# Disable default rng

**URL:** <https://discourse.julialang.org/t/disable-default-rng/128790>\
**Category:** General Usage\
**Tags:** question, random\
**Created:** [May 7, 2025, 12:25pm UTC](https://discourse.julialang.org/t/disable-default-rng/128790 "2025-05-07T12:25:52Z")\
**Posts on this page:** 11\
**Page:** 1

<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [May 7, 2025, 12:25pm UTC](https://discourse.julialang.org/t/disable-default-rng/128790/1 "2025-05-07T12:25:52Z")

</div>

In a test suite I want to make sure that the default RNG is never called (clarification: even indirectly, somewhere within a package), ie that all calls to `rand` & friends have an explicit rng as their first argument.

[This old topic](https://discourse.julialang.org/t/disable-global-rng/20690) has some hacks, but the one I suggested is outdated (`GLOBAL_RNG` is no longer part of the API, and code may be threaded), and the other relies on redefining `rand`, which does not help as the code I am testing is in a package I don’t want to modify like this (recall, this is for CI).

An API that would allow setting the default RNG to an instance of

```julia
struct DummyRNG <: AbstractRNG end # I define no methods so it errors

```

would solve the problem, but I there is no `default_rng!`.

I am interested in all suggestions which would work in 1.10+, even hacks.

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<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [May 7, 2025, 12:34pm UTC](https://discourse.julialang.org/t/disable-default-rng/128790/2 "2025-05-07T12:34:58Z")

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> [@Tamas\_Papp](#):
>
> I am interested in all suggestions which would work in 1.10+, even hacks.

A hack would be to overwrite the method definition of `Random.default_rng()`:

```julia
julia> Random.default_rng() = nothing

julia> rand()
ERROR: MethodError: no method matching rand(::Nothing, ::Type{Float64})
The function `rand` exists, but no method is defined for this combination of argument types.
[...]

julia> rand(Xoshiro())
0.7427065090263826

```

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<div class="post-metadata">

**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [May 7, 2025, 12:47pm UTC](https://discourse.julialang.org/t/disable-default-rng/128790/3 "2025-05-07T12:47:06Z")

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Is the intention to make sure that CI runs are deterministic by disallowing calls to `rand` involving that RNG?

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<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [May 7, 2025, 12:52pm UTC](https://discourse.julialang.org/t/disable-default-rng/128790/4 "2025-05-07T12:52:16Z")

</div>

> [@stevengj](#):
>
> A hack would be to overwrite the method definition of `Random.default_rng()`:

I thought of that too, but unfortunately `@testset` also uses it:

```julia
julia> Random.default_rng() = error("disabled for testing") # overwrite so it errors

julia> @testset "foo" begin
       @test 1 + 1 == 2
       end
ERROR: disabled for testing
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] default_rng()
   @ Main ./REPL[36]:1
 [3] macro expansion
   @ ~/.julia/juliaup/julia-1.11.5+0.x64.linux.gnu/share/julia/stdlib/v1.11/Test/src/Test.jl:1698 [inlined]
 [4] top-level scope
   @ REPL[37]:2

```

(if you return `nothing` instead of erroring, it will just complain that it cannot `copy` it, etc, the setup & teardown code is quite hard to fool)

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<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [May 7, 2025, 12:53pm UTC](https://discourse.julialang.org/t/disable-default-rng/128790/5 "2025-05-07T12:53:09Z")

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> [@Sukera](#):
>
> Is the intention to make sure that CI runs are deterministic by disallowing calls to `rand` involving that RNG?

No, the intention is to test that all computations that use random numbers do so with an explicit RNG.

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<div class="post-metadata">

**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [May 7, 2025, 1:01pm UTC](https://discourse.julialang.org/t/disable-default-rng/128790/6 "2025-05-07T13:01:13Z")

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I see - you could create a copy of the default rng before & after the test and see whether it changed. That won’t catch rng in spawned tasks (the splitting RNG used for that is not exposed in the API, it’s hidden state in the runtime), but should cover any calls to `rand` in the original task.

