Unavoidable warnings with pybind11 main branch

I am seeing warnings like

<ipython-input-4-1eb00ff78cf2>:1: MatplotlibDeprecationWarning: The x parameter as float was deprecated in Matplotlib 3.10 and will be removed in 3.12. Use int(x) instead.
  plt.show()
<ipython-input-4-1eb00ff78cf2>:1: MatplotlibDeprecationWarning: The y parameter as float was deprecated in Matplotlib 3.10 and will be removed in 3.12. Use int(y) instead.
  plt.show()

that come out on every draw ever for the simplest of plots (e.g. plt.gac(); plt.show()).

These are coming from

matplotlib/src/_backend_agg_wrapper.cpp

Lines 68 to 90 in 59eafb8

    
           if (auto value = std::get_if<double>(&vx)) { 
        
               auto api = py::module_::import("matplotlib._api"); 
        
               auto warn = api.attr("warn_deprecated"); 
        
               warn("since"_a="3.10", "name"_a="x", "obj_type"_a="parameter as float", 
        
                    "alternative"_a="int(x)"); 
        
               x = static_cast<int>(*value); 
        
           } else if (auto value = std::get_if<int>(&vx)) { 
        
               x = *value; 
        
           } else { 
        
               throw std::runtime_error("Should not happen"); 
        
           } 
        
           if (auto value = std::get_if<double>(&vy)) { 
        
               auto api = py::module_::import("matplotlib._api"); 
        
               auto warn = api.attr("warn_deprecated"); 
        
               warn("since"_a="3.10", "name"_a="y", "obj_type"_a="parameter as float", 
        
                    "alternative"_a="int(y)"); 
        
               y = static_cast<int>(*value); 
        
           } else if (auto value = std::get_if<int>(&vy)) { 
        
               y = *value; 
        
           } else { 
        
               throw std::runtime_error("Should not happen"); 
        
           }

these warnings triggering are due to a change in pybind11 (only see this with their development branch) and which I bisected to

e8e8d6ab22f39938607f31d457d4d359c130f078 is the first bad commit
commit e8e8d6ab22f39938607f31d457d4d359c130f078
Author: Michael Carlstrom <rmc@carlstrom.com>
Date:   Mon Feb 16 23:00:01 2026 -0800

which came in via

matplotlib/src/_backend_agg_wrapper.cpp

Lines 68 to 90 in 59eafb8

    
           if (auto value = std::get_if<double>(&vx)) { 
        
               auto api = py::module_::import("matplotlib._api"); 
        
               auto warn = api.attr("warn_deprecated"); 
        
               warn("since"_a="3.10", "name"_a="x", "obj_type"_a="parameter as float", 
        
                    "alternative"_a="int(x)"); 
        
               x = static_cast<int>(*value); 
        
           } else if (auto value = std::get_if<int>(&vx)) { 
        
               x = *value; 
        
           } else { 
        
               throw std::runtime_error("Should not happen"); 
        
           } 
        
           if (auto value = std::get_if<double>(&vy)) { 
        
               auto api = py::module_::import("matplotlib._api"); 
        
               auto warn = api.attr("warn_deprecated"); 
        
               warn("since"_a="3.10", "name"_a="y", "obj_type"_a="parameter as float", 
        
                    "alternative"_a="int(y)"); 
        
               y = static_cast<int>(*value); 
        
           } else if (auto value = std::get_if<int>(&vy)) { 
        
               y = *value; 
        
           } else { 
        
               throw std::runtime_error("Should not happen"); 
        
           }

pybind/pybind11#5879

We either need to sort out how to adjust the pybind11 code or remove this warning.

@tacaswell

@tacaswell This seems to be caused by pybind11 resolving values as double, so std::get_if matches even for integers. We should either prioritize int over double in the variant handling or only warn when the float value is non-integral. Alternatively, fixing pybind11 to preserve integer types during casting would avoid this issue

Should we report an issue upstream first? From the upstream PR, this affects noconvert args as well, so I don't know how we'd modify the type conversion priority.

I am not sure I have enough of a grasp on the problem to make a coherent upstream bug report nor the ability to write a minimal reproducer for them.

I wrote up a minimal reproducer:

#include <pybind11/pybind11.h>
#include <pybind11/stl.h>

#include <variant>

namespace py = pybind11;
using namespace pybind11::literals;

template <typename T>
using double_or_ = std::variant<double, T>;

static auto
check_type(double_or_<int> var)
{
    if (auto value = std::get_if<int>(&var)) {
        return "int";
    } else if (auto value = std::get_if<double>(&var)) {
        return "double";
    } else {
        // pybind11 will have only allowed types that match the variant, so this `else`
        // can't happen. We only have this case because older macOS doesn't support
        // `std::get` and using the conditional `std::get_if` means an `else` to silence
        // compiler warnings about "unhandled" cases.
        throw std::runtime_error("Should not happen");
    }
}

PYBIND11_MODULE(check, m)
{
    m.def("check_type", &check_type, "var"_a.noconvert(true));
}

but as I was writing up the issue, I had an idea: if we swap the order of the std::variant, then hopefully pybind11 will pick the non-double option first, and it seems to have worked in my testing.

Re-order variants to prioritize narrower types

Re-order variants to prioritize narrower types (#31504)

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Unavoidable warnings with pybind11 main branch #31495

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	if (auto value = std::get_if<double>(&vx)) {
	auto api = py::module_::import("matplotlib._api");
	auto warn = api.attr("warn_deprecated");
	warn("since"_a="3.10", "name"_a="x", "obj_type"_a="parameter as float",
	"alternative"_a="int(x)");
	x = static_cast<int>(*value);
	} else if (auto value = std::get_if<int>(&vx)) {
	x = *value;
	} else {
	throw std::runtime_error("Should not happen");
	}

	if (auto value = std::get_if<double>(&vy)) {
	auto api = py::module_::import("matplotlib._api");
	auto warn = api.attr("warn_deprecated");
	warn("since"_a="3.10", "name"_a="y", "obj_type"_a="parameter as float",
	"alternative"_a="int(y)");
	y = static_cast<int>(*value);
	} else if (auto value = std::get_if<int>(&vy)) {
	y = *value;
	} else {
	throw std::runtime_error("Should not happen");
	}

Unavoidable warnings with pybind11 main branch #31495

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Chirag3841 commented on Apr 13, 2026

QuLogic commented on Apr 13, 2026

tacaswell commented on Apr 14, 2026

QuLogic commented on Apr 16, 2026

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