diff --git a/programmer/MANIFEST.in b/programmer/MANIFEST.in
index a2068e8..b411759 100644
--- a/programmer/MANIFEST.in
+++ b/programmer/MANIFEST.in
@@ -4,6 +4,7 @@ include *.md
 # Include the license file
 include LICENSE
 
+include *.py
 include *.sh
 include *.toml
 include .coveragerc
diff --git a/programmer/check_doctests.py b/programmer/check_doctests.py
new file mode 100755
index 0000000..3696011
--- /dev/null
+++ b/programmer/check_doctests.py
@@ -0,0 +1,4 @@
+#!/usr/bin/env python
+import doctest
+import tinyprog
+doctest.testmod(tinyprog)
diff --git a/programmer/tinyprog/__init__.py b/programmer/tinyprog/__init__.py
index 373451d..451e8de 100644
--- a/programmer/tinyprog/__init__.py
+++ b/programmer/tinyprog/__init__.py
@@ -33,10 +33,22 @@
 
 
 def pretty_hex(data):
-    output = ""
+    """
+    >>> print(pretty_hex("abc123"))
+    61 62 63 31 32 33
+    >>> print(pretty_hex(b"abc123"))
+    61 62 63 31 32 33
+    >>> print(pretty_hex(u"abc123"))
+    61 62 63 31 32 33
+    >>> print(pretty_hex("\\x00a\\x02"*12))
+    00 61 02 00 61 02 00 61 02 00 61 02 00 61 02 00
+    61 02 00 61 02 00 61 02 00 61 02 00 61 02 00 61
+    02 00 61 02
+    """
+    output = []
     for i in range(0, len(data), 16):
-        output += " ".join(["%02x" % ord(x) for x in data[i:i + 16]]) + "\n"
-    return output
+        output.append(" ".join("%02x" % to_int(x) for x in data[i:i + 16]))
+    return "\n".join(output)
 
 
 def to_int(value):
@@ -44,7 +56,7 @@ def to_int(value):
     >>> to_int('A')
     65
     >>> to_int(0xff)
-    256
+    255
     >>> list(to_int(i) for i in ['T', 'i', 'n', 'y', 0xff, 0, 0])
     [84, 105, 110, 121, 255, 0, 0]
     """
@@ -71,7 +83,8 @@ def get_ports(device_id):
         try:
             ports += [
                 UsbPort(usb, d)
-                for d in usb.core.find(idVendor=vid, idProduct=pid, find_all=True)
+                for d in usb.core.find(
+                    idVendor=vid, idProduct=pid, find_all=True)
                 if not d.is_kernel_driver_active(1)
             ]
         except usb.core.USBError as e:
@@ -90,6 +103,7 @@ def get_ports(device_id):
 class PortError(Exception):
     pass
 
+
 class SerialPort(object):
     def __init__(self, port_name):
         self.port_name = port_name
@@ -129,6 +143,7 @@ def read(self, length):
         except serial.SerialException as e:
             raise PortError("Failed to read from serial port:\n%s" % str(e))
 
+
 class UsbPort(object):
     def __init__(self, usb, device):
         self.usb = usb
@@ -166,6 +181,7 @@ def read(self, length):
         except self.usb.core.USBError as e:
             raise PortError("Failed to read from USB:\n%s" % str(e))
 
+
 def _mirror_byte(b):
     return bit_reverse_table[to_int(b)]
 
@@ -185,7 +201,11 @@ def __init__(self, prog):
 
     def _parse_json(self, data):
         try:
-            return json.loads(bytes(data).replace(b"\x00", b"").replace(b"\xff", b"").decode("utf-8"))
+            data = bytes(data)
+            data = data.replace(b"\x00", b"")
+            data = data.replace(b"\xff", b"")
+            data = data.decode("utf-8")
+            return json.loads(data)
         except BaseException:
             return None
 
@@ -239,7 +259,8 @@ def userdata_addr_range(self):
 
     def _get_addr_range(self, name):
         # get the bootmeta's addrmap or fallback to the root's addrmap.
-        addr_map = self.root.get(u"bootmeta", {}).get(u"addrmap", self.root.get(u"addrmap", None))
+        addr_map = self.root.get(u"bootmeta", {}).get(
+            u"addrmap", self.root.get(u"addrmap", None))
         if addr_map is None:
             raise Exception("Missing address map from device metadata")
         addr_str = addr_map.get(name, None)
diff --git a/programmer/tinyprog/__main__.py b/programmer/tinyprog/__main__.py
index b10b04c..cf45634 100644
--- a/programmer/tinyprog/__main__.py
+++ b/programmer/tinyprog/__main__.py
@@ -1,3 +1,7 @@
+#!/usr/bin/env python
+
+from __future__ import print_function
+
 import sys
 import argparse
 import json
@@ -355,7 +359,8 @@ def parse_int(str_value):
 
         if args.update_bootloader:
             boards_needing_update = (
-                check_for_wrong_tinyfpga_bx_vidpid() + check_for_new_bootloader())
+                check_for_wrong_tinyfpga_bx_vidpid()
+                + check_for_new_bootloader())
 
             if len(boards_needing_update) == 0:
                 print("""\
@@ -365,7 +370,8 @@ def parse_int(str_value):
                     perform_bootloader_update(port)
 
         # program the flash memory
-        if (args.program is not None) or (args.program_userdata is not None) or (
+        if (args.program is not None) or (
+                args.program_userdata is not None) or (
                 args.program_image is not None):
             boot_fpga = False
 
@@ -422,7 +428,8 @@ def progress(info):
                         sys.exit(1)
 
                     if check_if_overwrite_bootloader(
-                            addr, len(bitstream), fpga.meta.userdata_addr_range()):
+                            addr, len(bitstream),
+                            fpga.meta.userdata_addr_range()):
                         boot_fpga = True
                         print("    Programming at addr {:06x}".format(addr))
                         if not fpga.program_bitstream(addr, bitstream):
diff --git a/programmer/tox.ini b/programmer/tox.ini
index 26f55f5..56c2f7c 100644
--- a/programmer/tox.ini
+++ b/programmer/tox.ini
@@ -16,11 +16,13 @@ commands =
     python setup.py --version
     check-manifest --ignore tox.ini,tinyprog/full_version.py
     python setup.py check -m -s
-    flake8 --exclude=data_tables.py setup.py tinyprog
+    python check_doctests.py
+    flake8 setup.py tinyprog
     python setup.py install
     tinyprog --help
     tinyprog --version
 
 [flake8]
-exclude = .tox,*.egg,build,data
+exclude = .tox,*.egg,build,data,data_tables.py
 select = E,W,F
+ignore=W503