diff --git a/adafruit_bitmapsaver.py b/adafruit_bitmapsaver.py
index f0231e8..70a119f 100644
--- a/adafruit_bitmapsaver.py
+++ b/adafruit_bitmapsaver.py
@@ -1,4 +1,5 @@
 # SPDX-FileCopyrightText: 2019 Dave Astels for Adafruit Industries
+# SPDX-FileCopyrightText: 2022 Matt Land
 #
 # SPDX-License-Identifier: MIT
 
@@ -10,7 +11,7 @@
 Make a screenshot (the contents of a displayio.Display) and save in a BMP file.
 
 
-* Author(s): Dave Astels
+* Author(s): Dave Astels, Matt Land
 
 Implementation Notes
 --------------------
@@ -32,11 +33,17 @@
 import board
 from displayio import Bitmap, Palette, Display
 
+try:
+    from typing import Tuple, Optional, Union
+    from io import BufferedWriter
+except ImportError:
+    pass
+
 __version__ = "0.0.0-auto.0"
 __repo__ = "https://github.com/adafruit/Adafruit_CircuitPython_BitmapSaver.git"
 
 
-def _write_bmp_header(output_file, filesize):
+def _write_bmp_header(output_file: BufferedWriter, filesize: int) -> None:
     output_file.write(bytes("BM", "ascii"))
     output_file.write(struct.pack("<I", filesize))
     output_file.write(b"\00\x00")
@@ -44,7 +51,7 @@ def _write_bmp_header(output_file, filesize):
     output_file.write(struct.pack("<I", 54))
 
 
-def _write_dib_header(output_file, width, height):
+def _write_dib_header(output_file: BufferedWriter, width: int, height: int) -> None:
     output_file.write(struct.pack("<I", 40))
     output_file.write(struct.pack("<I", width))
     output_file.write(struct.pack("<I", height))
@@ -54,42 +61,48 @@ def _write_dib_header(output_file, width, height):
         output_file.write(b"\x00")
 
 
-def _bytes_per_row(source_width):
+def _bytes_per_row(source_width: int) -> int:
     pixel_bytes = 3 * source_width
     padding_bytes = (4 - (pixel_bytes % 4)) % 4
     return pixel_bytes + padding_bytes
 
 
-def _rotated_height_and_width(pixel_source):
+def _rotated_height_and_width(pixel_source: Union[Bitmap, Display]) -> Tuple[int, int]:
     # flip axis if the display is rotated
     if isinstance(pixel_source, Display) and (pixel_source.rotation % 180 != 0):
-        return (pixel_source.height, pixel_source.width)
-    return (pixel_source.width, pixel_source.height)
+        return pixel_source.height, pixel_source.width
+    return pixel_source.width, pixel_source.height
 
 
-def _rgb565_to_bgr_tuple(color):
-    blue = (color << 3) & 0x00F8  # extract each of the RGB tripple into it's own byte
+def _rgb565_to_bgr_tuple(color: int) -> Tuple[int, int, int]:
+    blue = (color << 3) & 0x00F8  # extract each of the RGB triple into it's own byte
     green = (color >> 3) & 0x00FC
     red = (color >> 8) & 0x00F8
-    return (blue, green, red)
+    return blue, green, red
 
 
 # pylint:disable=too-many-locals
-def _write_pixels(output_file, pixel_source, palette):
+def _write_pixels(
+    output_file: BufferedWriter,
+    pixel_source: Union[Bitmap, Display],
+    palette: Optional[Palette],
+) -> None:
     saving_bitmap = isinstance(pixel_source, Bitmap)
     width, height = _rotated_height_and_width(pixel_source)
     row_buffer = bytearray(_bytes_per_row(width))
     for y in range(height, 0, -1):
         buffer_index = 0
         if saving_bitmap:
+            # pixel_source: Bitmap
             for x in range(width):
                 pixel = pixel_source[x, y - 1]
-                color = palette[pixel]
+                color = palette[pixel]  # handled by save_pixel's guardians
                 for _ in range(3):
                     row_buffer[buffer_index] = color & 0xFF
                     color >>= 8
                     buffer_index += 1
         else:
+            # pixel_source: Display
             result_buffer = bytearray(2048)
             data = pixel_source.fill_row(y - 1, result_buffer)
             for i in range(width):
@@ -106,7 +119,11 @@ def _write_pixels(output_file, pixel_source, palette):
 # pylint:enable=too-many-locals
 
 
-def save_pixels(file_or_filename, pixel_source=None, palette=None):
+def save_pixels(
+    file_or_filename: Union[str, BufferedWriter],
+    pixel_source: Union[Display, Bitmap] = None,
+    palette: Optional[Palette] = None,
+) -> None:
     """Save pixels to a 24 bit per pixel BMP file.
     If pixel_source if a displayio.Bitmap, save it's pixels through palette.
     If it's a displayio.Display, a palette isn't required.
@@ -116,10 +133,10 @@ def save_pixels(file_or_filename, pixel_source=None, palette=None):
     :param palette: the Palette to use for looking up colors in the bitmap
     """
     if not pixel_source:
-        if "DISPLAY" in dir(board):
-            pixel_source = board.DISPLAY
-        else:
+        if not hasattr(board, "DISPLAY"):
             raise ValueError("Second argument must be a Bitmap or Display")
+        pixel_source = board.DISPLAY
+
     if isinstance(pixel_source, Bitmap):
         if not isinstance(palette, Palette):
             raise ValueError("Third argument must be a Palette for a Bitmap save")
diff --git a/mypy.ini b/mypy.ini
new file mode 100644
index 0000000..5e6c3a5
--- /dev/null
+++ b/mypy.ini
@@ -0,0 +1,8 @@
+# SPDX-FileCopyrightText: 2022 Matt Land
+#
+# SPDX-License-Identifier: Unlicense
+[mypy]
+python_version = 3.7
+disallow_untyped_defs = True
+disable_error_code = no-redef
+exclude = (examples|tests|setup.py|docs)