cosmo: `comoving_transverse_distance(z1, z2)` by nstarman · Pull Request #18803 · astropy/astropy

nstarman · 2025-10-30T02:29:04Z

Make the 2-argument form of comoving_transverse_distance public.

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github-actions · 2025-10-30T02:29:18Z

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github-actions · 2025-10-30T02:29:19Z

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github-actions · 2026-04-18T06:10:01Z

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Copilot

Pull request overview

This PR makes FLRW.comoving_transverse_distance support the public 2-argument calling form (z1, z2), aligning it with existing 2-argument distance APIs in astropy.cosmology.

Changes:

Update FLRW.comoving_transverse_distance to accept an optional second redshift argument (z2) and implement the (z1, z2) behavior directly.
Update cosmology test suite to use the public 2-argument method instead of the former private helper.
Add a cosmology changelog fragment documenting the new capability.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
`docs/changes/cosmology/18803.feature.rst`	Documents the new optional `z2` argument for `comoving_transverse_distance`.
`astropy/cosmology/_src/tests/funcs/test_funcs.py`	Switches round-trip tests to use the public 2-argument API.
`astropy/cosmology/_src/tests/flrw/test_lambdacdm.py`	Updates transverse distance tests to call the public 2-argument API.
`astropy/cosmology/_src/flrw/base.py`	Implements the optional-`z2` signature and updates related call sites/docstrings.

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nstarman · 2026-04-28T01:25:04Z

pre-commit.ci autofix

nstarman · 2026-04-28T02:21:01Z

@neutrinoceros, ready :)

nstarman · 2026-05-08T20:58:09Z

@neutrinoceros this PR simplifies the comoving_transverse_distance function stack while also enabling the 2-argument form.

neutrinoceros · 2026-05-21T13:12:17Z

-    def comoving_transverse_distance(self, z: u.Quantity | ArrayLike, /) -> u.Quantity:
-        r"""Comoving transverse distance in Mpc at a given redshift.
+    def comoving_transverse_distance(
+        self, z: _InputT, z2: _InputT | None = None, /


The annotations look either incorrect or at odds with how the arguments are actually handled. A type var implies that, as long as z2 != None, z2 and z1,always adhere to some common type (or interface); but then z1 is conditionally set to 0.0 (without any regard to z2's type).

I think relaxing the type annotations to be _InputT | float should work. WDYT?

unioning a typevar seems like an antipattern to me: in most scenarios, it just kills the benefits from a typevar, and in the rare cases it might be correct, it still looks fishy enough that I'd rather avoid it to save mental bandwidth in readers.
What type checker (if any) are you using to check these ?

In this case we always want to be able to accept floats. We also want to support higher types. What I think we want to discourage is a lot of type mixing.
I think that's a TypeVar|float.

I use mypy on Astropy and ty on pretty much everything else.

WDYT?

so TypeVar|float basically means "this can always be a float, but if it isn't then it needs to be consistent with other non-float variables that have this type" ? I think I get the intention but I'm not sure that type checkers understand this (or that it's supported by the spec). Do you have any source to back it up ? I'll play around with it and see what happens.

I use mypy on Astropy and ty on pretty much everything else.

Out of curiosity, I ran ty check astropy/cosmology and pyrefly check astropy/cosmology. ty seems to pick up many problems pyright does not, though I don't know wether they are real of false positives.

I've fixed all errors to do with cosmoving_transverse_distance

ty check astropy/cosmology 2>&1 | grep -A 5 -B 5 "comoving_transverse_distance"

neutrinoceros

sorry for taking so long to reply. On paper this is perfectly fine to me. The only important thing to fix IMO are the type annotations.

nstarman · 2026-05-26T22:23:21Z

Ping @neutrinoceros for thoughts.

nstarman · 2026-07-06T19:30:42Z

@neutrinoceros all ty complaints about comoving_transverse_distance specifically are now fixed: ty check astropy/cosmology 2>&1 | grep -A 5 -B 5 "comoving_transverse_distance"

Signed-off-by: nstarman <[email protected]>

…ccept float

Signed-off-by: nstarman <[email protected]>

neutrinoceros

Only minor comments here. LGTM otherwise

neutrinoceros · 2026-07-08T18:26:53Z


 @pytest.mark.skipif(not HAS_SCIPY, reason="test requires scipy")
-def test_comoving_transverse_distance_z1z2():
+def test_comoving_transverse_distance():


I think I would actually keep the original test name here

I like the new name. Unified function w/ 2 args.

