Fix for format with pattern only.

Fix issue where pattern formats without colours where given a
default black fill colour.

src/styles.c

@@ -641,7 +641,7 @@ _write_fg_color(lxw_styles *self, lxw_color_t color)
  * Write the <bgColor> element.
  */
 STATIC void
-_write_bg_color(lxw_styles *self, lxw_color_t color)
+_write_bg_color(lxw_styles *self, lxw_color_t color, uint8_t pattern)
 {
     struct xml_attribute_list attributes;
     struct xml_attribute *attribute;
@@ -650,15 +650,17 @@ _write_bg_color(lxw_styles *self, lxw_color_t color)
     LXW_INIT_ATTRIBUTES();
 
     if (color == LXW_COLOR_UNSET) {
-        LXW_PUSH_ATTRIBUTES_STR("indexed", "64");
+        if (pattern <= LXW_PATTERN_SOLID) {
+            LXW_PUSH_ATTRIBUTES_STR("indexed", "64");
+            lxw_xml_empty_tag(self->file, "bgColor", &attributes);
+        }
     }
     else {
         lxw_snprintf(rgb_str, LXW_ATTR_32, "FF%06X", color & LXW_COLOR_MASK);
         LXW_PUSH_ATTRIBUTES_STR("rgb", rgb_str);
+        lxw_xml_empty_tag(self->file, "bgColor", &attributes);
     }
 
-    lxw_xml_empty_tag(self->file, "bgColor", &attributes);
-
     LXW_FREE_ATTRIBUTES();
 }
 
@@ -704,6 +706,13 @@ _write_fill(lxw_styles *self, lxw_format *format, uint8_t is_dxf)
 
     LXW_INIT_ATTRIBUTES();
 
+    /* Special handling for pattern only case. */
+    if (!bg_color && !fg_color && pattern) {
+        _write_default_fill(self, patterns[pattern]);
+        LXW_FREE_ATTRIBUTES();
+        return;
+    }
+
     lxw_xml_start_tag(self->file, "fill", NULL);
 
     /* None/Solid patterns are handled differently for dxf formats. */
@@ -715,7 +724,7 @@ _write_fill(lxw_styles *self, lxw_format *format, uint8_t is_dxf)
     if (fg_color != LXW_COLOR_UNSET)
         _write_fg_color(self, fg_color);
 
-    _write_bg_color(self, bg_color);
+    _write_bg_color(self, bg_color, pattern);
 
     lxw_xml_end_tag(self->file, "patternFill");
     lxw_xml_end_tag(self->file, "fill");

test/functional/src/test_format16.c

@@ -0,0 +1,23 @@
+/*****************************************************************************
+ * Test cases for libxlsxwriter.
+ *
+ * Test to compare output against Excel files.
+ *
+ * Copyright 2014-2020, John McNamara, jmcnamara@cpan.org
+ *
+ */
+
+#include "xlsxwriter.h"
+
+int main() {
+
+    lxw_workbook  *workbook  = workbook_new("test_format16.xlsx");
+    lxw_worksheet *worksheet = workbook_add_worksheet(workbook, NULL);
+
+    lxw_format *pattern = workbook_add_format(workbook);
+    format_set_pattern(pattern, LXW_PATTERN_MEDIUM_GRAY);
+
+    worksheet_write_string(worksheet, CELL("A1"), "", pattern);
+
+    return workbook_close(workbook);
+}

test/functional/src/test_format17.c

@@ -0,0 +1,24 @@
+/*****************************************************************************
+ * Test cases for libxlsxwriter.
+ *
+ * Test to compare output against Excel files.
+ *
+ * Copyright 2014-2020, John McNamara, jmcnamara@cpan.org
+ *
+ */
+
+#include "xlsxwriter.h"
+
+int main() {
+
+    lxw_workbook  *workbook  = workbook_new("test_format17.xlsx");
+    lxw_worksheet *worksheet = workbook_add_worksheet(workbook, NULL);
+
+    lxw_format *pattern = workbook_add_format(workbook);
+    format_set_pattern(pattern, LXW_PATTERN_MEDIUM_GRAY);
+    format_set_fg_color(pattern, LXW_COLOR_RED);
+
+    worksheet_write_string(worksheet, CELL("A1"), "", pattern);
+
+    return workbook_close(workbook);
+}

test/functional/test_formatting.py

@@ -40,6 +40,12 @@ class TestCompareXLSXFiles(base_test_class.XLSXBaseTest):
     def test_format15(self):
         self.run_exe_test('test_format15')
 
+    def test_format16(self):
+        self.run_exe_test('test_format16')
+
+    def test_format17(self):
+        self.run_exe_test('test_format17')
+
     def test_format50(self):
         self.run_exe_test('test_format50')