html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/pull/4064#issuecomment-633710066,https://api.github.com/repos/pydata/xarray/issues/4064,633710066,MDEyOklzc3VlQ29tbWVudDYzMzcxMDA2Ng==,56925856,2020-05-25T20:38:49Z,2020-05-25T20:38:49Z,CONTRIBUTOR,No problem ! Thanks everyone for helping me get up to speed :),"{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,618828102
https://github.com/pydata/xarray/pull/4064#issuecomment-632128807,https://api.github.com/repos/pydata/xarray/issues/4064,632128807,MDEyOklzc3VlQ29tbWVudDYzMjEyODgwNw==,56925856,2020-05-21T14:49:37Z,2020-05-21T14:49:37Z,CONTRIBUTOR,"@keewis thanks for this! I've added what I think is a suitable test for `DataArray`s, do you think it's also a good idea to have a `DataSet` test? ","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,618828102
https://github.com/pydata/xarray/pull/4064#issuecomment-632090407,https://api.github.com/repos/pydata/xarray/issues/4064,632090407,MDEyOklzc3VlQ29tbWVudDYzMjA5MDQwNw==,56925856,2020-05-21T13:36:41Z,2020-05-21T13:36:41Z,CONTRIBUTOR,"> This could test that `dataarray.chunk(""auto"").data` is the same as `dataarray.data.rechunk(""auto"")` (or something like that).

@dcherian Thanks for the tip:) Quick question: Is there a reason why you're specifying the `.data` here? Also I think I'm missing something because I don't get what the difference between `.chunk()` and `.rechunk()` would be in this case. ","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,618828102
https://github.com/pydata/xarray/pull/4064#issuecomment-632035116,https://api.github.com/repos/pydata/xarray/issues/4064,632035116,MDEyOklzc3VlQ29tbWVudDYzMjAzNTExNg==,56925856,2020-05-21T11:30:01Z,2020-05-21T11:30:01Z,CONTRIBUTOR,"Cheers! I forgot about the tests, will add them this week or next hopefully","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,618828102
https://github.com/pydata/xarray/pull/4064#issuecomment-629390609,https://api.github.com/repos/pydata/xarray/issues/4064,629390609,MDEyOklzc3VlQ29tbWVudDYyOTM5MDYwOQ==,56925856,2020-05-15T17:40:39Z,2020-05-15T17:41:25Z,CONTRIBUTOR,"@dcherian do you have any idea about this `mypy` Type error? I can't find much (accessible) documentation on how the `Union[]` is working in this context. 

```
xarray/core/dataset.py:1737: error: Argument 2 to ""fromkeys"" of ""dict"" has incompatible type ""Union[Number, Mapping[Hashable, Union[None, Number, Tuple[Number, ...]]]]""; expected ""Union[None, Number, Tuple[Number, ...]]""
xarray/core/dataset.py:1740: error: Item ""Number"" of ""Union[Number, Mapping[Hashable, Union[None, Number, Tuple[Number, ...]]]]"" has no attribute ""keys""
```

Edit: thanks to everyone for your help so far! ","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,618828102
https://github.com/pydata/xarray/pull/4064#issuecomment-629346101,https://api.github.com/repos/pydata/xarray/issues/4064,629346101,MDEyOklzc3VlQ29tbWVudDYyOTM0NjEwMQ==,56925856,2020-05-15T16:11:04Z,2020-05-15T16:22:24Z,CONTRIBUTOR,"Okay so I've traced the error back to the `map_blocks()` function. I don't fully understand the code for this function in `xarray/core/parallel.py`, but here's a quick report on the different behaviours.

