add_enabled_units leaves a registry context active when alias validation raises

Description

add_enabled_units can leave a unit-registry context active when enabling a conflicting alias raises. After an enclosing application's unit context exits, its custom definition unexpectedly remains enabled.

Steps to reproduce

In a fresh process, without PyUnitWizard or any other consumer library:

from astropy import units as u

# u.Unit('kcal') initially raises ValueError in the default registry.
bad = u.def_unit('kcal', 1000 * u.J)
with u.add_enabled_units([bad]):
    try:
        with u.add_enabled_units([u.imperial.kcal]):
            pass
    except ValueError:
        pass

# Expected: the default registry was restored, so this raises ValueError.
# Observed: it succeeds and prints 1000.0.
print(u.Unit('kcal').to(u.J))

Expected and observed behavior

The failed inner activation should preserve the pre-call registry, and the successful outer context should restore the original registry on exit. Instead the custom kcal definition remains active after the outer context exits.

Source inspection suggests that the public context factory constructs/pushes _UnitContext before validating the supplied aliases. If validation raises, the caller never receives the new context manager to unwind; the enclosing application's exit then pops the inner level instead. The current development source appears to retain that construction order; I have not executed this reproducer on a development checkout.

Versions and consumer scope

The Astropy-only reproducer above was executed with Python 3.14.7 / Astropy 8.0.1. The corresponding consumer regression also executes with Python 3.11.18 / Astropy 6.1.7. The finding arose while implementing bounded calorie interoperability in uibcdf/pyunitwizard#137. PyUnitWizard now prevalidates native caloric aliases before entering the context, so it avoids the failing constructor path without mutating private registries. This report concerns the public context's exception-restoration behavior, not whether imperial units should be enabled by default.

No external scientific dataset or credentials are involved.

Indeed, I see this behavior in astropy v8.1.dev as well.

with u.add_enabled_units([bad]):
    with u.add_enabled_units([u.imperial.kcal]):   # <--- Error
        print("inside", u.Unit("kcal"))  # <--- Never runs
    print("outside", u.Unit("kcal"))  # <--- Never runs
print("ooops", u.Unit("kcal"))   # <--- kcal still there

ValueError: Object with name 'kcal' already exists in namespace. Filter the set of units to avoid name clashes before enabling them.

@mhvk

I am actually not sure if it is ever designed to be used nested like that. It changes the global registry in real time. And even though astropy.units.core.add_enabled_units() says you can use it with with, it is not marked explicitly as a context manager, nor has context enter/exit coded in, nor does it yield. @mhvk , maybe you can clarify?

Thanks for reporting the bug (and I agree it is a bug). You are right about where the problem is. The simplest solution is that of #20795, but it means basically working around what really is a design flaw. I'd rather update _UnitContext to be more helpful, so that, e.g., code would become something like,

class _UnitContext(ContextDecorator):
    def __init__(self, init=[], equivalencies=[], command=None, *args, **kwargs):
        new_registry = _UnitRegistry(init=init, equivalencies=equivalencies)
        getattr(new_registry, command)(*args, **kwargs)
        _unit_registries.append(new_registry)
...


def add_enabled_units(units: object) -> _UnitContext:
    return _UnitContext(get_current_unit_registry(), command="add_enabled_units), units)

Note that I didn't test it, but it would seem cleaner.

@mhvk

@mhvk does #20803 look about right?

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add_enabled_units leaves a registry context active when alias validation raises #20790

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add_enabled_units leaves a registry context active when alias validation raises #20790

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pllim commented on Nov 5, 2026

pllim commented on Nov 5, 2026

mhvk commented on Nov 6, 2026

pllim commented on Nov 7, 2026

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