id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
299346082,MDU6SXNzdWUyOTkzNDYwODI=,1935,Not compatible with PyPy and dask.array.,1102577,closed,0,,,11,2018-02-22T13:05:42Z,2018-02-26T23:22:37Z,2018-02-26T23:22:37Z,NONE,,,,"Hi,

#### Problem description
With a fresh pypy2 installation, i installed dask and xarray then tried to import xarray but it failed. According to the logs dask trigger a ""weak reference"" exception.

- Dask works by himself (with or without xarray installed)
- Without installing dask, xarray works.
- But both installed xarray don't work.

#### Question
Is there a compatibility problem with PyPy ? With the standard python everything works, but i have performance issues.

#### Example

```bash
$ pip install dask[array]
   ... OK
$ pip install xarray
   ... OK
$ pypy -c ""import dask""
  ... OK
$ pypy -c ""import xarray""
Traceback (most recent call last):
  File ""<module>"", line 1, in <module>
  File ""/opt/pypy/site-packages/xarray/__init__.py"", line 18, in <module>
    from .backends.api import (open_dataset, open_dataarray, open_mfdataset,
  File ""/opt/pypy/site-packages/xarray/backends/__init__.py"", line 6, in <module>
    from .common import AbstractDataStore
  File ""/opt/pypy/site-packages/xarray/backends/common.py"", line 31, in <module>
    GLOBAL_LOCK = Lock()
  File ""/opt/pypy/site-packages/dask/utils.py"", line 771, in __init__
    SerializableLock._locks[self.token] = self.lock
  File ""/opt/pypy/lib-python/2.7/weakref.py"", line 131, in __setitem__
    self.data[key] = KeyedRef(value, self._remove, key)
  File ""/opt/pypy/lib-python/2.7/weakref.py"", line 304, in __new__
    self = ref.__new__(type, ob, callback)
TypeError: cannot create weak reference to 'thread.lock' object

```

Thanks for your help","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/1935/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue