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
484499801,MDExOlB1bGxSZXF1ZXN0MzEwMzYxOTMz,3250,__slots__,6213168,closed,0,,,10,2019-08-23T12:16:44Z,2019-08-30T12:13:28Z,2019-08-29T17:14:20Z,MEMBER,,0,pydata/xarray/pulls/3250,"What changes:
- Most classes now define ``__slots__``
- removed ``_initialized`` property
- Enforced checks that all subclasses must also define ``__slots__``. For third-party subclasses, this is for now a DeprecationWarning and should be changed into a hard crash later on.
- 22% reduction in RAM usage
- 5% performance speedup for a DataArray method that performs a ``_to_temp_dataset`` roundtrip 

**DISCUSS:** support for third party subclasses is very poor at the moment (#1097). Should we skip the deprecation altogether?

Performance benchmark:
```python
import timeit
import psutil
import xarray

a = xarray.DataArray([1, 2], dims=['x'], coords={'x': [10, 20]})
RUNS = 10000
t = timeit.timeit(""a.roll(x=1, roll_coords=True)"", globals=globals(), number=RUNS)
print(""{:.0f} us"".format(t / RUNS * 1e6))

p = psutil.Process()
N = 100000
rss0 = p.memory_info().rss
x = [
    xarray.DataArray([1, 2], dims=['x'], coords={'x': [10, 20]})
    for _ in range(N)
]
rss1 = p.memory_info().rss
print(""{:.0f} bytes"".format((rss1 - rss0) / N))
```
Output:

| test          | env | master     | slots      |
|:-------------:|:---:|:----------:| ----------:|
| DataArray.roll | py35-min | 332 us     | 360 us     |
|  DataArray.roll | py37 | 354 us     | 337 us     |
| RAM usage of a DataArray | py35-min     | 2755 bytes | 2074 bytes |
| RAM usage of a DataArray | py37     | 1970 bytes | 1532 bytes |

The performance degradation on Python 3.5 is caused by the deprecation mechanism - see changes to common.py.

I honestly never realised that xarray objects are measured in kilobytes (vs. 32 bytes of underlying buffers!)

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467756080,MDExOlB1bGxSZXF1ZXN0Mjk3MzQwNTEy,3112,More annotations in Dataset,6213168,closed,0,,,10,2019-07-13T19:06:49Z,2019-08-01T10:41:51Z,2019-07-31T17:48:00Z,MEMBER,,0,pydata/xarray/pulls/3112,,"{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3112/reactions"", ""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull