.
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libnotmuch uses a hierarchical memory allocator, where freeing the
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memory of a parent object also frees the memory of all child
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objects. To make this work seamlessly in Python this package
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keeps references to parent objects which makes them stay alive
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correctly under normal circumstances. When an object finally gets
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deleted the :meth:`__del__` method will be called to free the
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memory.
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However during some peculiar situations, e.g. interpreter
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shutdown, it is possible for the :meth:`__del__` method to have
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been called, whele there are still references to an object. This
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could result in child objects asking their memory to be freed
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after the parent has already freed the memory, making things
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rather unhappy as double frees are not taken lightly in C. To
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handle this case all objects need to follow the same protocol to
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destroy themselves, see :meth:`destroy`.
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Once an object has been destroyed trying to use it should raise
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the :exc:`ObjectDestroyedError` exception. For this see also the
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convenience :class:`MemoryPointer` descriptor in this module which
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can be used as a pointer to libnotmuch memory.
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"""
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@abc.abstractmethod
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def __init__(self, parent, *args, **kwargs):
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"""Create a new object.
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Other then for the toplevel :class:`Database` object
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constructors are only ever called by internal code and not by
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the user. Per convention their signature always takes the
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parent object as first argument. Feel free to make the rest
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of the signature match the object's requirement. The object
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needs to keep a reference to the parent, so it can check the
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parent is still alive.
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"""
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@property
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@abc.abstractmethod
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def alive(self):
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"""Whether the object is still alive.
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This indicates whether the object is still alive. The first
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thing this needs to check is whether the parent object is
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still alive, if it is not then this object can not be alive
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either. If the parent is alive then it depends on whether the
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memory for this object has been freed yet or not.
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"""
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def __del__(self):
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self._destroy()
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@abc.abstractmethod
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def _destroy(self):
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"""Destroy the object, freeing all memory.
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This method needs to destroy the object on the
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libnotmuch-level. It must ensure it's not been destroyed by
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it's parent object yet before doing so. It also must be
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idempotent.
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"""
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class MemoryPointer:
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"""Data Descriptor to handle accessing libnotmuch pointers.
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Most :class:`NotmuchObject` instances will have one or more CFFI
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pointers to C-objects. Once an object is destroyed this pointer
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should no longer be used and a :exc:`ObjectDestroyedError`
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exception should be raised on trying to access it. This
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descriptor simplifies implementing this, allowing the creation of
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an attribute which can be assigned to, but when accessed when the
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stored value is *None* it will raise the
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:exc:`ObjectDestroyedError` exception::
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class SomeOjb:
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_ptr = MemoryPointer()
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def __init__(self, ptr):
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self._ptr = ptr
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def destroy(self):
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somehow_free(self._ptr)
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self._ptr = None
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def do_something(self):
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return some_libnotmuch_call(self._ptr)
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"""
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def __get__(self, instance, owner):
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try:
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val = getattr(instance, self.attr_name, None)
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except AttributeError:
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# We're not on 3.6+ and self.attr_name does not exist
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self.__set_name__(instance, 'dummy')
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val = getattr(instance, self.attr_name, None)
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if val is None:
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raise errors.ObjectDestroyedError()
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return val
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def __set__(self, instance, value):
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try:
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setattr(instance, self.attr_name, value)
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except AttributeError:
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# We're not on 3.6+ and self.attr_name does not exist
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self.__set_name__(instance, 'dummy')
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setattr(instance, self.attr_name, value)
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def __set_name__(self, instance, name):
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self.attr_name = '_memptr_{}_{:x}'.format(name, id(instance))
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class BinString(str):
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"""A str subclass with binary data.
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Most data in libnotmuch should be valid ASCII or valid UTF-8.
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However since it is a C library these are represented as
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bytestrings instead which means on an API level we can not
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guarantee that decoding this to UTF-8 will both succeed and be
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lossless. This string type converts bytes to unicode in a lossy
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way, but also makes the raw bytes available.
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This object is a normal unicode string for most intents and
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purposes, but you can get the original bytestring back by calling
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``bytes()`` on it.
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"""
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def __new__(cls, data, encoding='utf-8', errors='ignore'):
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if not isinstance(data, bytes):
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data = bytes(data, encoding=encoding)
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strdata = str(data, encoding=encoding, errors=errors)
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inst = super().__new__(cls, strdata)
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inst._bindata = data
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return inst
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@classmethod
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def from_cffi(cls, cdata):
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"""Create a new string from a CFFI cdata pointer."""
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return cls(capi.ffi.string(cdata))
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def __bytes__(self):
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return self._bindata
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class NotmuchIter(NotmuchObject, collections.abc.Iterator):
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"""An iterator for libnotmuch iterators.
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It is tempting to use a generator function instead, but this would
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not correctly respect the :class:`NotmuchObject` memory handling
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protocol and in some unsuspecting cornercases cause memory
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trouble. You probably want to sublcass this in order to wrap the
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value returned by :meth:`__next__`.
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:param parent: The parent object.
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:type parent: NotmuchObject
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:param iter_p: The CFFI pointer to the C iterator.
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:type iter_p: cffi.cdata
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:param fn_destory: The CFFI notmuch_*_destroy function.
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:param fn_valid: The CFFI notmuch_*_valid function.
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:param fn_get: The CFFI notmuch_*_get function.
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:param fn_next: The CFFI notmuch_*_move_to_next function.
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"""
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_iter_p = MemoryPointer()
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def __init__(self, parent, iter_p,
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*, fn_destroy, fn_valid, fn_get, fn_next):
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self._parent = parent
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self._iter_p = iter_p
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self._fn_destroy = fn_destroy
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self._fn_valid = fn_valid
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self._fn_get = fn_get
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self._fn_next = fn_next
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def __del__(self):
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self._destroy()
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@property
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def alive(self):
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if not self._parent.alive:
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return False
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try:
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self._iter_p
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except errors.ObjectDestroyedError:
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return False
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else:
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return True
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def _destroy(self):
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if self.alive:
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try:
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self._fn_destroy(self._iter_p)
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except errors.ObjectDestroyedError:
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pass
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self._iter_p = None
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def __iter__(self):
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"""Return the iterator itself.
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Note that as this is an iterator and not a container this will
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not return a new iterator. Thus any elements already consumed
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will not be yielded by the :meth:`__next__` method anymore.
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"""
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return self
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def __next__(self):
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if not self._fn_valid(self._iter_p):
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self._destroy()
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raise StopIteration()
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obj_p = self._fn_get(self._iter_p)
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self._fn_next(self._iter_p)
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return obj_p
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def __repr__(self):
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try:
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self._iter_p
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except errors.ObjectDestroyedError:
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return '<NotmuchIter (exhausted)>'
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else:
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return '<NotmuchIter>'
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