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Prevent unit equivalencies from interfering with an error message#19043

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astropy:mainfrom
eerovaher:skycoord-velocity-units-error-message
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Prevent unit equivalencies from interfering with an error message#19043
eerovaher wants to merge 2 commits into
astropy:mainfrom
eerovaher:skycoord-velocity-units-error-message

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@eerovaher eerovaher commented Dec 9, 2025

Description

If a user tries to create a SkyCoord with positions and velocities that have incompatible units then normally they get an informative error message:

>>> from astropy import units as u
>>> from astropy.coordinates import SkyCoord
>>> SkyCoord(0 * u.deg, 0 * u.deg, 1 * u.kpc, radial_velocity=1)
Traceback (most recent call last):
  ...
ValueError: distance has unit "kpc" with physical type "length", but radial_velocity has incompatible unit "" with physical type "dimensionless" instead of the expected "speed/velocity".

But unit equivalencies can interfere with that, in which case the error message will be quite cryptic:

>>> with u.set_enabled_equivalencies(u.doppler_redshift()):
...     SkyCoord(0 * u.deg, 0 * u.deg, 1 * u.kpc, radial_velocity=1)
... 
Traceback (most recent call last):
  ...
ValueError: For differential object '<RadialDifferential (d_distance) [dimensionless]
    (1.,)>', expected unit key = 'm' but received key = 's'

The problem can be solved by using physical types, which are not affected by unit equivalencies.

  • By checking this box, the PR author has requested that maintainers do NOT use the "Squash and Merge" button. Maintainers should respect this when possible; however, the final decision is at the discretion of the maintainer that merges the PR.

@eerovaher eerovaher added this to the v7.2.1 milestone Dec 9, 2025
@eerovaher eerovaher requested a review from mhvk December 9, 2025 20:59
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github-actions Bot commented Dec 9, 2025

Thank you for your contribution to Astropy! 🌌 This checklist is meant to remind the package maintainers who will review this pull request of some common things to look for.

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@eerovaher
Reveal a coordinate creation error message bug
bc5c457 
If a user tries to create a `SkyCoord` with positions and velocities
that have incompatible units then normally they get an informative error
message. The updated tests reveal that if the `u.doppler_redshift()`
unit equivalency is enabled then they get a much more cryptic error
message instead.
@eerovaher
Ensure the expected error message is always used
2148f34 
Previously trying to create a `SkyCoord` with positions and velocities
that have incompatible units could cause the error message to be
needlessly cryptic if unit equivalencies were enabled, but now the error
message for incompatible units is the intended one regardless of
equivalencies.
@eerovaher eerovaher force-pushed the skycoord-velocity-units-error-message branch from 5f3a62b to 2148f34 Compare December 9, 2025 21:03
@github-actions github-actions Bot added the Close? Tell stale bot that this issue/PR is stale label May 9, 2026
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github-actions Bot commented May 9, 2026

Hi humans 👋 - this pull request hasn't had any new commits for approximately 5 months. I plan to close this in a month if the pull request doesn't have any new commits by then.

In lieu of a stalled pull request, please consider closing this and open an issue instead if a reminder is needed to revisit in the future. Maintainers may also choose to add keep-open label to keep this PR open but it is discouraged unless absolutely necessary.

If this PR still needs to be reviewed, as an author, you can rebase it to reset the clock.

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