New submission from Jason R. Coombs:
Beginning with Python 3.5, an lru_cache no longer survives a deepcopy.
```
$ cat >> cache_copy.py
import copy
from functools import lru_cache
foo = lru_cache()(lambda: None)
copy.deepcopy(foo)
$ python3.5 cache_copy.py
Traceback (most recent call last):
File "cache_copy.py", line 4, in <module>
copy.deepcopy(foo)
File "/usr/lib/python3.5/copy.py", line 182, in deepcopy
y = _reconstruct(x, rv, 1, memo)
File "/usr/lib/python3.5/copy.py", line 293, in _reconstruct
y = callable(*args)
File "/usr/lib/python3.5/copyreg.py", line 88, in __newobj__
return cls.__new__(cls, *args)
TypeError: Required argument 'user_function' (pos 1) not found
```
I suspect this is due to the C implementation per issue14373.
I realize one's first reaction might be "why are you deep copying a function,"
so here's the [downstream
bug](https://bitbucket.org/jaraco/jaraco.functools/issues/1/method_cache-causes-typeerror-when)
and [offended
code](https://bitbucket.org/jaraco/jaraco.functools/src/68d1fda21af7e7b4c36577f953f382270bdc1e05/jaraco/functools.py?at=default&fileviewer=file-view-default#functools.py-72:138).
That decorator is designed to wrap a method and to store the cache on the
instance object, an object which is liable to be deep copied. As a result,
production code using this technique is failing on Python 3.5.
Although it was apparently an artifact of previous implementations that the
cache happened to be deep-copyable, I hope this expectation can be restored
with minimal fuss.
----------
assignee: rhettinger
components: Library (Lib)
keywords: 3.5regression
messages: 253233
nosy: jason.coombs, rhettinger
priority: high
severity: normal
status: open
title: TypeError invoking deepcopy on lru_cache
type: behavior
versions: Python 3.5, Python 3.6
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue25447>
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