Disallowing external RNG entirely is unfortunately quite difficult to do without some form of hook into the runtime.

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<div class="post-metadata">

**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [May 7, 2025, 1:11pm UTC](https://discourse.julialang.org/t/disable-default-rng/128790/7 "2025-05-07T13:11:15Z")

</div>

Btw, with “deterministic” I basically meant the same thing as you did - if all RNG is only controlled by the instance that’s being passed in explicitly, passing in the same exact RNG (including internal state!) should lead to the same exact result when the test is executed twice. If the results differ, the test must have accessed some other RNG/source of non-determinism.

Detecting this kind of non-determinism is an upcoming feature of the next release of Supposition.jl, which is currently WIP. It’s not a drop-in solution though.

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<div class="post-metadata">

**Author:** ![rfourquet](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rfourquet/32/3610_2.png) [@rfourquet](https://discourse.julialang.org/u/rfourquet)\
**Post date:** [May 7, 2025, 1:12pm UTC](https://discourse.julialang.org/t/disable-default-rng/128790/8 "2025-05-07T13:12:19Z")

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Here is a version which at least doesn’t completely break `@testset`:

```julia
Random.rand(::TaskLocalRNG, ::Random.SamplerType{UInt64}) = error("disabled")

```

This is the primary generating method for `TaskLocalRNG`, so it will block any other rand method.

Compared to the excellent other suggestion overwriting `default_rng()`, this will also catch uses in library via `GLOBAL_RNG`, which is set at startup to `TaskLocalRNG()` (like in `somemethod(rng=Random.GLOBAL_RNG)`).

EDIT: for clarification, `GLOBAL_RNG` was never part of the API, but was obvioulsy used by libraries. The API is now `default_rng()`, but to not break these libraries, `GLOBAL_RNG` was set equal to `TaskLocalRNG()`.

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<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [May 7, 2025, 2:20pm UTC](https://discourse.julialang.org/t/disable-default-rng/128790/9 "2025-05-07T14:20:47Z")

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> [@Tamas\_Papp](#):
>
> I thought of that too, but unfortunately `@testset` also uses it:

You can hack around that by returning your `DummyRNG()` instance, and defining:

```julia
using Random
struct DummyRNG <: AbstractRNG end
Random.default_rng() = DummyRNG()
Base.copy(rng::DummyRNG) = rng
Base.copy!(::DummyRNG, rng::AbstractRNG) = nothing

```

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<div class="post-metadata">

**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [May 8, 2025, 8:10am UTC](https://discourse.julialang.org/t/disable-default-rng/128790/10 "2025-05-08T08:10:03Z")

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> [@rfourquet](#):
>
> This is the primary generating method for `TaskLocalRNG`, so it will block any other rand method.

Can you please explain why this works? Is it because implicitly, `default_rng()` returns a `TaskLocalRNG`?

(It looks like a reasonable workaround, I am just checking if I am relying on something that is documented, or internals)

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<div class="post-metadata">

**Author:** ![rfourquet](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rfourquet/32/3610_2.png) [@rfourquet](https://discourse.julialang.org/u/rfourquet)\
**Post date:** [May 8, 2025, 11:26am UTC](https://discourse.julialang.org/t/disable-default-rng/128790/11 "2025-05-08T11:26:22Z")

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Yes, `default_rng()` returns `TaskLocalRNG()`, probably since `TaskLocalRNG()` was introduced, so is true in particular since v1.10 (which is what you wanted to support iirc). It’s not a contract though, and could change in the future, but I don’t see a change coming soon.

I think `default_rng()` was introduced when we started to have a thread-safe “global RNG” implemented with multiple `MersenneTwister` objects, once per thread, in v1.13. The old undocumented `Random.GLOBAL_RNG` was not the correct object to use anymore; in contrast, `default_rng()` could look-up the thread ID and return a valid object.