[Python] Fix regression for %A format specifier to output booleans as lowercase (#4289)

* fix(python): Fix `%A` format specifier to output booleans as lowercase

* doc: Fix changelog entries

Author: dbrattli

src/Fable.Cli/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Fixed
+
+* [Python] Fix regression `%A` format specifier to output booleans as lowercase `true`/`false` (by @dbrattli)
+
 ## 5.0.0-alpha.19 - 2025-12-04
 
 ### Fixed
@@ -15,8 +19,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-* [Python] Add support for `[<Py.Decorate>]` attribute on methods (previously only worked on classes)
-* [Python] Add new `[<Py.ClassMethod>]` attribute to emit @classmethod instead of @staticmethod
+* [Python] Add support for `[<Py.Decorate>]` attribute on methods (previously only worked on classes) (by @dbrattli)
+* [Python] Add new `[<Py.ClassMethod>]` attribute to emit @classmethod instead of @staticmethod (by @dbrattli)
 * [Python] Added support for Pydantic serialization of core numeric and array types (by @dbrattli)
 
 ## 5.0.0-alpha.18 - 2025-12-03

src/Fable.Compiler/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Fixed
+
+* [Python] Fix regression `%A` format specifier to output booleans as lowercase `true`/`false` (by @dbrattli)
+
 ## 5.0.0-alpha.18 - 2025-12-04
 
 ### Fixed
@@ -15,8 +19,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-* [Python] Add support for `[<Py.Decorate>]` attribute on methods (previously only worked on classes)
-* [Python] Add new `[<Py.ClassMethod>]` attribute to emit @classmethod instead of @staticmethod
+* [Python] Add support for `[<Py.Decorate>]` attribute on methods (previously only worked on classes) (by @dbrattli)
+* [Python] Add new `[<Py.ClassMethod>]` attribute to emit @classmethod instead of @staticmethod (by @dbrattli)
 * [Python] Added support for Pydantic serialization of core numeric and array types (by @dbrattli)
 
 ## 5.0.0-alpha.17 - 2025-12-03

src/Fable.Core/CHANGELOG.md

@@ -11,8 +11,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-* [Python] Add support for [<Py.Decorate>] attribute on methods (previously only worked on classes)
-* [Python] Add new [<Py.ClassMethod>] attribute to emit @classmethod instead of @staticmethod
+* [Python] Add support for [<Py.Decorate>] attribute on methods (previously only worked on classes) (by @dbrattli)
+* [Python] Add new [<Py.ClassMethod>] attribute to emit @classmethod instead of @staticmethod (by @dbrattli)
 
 ## 5.0.0-beta.2 - 2025-11-19
 

src/fable-library-py/src/strings.rs

@@ -160,6 +160,14 @@ mod printf {
                     "Int64" => format!("{}:i64", arg.str()?),
                     "UInt32" => format!("{}:u32", arg.str()?),
                     "UInt64" => format!("{}:u64", arg.str()?),
+                    // Handle booleans with F# lowercase representation (true/false)
+                    "bool" => {
+                        if arg.is_truthy()? {
+                            TRUE_STR.to_string()
+                        } else {
+                            FALSE_STR.to_string()
+                        }
+                    }
                     _ => arg.str()?.to_string(),
                 }
             };

tests/Python/TestString.fs

@@ -127,6 +127,19 @@ let ``test Extended string interpolation syntax`` () =
     let cssNew = $$""".{{classAttr}}:hover {background-color: #eee;}"""
     cssNew |> equal ".item-panel:hover {background-color: #eee;}"
 
+[<Fact>]
+let ``test sprintf \"%A\" formats booleans as lowercase`` () =
+    // Test that %A formats booleans as lowercase true/false (F# semantics)
+    let result = sprintf "%A" true
+    equal "true" result
+
+    let result2 = sprintf "%A" false
+    equal "false" result2
+
+    // Also test in composed context
+    let optResult = sprintf "Some(%A)" true
+    equal "Some(true)" optResult
+
 [<Fact>]
 let ``test sprintf \"%A\" with lists works`` () =
     let xs = ["Hi"; "Hello"; "Hola"]