issues: 484089737

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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
484089737 MDU6SXNzdWU0ODQwODk3Mzc= 3240 assign_coords adds coordinates without a new dimension if the value is scalar 2818208 closed 0     5 2019-08-22T16:14:05Z 2019-08-26T08:16:37Z 2019-08-26T08:16:37Z CONTRIBUTOR      

Problem

assign_corods correctly disallows creating a new dimension when assigning list-like coords with a name that does not match an existing dimension. However, it does allow this operation if the value is scalar.

MCVE

Consider the following DataArray:

```python

coords = {"fruit": ("x", ["apple", "banana"])} arr = xr.DataArray([[1, 2, 3], [4, 5, 6]], dims=("x", "y"), coords=coords) arr <xarray.DataArray (x: 2, y: 3)> array([[1, 2, 3], [4, 5, 6]]) Coordinates: fruit (x) <U6 'apple' 'banana' Dimensions without coordinates: x, y ```

I can assign new coordinates to an existing dimension:

```python

arr.assign_coords(color=("x", ["red", "yellow"])) <xarray.DataArray (x: 2, y: 3)> array([[1, 2, 3], [4, 5, 6]]) Coordinates: fruit (x) <U6 'apple' 'banana' color (x) <U6 'red' 'yellow' Dimensions without coordinates: x, y ```

And I cannot (correctly) assign coordinates to a new (nonexistent) dimension:

```python

arr.assign_coords(color=["red", "yellow"]) ... ValueError: cannot add coordinates with new dimensions to a DataArray ```

The above fails because Xarray, in the absence of an explicit dimension, tries to assign the new coordinates to a color dimension which does not exist. So far so good. But why does this work?

```python

arr = arr.assign_coords(color="red") arr <xarray.DataArray (x: 2, y: 3)> array([[1, 2, 3], [4, 5, 6]]) Coordinates: fruit (x) <U6 'apple' 'banana' color <U3 'red' Dimensions without coordinates: x, y ```

I would expect this to fail because color is not a dimension. But these appear to be newly added coordinates without a dimension?

```python

arr.coords Coordinates: fruit (x) <U6 'apple' 'banana' color <U3 'red' ```

 
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