Co-authored-by: Clément Robert <[email protected]>

nstarman · 2026-07-13T03:06:03Z

Thanks @neutrinoceros for the reviews!

nstarman · 2026-07-13T03:23:08Z

@neutrinoceros / @pllim can you confirm the CI failures are unrelated. Then I can bypass and merge. :) Thanks.

neutrinoceros · 2026-07-13T09:11:20Z

I confirm. These are known failures unrelated to your PR.

nstarman added this to the v8.0.0 milestone Oct 30, 2025

nstarman added this to Cosmology, the Expansion Oct 30, 2025

github-project-automation Bot moved this to Triage in Cosmology, the Expansion Oct 30, 2025

github-actions Bot added the cosmology label Oct 30, 2025

nstarman moved this from Triage to In Progress in Cosmology, the Expansion Oct 30, 2025

nstarman force-pushed the cosmo/api/comoving_transverse_distance_z1_z2 branch 2 times, most recently from b5a7998 to df807a5 Compare October 30, 2025 16:31

nstarman mentioned this pull request Oct 30, 2025

feat(cosmo): DistanceMeasures trait class #18808

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nstarman force-pushed the cosmo/api/comoving_transverse_distance_z1_z2 branch 2 times, most recently from 99bf161 to 1500054 Compare October 30, 2025 17:37

nstarman mentioned this pull request Nov 11, 2025

feat(cosmo): matter-only pedagogical cosmology #18876

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nstarman force-pushed the cosmo/api/comoving_transverse_distance_z1_z2 branch from 1500054 to 8194bc5 Compare November 16, 2025 18:30

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nstarman added keep-open and removed Close? Tell stale bot that this issue/PR is stale labels Apr 20, 2026

nstarman force-pushed the cosmo/api/comoving_transverse_distance_z1_z2 branch 5 times, most recently from 099554e to 44efc73 Compare April 27, 2026 15:16

nstarman marked this pull request as ready for review April 27, 2026 15:58

nstarman requested a review from a team as a code owner April 27, 2026 15:58

nstarman requested review from Copilot and neutrinoceros April 27, 2026 15:58

Copilot AI reviewed Apr 27, 2026

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Comment thread astropy/cosmology/_src/flrw/base.py Resolved

Comment thread astropy/cosmology/_src/flrw/base.py Resolved

Comment thread astropy/cosmology/_src/flrw/base.py Resolved

Comment thread astropy/cosmology/_src/flrw/base.py Resolved

astrofrog modified the milestones: v8.0.0, v8.1.0 May 5, 2026

nstarman force-pushed the cosmo/api/comoving_transverse_distance_z1_z2 branch from f023eb1 to 2b2170b Compare May 8, 2026 20:54

neutrinoceros reviewed May 21, 2026

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Comment thread astropy/cosmology/_src/flrw/base.py Resolved

neutrinoceros requested changes May 21, 2026

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github-project-automation Bot moved this from In Progress to Review in progress in Cosmology, the Expansion May 21, 2026

nstarman force-pushed the cosmo/api/comoving_transverse_distance_z1_z2 branch from 2b2170b to ad9afdd Compare May 21, 2026 14:19

nstarman requested a review from neutrinoceros May 22, 2026 02:51

nstarman added 3 commits July 6, 2026 15:36

feat(cosmo): comoving_transverse_distance(z1,z2)

127fc65

Signed-off-by: nstarman <[email protected]>

fix(cosmo): update comoving_transverse_distance method signature to a…

f9bbbf1

…ccept float

type: make ty happier

6689213

Signed-off-by: nstarman <[email protected]>

nstarman force-pushed the cosmo/api/comoving_transverse_distance_z1_z2 branch from e1b8e3c to 6689213 Compare July 6, 2026 19:36

neutrinoceros approved these changes Jul 8, 2026

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github-project-automation Bot moved this from Review in progress to Reviewer approved in Cosmology, the Expansion Jul 8, 2026

Update astropy/cosmology/_src/flrw/base.py

eeb4e4a

Co-authored-by: Clément Robert <[email protected]>

nstarman merged commit 49f3dab into astropy:main Jul 13, 2026
36 of 39 checks passed

nstarman deleted the cosmo/api/comoving_transverse_distance_z1_z2 branch July 13, 2026 13:43

github-project-automation Bot moved this from Reviewer approved to Done in Cosmology, the Expansion Jul 13, 2026

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