fix: build a write-only Python property from its setter alone [patch]

Coder.Test/Languages/PythonGeneratedSourceImportsTests.cs

@@ -196,6 +196,58 @@ assert isinstance(shapes.Point.zero(), shapes.Point)
 		});
 	}
 
+	/// <summary>
+	/// Tests that a class with a property that can be written but not read can be loaded and set.
+	/// </summary>
+	/// <remarks>
+	/// <c>@level.setter</c> names the property a getter made, so a property with no getter needs
+	/// another spelling or the class statement raises <c>NameError</c>.
+	/// </remarks>
+	[TestMethod]
+	public void WriteOnlyProperty_ImportsAndSets()
+	{
+		string? python = ToolchainHarness.FindOnPath("--version", "python3", "python");
+		if (python is null)
+		{
+			Assert.Inconclusive("No Python interpreter on the path, so nothing was run.");
+			return;
+		}
+
+		ClassDeclaration sink = new("Sink");
+		PropertyDeclaration level = new("level", "int") { HasGetter = false, HasSetter = true };
+		CallExpression record = new("record");
+		record.Arguments.Add(new VariableReference("value"));
+		level.SetterBody.Add(new ExpressionStatement(record));
+		sink.Members.Add(level);
+
+		ToolchainHarness.InTemporaryDirectory(directory =>
+		{
+			File.WriteAllText(Path.Combine(directory, "sink.py"), new PythonGenerator().Generate(sink));
+			File.WriteAllText(
+				Path.Combine(directory, "driver.py"),
+				"""
+				import runpy
+
+				recorded = []
+				module = runpy.run_path("sink.py", init_globals={"record": recorded.append})
+
+				instance = module["Sink"]()
+				instance.level = 3
+				assert recorded == [3], recorded
+
+				try:
+				    instance.level
+				except AttributeError:
+				    pass
+				else:
+				    raise AssertionError("a write-only property could be read")
+				""");
+
+			(int exitCode, string output) = ToolchainHarness.Run(python, "driver.py", directory);
+			Assert.AreEqual(0, exitCode, $"Python rejected the generated module:{Environment.NewLine}{output}");
+		});
+	}
+
 	/// <summary>
 	/// Builds a file whose declarations are each written over themselves.
 	/// </summary>

Coder/Languages/PythonGenerator.cs

@@ -131,6 +131,18 @@
 			WriteAccessorBody(declaration.GetterBody, code);
 		}
 
+		if (declaration.CanWrite && !declaration.CanRead)
+		{
+			// `@name.setter` is an attribute of the property the getter made, so with no getter there
+			// is nothing for the decorator to name and the class statement raises NameError. The
+			// property is built from the setter alone instead.
+			string setter = $"_set_{name}";
+			code.Write($"def {setter}(self, value{annotation}) -> None:");
+			WriteAccessorBody(declaration.SetterBody, code);
+			code.WriteLine($"{name} = property(fset={setter})");
+			return;
+		}
+
 		if (declaration.CanWrite)
 		{
 			code.WriteLine($"@{name}.setter");
@@ -207,7 +219,7 @@
 	/// <c>float("nan")</c> does not.
 	/// </remarks>
 	protected override string SpellNonFiniteDouble(double value) =>
 		double.IsNaN(value) ? "float(\"nan\")" : value > 0 ? "float(\"inf\")" : "float(\"-inf\")";
 
 	/// <inheritdoc/>
 	protected override IEnumerable<string> RequiredImports(SourceFile file)