Add support for transparent backgrounds

Fixes #64

README.rst

@@ -135,10 +135,16 @@ Commandline::
 Changelog
 ---------
 
+v0.13.0
+~~~~~~~
+
+* Added support for transparent backgrounds. This is done by setting the ``mode`` option
+  for a writer to ``RGBA``.
+
 v0.12.0
 ~~~~~~~
 
-* Removed `writer_options` from `barcode.get`. This parameter was not used.
+* Removed ``writer_options`` from ``barcode.get``. This parameter was not used.
 * Add a ``with_doctype`` flag to ``SVGWriter``. Set this to false to avoid including a
   ``DOCTYPE`` in the resulting SVG.
 * Add support for ``Pillow>=8.0.0``.

barcode/writer.py

@@ -332,18 +332,30 @@ def write(self, content, fp: BinaryIO):
 else:
 
     class ImageWriter(BaseWriter):  # type: ignore
-        def __init__(self):
+        format: str
+        mode: str
+        dpi: int
+
+        def __init__(self, format="PNG", mode="RGB"):
+            """Initialise a new write instance.
+
+            :params format: The file format for the generated image. This parameter can
+                take any value that Pillow accepts.
+            :params mode: The colour-mode for the generated image. Set this to RGBA if
+                you wish to use colours with transparency.
+            """
             BaseWriter.__init__(
                 self, self._init, self._paint_module, self._paint_text, self._finish
             )
-            self.format = "PNG"
+            self.format = format
+            self.mode = mode
             self.dpi = 300
             self._image = None
             self._draw = None
 
         def _init(self, code):
             size = self.calculate_size(len(code[0]), len(code), self.dpi)
-            self._image = Image.new("RGB", size, self.background)
+            self._image = Image.new(self.mode, size, self.background)
             self._draw = ImageDraw.Draw(self._image)
 
         def _paint_module(self, xpos, ypos, width, color):

tests/test_writers.py

@@ -18,6 +18,17 @@ def test_saving_image_to_byteio():
         with open(f"{TESTPATH}/somefile.jpeg", "wb") as f:
             EAN13("100000011111", writer=ImageWriter()).write(f)
 
+    def test_saving_rgba_image():
+        rv = BytesIO()
+        EAN13(str(100000902922), writer=ImageWriter()).write(rv)
+
+        with open(f"{TESTPATH}/ean13-with-transparent-bg.png", "wb") as f:
+            writer = ImageWriter(mode="RGBA")
+
+            EAN13("100000011111", writer=writer).write(
+                f, options={"background": "rgba(255,0,0,0)"}
+            )
+
 
 def test_saving_svg_to_byteio():
     rv = BytesIO()