Normally, when using the `make_ds()` and `make_da()` functions in `test_dask.py`, **without any changes to `ds.chunk()`** we have:

```python
>>> def func(obj):
...         result = obj + obj.x + 5 * obj.y
...         return result
...
>>> xr.map_blocks(func, ds).unify_chunks().chunks
Frozen(SortedKeysDict({'x': (4, 4, 2), 'y': (5, 5, 5, 5), 'z': (4,)}))
>>> func(ds).chunk().unify_chunks().chunks
Frozen(SortedKeysDict({'x': (4, 4, 2), 'y': (5, 5, 5, 5), 'z': (4,)}))
```

However, when I use the changes I've made to `dataset.py` (changing `isinstance(chunks, Number)` to `is_scalar(chunks)`), the behaviour becomes:

```python
>>> xr.map_blocks(func, ds).unify_chunks().chunks
Frozen(SortedKeysDict({'x': (4, 4, 2), 'y': (5, 5, 5, 5), 'z': (4,)}))
>>> func(ds).chunk().unify_chunks().chunks
Frozen(SortedKeysDict({'x': (10,), 'y': (20,), 'z': (4,)}))
```

Which means that it now fails the `test_map_blocks()` call in `test_dask.py` line 1077. 

I've tried to follow through the code and see what is actually happening when this change is made, but I'm out of my depth here. My guess is that `is_scalar(chunks)` is giving the wrong behaviour when `chunks=None` ?

**Edit: I think that's the problem!**  
```python
>>> isinstance(None, numbers.Number)
False
>>> is_scalar(None)
True
```

I'll add in something to catch `None`s and see if it fixes the error... 
","{""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,618828102
https://github.com/pydata/xarray/pull/4064#issuecomment-629197362,https://api.github.com/repos/pydata/xarray/issues/4064,629197362,MDEyOklzc3VlQ29tbWVudDYyOTE5NzM2Mg==,56925856,2020-05-15T12:05:22Z,2020-05-15T12:05:22Z,CONTRIBUTOR,No unpushed commits ,"{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,618828102
https://github.com/pydata/xarray/pull/4064#issuecomment-629191037,https://api.github.com/repos/pydata/xarray/issues/4064,629191037,MDEyOklzc3VlQ29tbWVudDYyOTE5MTAzNw==,56925856,2020-05-15T11:49:23Z,2020-05-15T11:49:23Z,CONTRIBUTOR,"Do you mean the master merge? If that's wrong would you be able to fix it for me? My bad, hopefully i'll be able to do it more cleanly in future","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,618828102
https://github.com/pydata/xarray/pull/4064#issuecomment-629168282,https://api.github.com/repos/pydata/xarray/issues/4064,629168282,MDEyOklzc3VlQ29tbWVudDYyOTE2ODI4Mg==,56925856,2020-05-15T10:49:43Z,2020-05-15T10:49:43Z,CONTRIBUTOR,"Okay, that makes sense. Though, it seems that I forked the master branch before @kmuehlbauer's commit, which fixed this flake8 issue? So I think I need to make a new fork?","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,618828102
https://github.com/pydata/xarray/pull/4064#issuecomment-629154336,https://api.github.com/repos/pydata/xarray/issues/4064,629154336,MDEyOklzc3VlQ29tbWVudDYyOTE1NDMzNg==,56925856,2020-05-15T10:15:50Z,2020-05-15T10:17:38Z,CONTRIBUTOR,"Okay cheers both! I'll have a look at these now. 

@keewis sorry I'm still getting used to using this side of Git at the moment, could you clarify what you mean by merge `master` ? Do you mean merge with my local `master`? ","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,618828102
https://github.com/pydata/xarray/pull/4064#issuecomment-629147818,https://api.github.com/repos/pydata/xarray/issues/4064,629147818,MDEyOklzc3VlQ29tbWVudDYyOTE0NzgxOA==,56925856,2020-05-15T10:00:35Z,2020-05-15T10:01:43Z,CONTRIBUTOR,"In my git clone, when I run the `flake8` and `black .` tests, I get the following messages.

```
(xarray-tests) Andrews-MacBook-Pro-2:xarray andrewwilliams$ black .
All done! ✨ 🍰 ✨
143 files left unchanged.
(xarray-tests) Andrews-MacBook-Pro-2:xarray andrewwilliams$ flake8
./xarray/backends/memory.py:43:32: E741 ambiguous variable name 'l'
./xarray/backends/common.py:244:32: E741 ambiguous variable name 'l'
./xarray/backends/.ipynb_checkpoints/memory-checkpoint.py:43:32: E741 ambiguous variable name 'l'
```

I'm not sure why something has changed in these files (I haven't touched them), I also can't work out what the `l` variable is meant to be doing there. 

Could this somehow be associated with loads of the checks failing below? Thanks! :) ","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,618828102