Add support for '%' type to output floating point values as a

percentage.

This helps with compatibility with Python's format strings.

include/fmt/format-inl.h

@@ -656,7 +656,14 @@ void sprintf_format(Double value, internal::buffer& buf,
     *format_ptr++ = '*';
   }
   if (std::is_same<Double, long double>::value) *format_ptr++ = 'L';
-  char type = spec.type ? spec.type : 'g';
+
+  char type = spec.type;
+
+  if (type == '%') {
+      type = 'f';
+  } else if (type == 0) {
+      type = 'g';
+  }
 #if FMT_MSC_VER
   if (type == 'F') {
     // MSVC's printf doesn't support 'F'.

include/fmt/format.h

@@ -1274,6 +1274,9 @@ FMT_CONSTEXPR void handle_float_type_spec(char spec, Handler&& handler) {
   case 'F':
     handler.on_fixed();
     break;
+  case '%':
+    handler.on_percent();
+    break;
   case 'a':
   case 'A':
     handler.on_hex();
@@ -1338,6 +1341,7 @@ class float_type_checker : private ErrorHandler {
   FMT_CONSTEXPR void on_general() {}
   FMT_CONSTEXPR void on_exp() {}
   FMT_CONSTEXPR void on_fixed() {}
+  FMT_CONSTEXPR void on_percent() {}
   FMT_CONSTEXPR void on_hex() {}
 
   FMT_CONSTEXPR void on_error() {
@@ -2808,10 +2812,11 @@ template <typename Range> class basic_writer {
 
 struct float_spec_handler {
   char type;
-  bool upper;
+  bool upper{false};
-  bool fixed;
+  bool fixed{false};
+  bool as_percentage{false};
 
-  explicit float_spec_handler(char t) : type(t), upper(false), fixed(false) {}
+  explicit float_spec_handler(char t) : type(t) {}
 
   void on_general() {
     if (type == 'G') upper = true;
@@ -2826,6 +2831,11 @@ struct float_spec_handler {
     if (type == 'F') upper = true;
   }
 
+  void on_percent() {
+      fixed = true;
+      as_percentage = true;
+  }
+
   void on_hex() {
     if (type == 'A') upper = true;
   }
@@ -2869,11 +2879,19 @@ void basic_writer<Range>::write_double(T value, const format_specs& spec) {
   memory_buffer buffer;
   int exp = 0;
   int precision = spec.has_precision() || !spec.type ? spec.precision : 6;
+
+  if (handler.as_percentage) value *= 100.;
+
   bool use_grisu = fmt::internal::use_grisu<T>() &&
                    (!spec.type || handler.fixed) &&
                    internal::grisu2_format(static_cast<double>(value), buffer,
                                            precision, handler.fixed, exp);
   if (!use_grisu) internal::sprintf_format(value, buffer, spec);
+
+  if (handler.as_percentage) {
+      buffer.push_back('%');
+      --exp; // Adjust decimal place position.
+  }
   align_spec as = spec;
   if (spec.align() == ALIGN_NUMERIC) {
     if (sign) {

test/format-test.cc

@@ -1443,7 +1443,7 @@ TEST(FormatterTest, FormatFloat) {
 }
 
 TEST(FormatterTest, FormatDouble) {
-  check_unknown_types(1.2, "eEfFgGaA", "double");
+  check_unknown_types(1.2, "eEfFgGaA%", "double");
   EXPECT_EQ("0.0", format("{:}", 0.0));
   EXPECT_EQ("0.000000", format("{:f}", 0.0));
   EXPECT_EQ("0", format("{:g}", 0.0));