Help deciphering code for swap_dims in dataset.py

[DanielAdriaansen]

I was working on some code dealing with dimension names. Here is a toy example:

import xarray as xr
da1 = xr.DataArray([[20,30,40],[50,60,70],[80,90,100]],dims=['x0','y0'],coords={'x0':[0,1,2],'y0':[0,1,2]})
da2 = xr.DataArray([[20,30,40],[50,60,70],[80,90,100]],dims=['x0','y1'],coords={'x0':[0,1,2],'y1':[0,1,2]})
ds = xr.Dataset(data_vars={'da1':da1,'da2':da2})
print(ds)

<xarray.Dataset>
Dimensions:  (x0: 3, y0: 3, y1: 3)
Coordinates:
  * x0       (x0) int64 0 1 2
  * y0       (y0) int64 0 1 2
  * y1       (y1) int64 0 1 2
Data variables:
    da1      (x0, y0) int64 20 30 40 50 60 70 80 90 100
    da2      (x0, y1) int64 20 30 40 50 60 70 80 90 100

In this case, I know that y0 = y1, and so I want to simply "swap" y1, with y0 everywhere. Something like:
ds.swap_dims({'y1':'y0'}).drop_vars(['y1'])

So to start, using 2023.1.0, I tried this:

ds.swap_dims({'y1':'y0'})

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/dadriaan/.conda/envs/icing/lib/python3.9/site-packages/xarray/core/dataset.py", line 3559, in swap_dims
    raise ValueError(
ValueError: replacement dimension 'y0' is not a 1D variable along the old dimension 'y1'

So instead, I tried this:

ds.rename_dims({'y1':'y0'})

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/dadriaan/.conda/envs/icing/lib/python3.9/site-packages/xarray/core/dataset.py", line 3439, in rename_dims
    raise ValueError(
ValueError: Cannot rename y1 to y0 because y0 already exists. Try using swap_dims instead.

If I look at dataset.py", line 3559, in swap_dims, I see this code:

        dims_dict = either_dict_or_kwargs(dims_dict, dims_kwargs, "swap_dims")
        for k, v in dims_dict.items():
            if k not in self.dims:
                raise ValueError(
                    f"cannot swap from dimension {k!r} because it is "
                    "not an existing dimension"
                )
            if v in self.variables and self.variables[v].dims != (k,):
                raise ValueError(
                    f"replacement dimension {v!r} is not a 1D "
                    f"variable along the old dimension {k!r}"
                )

If I simply comment out that second if block, then ds.swap_dims() works for my case. I was hoping someone could help me understand the situation or case that the second if block here:

            if v in self.variables and self.variables[v].dims != (k,):
                raise ValueError(
                    f"replacement dimension {v!r} is not a 1D "
                    f"variable along the old dimension {k!r}"
                )

is trying to catch or handle? I don't understand what the ValueError is trying to tell me nor do I follow the if statement. I'd like to try and improve the code so that my case of ds.swap_dims() works but don't want to simply remove code because presumably it's designed to catch something I am not thinking of.

On the other hand, I am aware there seems to be a push toward deprecating swap_dims() (#2838), but I would argue that there is a case for each:

1. swap_dims --> swap one existing dimension with another existing dimension
2. rename_dims --> rename an existing dimension with a new name that does not already exist

Thanks in advance for any help!