fix bug when calling propertynames on a different thread with gil handled properly (#770)

the fix is to do the propertynames logic directly on the current thread when it already holds the gil, rather than using on_main_thread

Co-authored-by: Christopher Rowley <github.com/cjdoris>

Author: cjdoris

CHANGELOG.md

@@ -3,8 +3,7 @@
 ## Unreleased
 * Add configuration via Preferences in addition to environment variables (e.g. `exe`
   rather than `JULIA_PYTHONCALL_EXE`.)
-* Internals: removed the cache of unused Python objects. This makes `pydel!` faster and
-  removes a race condition in free-threaded python.
+* Bug fixes.
 
 ## 0.9.32 (2026-05-14)
 * Added `juliacall.TypeValue.__numpy_dtype__` attribute to allow converting Julia types

src/Core/Py.jl

@@ -270,30 +270,35 @@ Base.hasproperty(x::Py, k::String) = pyhasattr(x, k)
 Base.setproperty!(x::Py, k::Symbol, v) = pysetattr(x, string(k), v)
 Base.setproperty!(x::Py, k::String, v) = pysetattr(x, k, v)
 
-function Base.propertynames(x::Py, private::Bool = false)
-    properties = C.on_main_thread() do
-        # this follows the logic of rlcompleter.py
-        function classmembers(c)
-            r = pydir(c)
-            if pyhasattr(c, "__bases__")
-                for b in c.__bases__
-                    r = pyiadd(r, classmembers(b))
-                end
+function _propertynames(x::Py, private::Bool)
+    # this follows the logic of rlcompleter.py
+    function classmembers(c)
+        r = pydir(c)
+        if pyhasattr(c, "__bases__")
+            for b in c.__bases__
+                r = pyiadd(r, classmembers(b))
             end
-            return r
         end
+        return r
+    end
 
-        words = pyset(pydir(x::Py))
-        words.discard("__builtins__")
-        if pyhasattr(x, "__class__")
-            words.add("__class__")
-            words.update(classmembers(x.__class__))
-        end
-        map(pystr_asstring, words)
-    end::Vector{String} # explicit type since on_main_thread() is type-unstable
+    words = pyset(pydir(x::Py))
+    words.discard("__builtins__")
+    if pyhasattr(x, "__class__")
+        words.add("__class__")
+        words.update(classmembers(x.__class__))
+    end
+    return Symbol[Symbol(pystr_asstring(word)) for word in words]
+end
 
-    # private || filter!(w->!startswith(w, "_"), words)
-    map(Symbol, properties)
+function Base.propertynames(x::Py, private::Bool = false)
+    if C.PyGILState_Check() == 1
+        _propertynames(x, private)
+    else
+        C.on_main_thread() do
+            _propertynames(x, private)
+        end::Vector{Symbol}
+    end
 end
 
 Base.Bool(x::Py) = pytruth(x)

test/Core.jl

@@ -832,12 +832,23 @@ end
 end
 
 @testitem "propertynames" begin
-    x = pyint(7)
-    task = Threads.@spawn propertynames(x)
-    properties = propertynames(x)
-    @test :__init__ in properties
-    prop_task = fetch(task)
-    @test properties == prop_task
+    @testset "basic" begin
+        x = pyint(7)
+        task = Threads.@spawn propertynames(x)
+        properties = propertynames(x)
+        @test :__init__ in properties
+        prop_task = fetch(task)
+        @test properties == prop_task
+    end
+    @testset "gil (#751)" begin
+        o = pyint(751)
+        t = Base.Threads.@spawn begin
+            PythonCall.GIL.@lock begin
+                propertynames(o)
+            end
+        end
+        PythonCall.GIL.@unlock wait(t)
+    end
 end
 
 @testitem "on_main_thread" begin