cloudpipe/cloudpickle

Unable to pickle functions that are decorated with `functools.lru_cache()`

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#178 opened on Jun 20, 2018

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bughelp wanted

Description

Observed behavior

The cloudpickle module is unable to pickle functions that are cached via the functools.lru_cache decorator function.

The following minimal example triggers the issue on Python 3.6.5 and cloudpickle version 0.5.3:

# minimal.py
from functools import lru_cache
import cloudpickle

@lru_cache()
def cached_func():
    pass

cloudpickle.dumps(cached_func)

Resulting in the following traceback:

$ python minimal.py
Traceback (most recent call last):
  File "minimal.py", line 8, in <module>
    cloudpickle.dumps(cached_func)
  File "/Users/csadorf/miniconda3/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 895, in dumps
    cp.dump(obj)
  File "/Users/csadorf/miniconda3/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 268, in dump
    return Pickler.dump(self, obj)
  File "/Users/csadorf/miniconda3/lib/python3.6/pickle.py", line 409, in dump
    self.save(obj)
  File "/Users/csadorf/miniconda3/lib/python3.6/pickle.py", line 507, in save
    self.save_global(obj, rv)
  File "/Users/csadorf/miniconda3/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 642, in save_global
    return self.save_dynamic_class(obj)
  File "/Users/csadorf/miniconda3/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 497, in save_dynamic_class
    self.save_reduce(tp, (obj.__name__, obj.__bases__, type_kwargs), obj=obj)
AttributeError: 'functools._lru_cache_wrapper' object has no attribute '__bases__'

Versions:

$ python -V
Python 3.6.5 :: Anaconda, Inc.
$ python -c "import cloudpickle; print(cloudpickle.__version__)"
0.5.3

Expected behavior

The cloudpickle module should handle cached functions by either pickling or ignoring the cache or at least should fail with a descriptive error message.

Contributor guide

Tech stack
python
Domain
data
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Pythonpicklecloudpickle codebase
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55
Research direction
Investigate the cloudpickle.py file, focusing on the save global and save dynamic class methods (lines around 642 and 497). Understand how other function wrappers are handled, such as partial or wraps. The lru cache wrapper should be treated similarly, possibly by extracting the original function or by using a custom reducer. Look at the Python docs for functools.lru cache to understand its wrapper behavior. Also review any existing comments or PRs related to this issue for prior attempts.