Display more helpful error message when two fields strings are used in condition #2979
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mattijn
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Mar 25, 2023
altair/vegalite/v5/api.py
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| if_true.update(kwargs) | ||
| if isinstance(if_false, str): | ||
| raise ValueError( | ||
| "A field cannot be used in both the `if_true` and `if_false` branches of a condition. One of the them has to specify a `value` or `datum` definition." |
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branches of a condition is this the correct way to describe this? I've never heard of this before.
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I was hesitant about writing it like that too, but went with it because the VegaLite docs use something similar"
When using a conditional field definition, only value or datum may be specified as the else (outer) branch.
Do you think changing the error message to the following would be clearer?
A field cannot be used as both the
if_trueandif_falsevalues of a condition. One of the them has to specify avalueordatumdefinition.
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Yes, I think that's better (single them).
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mattijn
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Mar 26, 2023
Co-authored-by: Mattijn van Hoek <mattijn@gmail.com>
mattijn
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Mar 26, 2023
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This is an attempt to resolve #2935.
If there are other string that can be passed to conditions that are not references for fields, then that would be an issue for this PR, but I can't think of any on the top of my head. It could also be argued that maybe this should be fixed in VegaLite, but it seems helpful and straightforward to fix it directly in Altair.