diff --git a/adafruit_pioasm.py b/adafruit_pioasm.py
index 8ed258a..514d1ba 100644
--- a/adafruit_pioasm.py
+++ b/adafruit_pioasm.py
@@ -11,6 +11,11 @@
 * Author(s): Scott Shawcroft
 """
 
+try:
+    from typing import List, MutableSequence
+except ImportError:
+    pass
+
 import array
 import re
 
@@ -40,14 +45,14 @@ class Program:  # pylint: disable=too-few-public-methods
 
     """
 
-    def __init__(self, text_program: str, *, build_debuginfo=False) -> None:
+    def __init__(self, text_program: str, *, build_debuginfo: bool = False) -> None:
         """Converts pioasm text to encoded instruction bytes"""
         # pylint: disable=too-many-branches,too-many-statements,too-many-locals
-        assembled = []
+        assembled: List[int] = []
         program_name = None
         labels = {}
         linemap = []
-        instructions = []
+        instructions: List[str] = []
         sideset_count = 0
         sideset_enable = 0
         wrap = None
@@ -86,9 +91,8 @@ def __init__(self, text_program: str, *, build_debuginfo=False) -> None:
 
         max_delay = 2 ** (5 - sideset_count - sideset_enable) - 1
         assembled = []
-        for instruction in instructions:
-            # print(instruction)
-            instruction = splitter(instruction.strip())
+        for line in instructions:
+            instruction = splitter(line.strip())
             delay = 0
             if instruction[-1].endswith("]"):  # Delay
                 delay = int(instruction[-1].strip("[]"), 0)
@@ -253,16 +257,13 @@ def __init__(self, text_program: str, *, build_debuginfo=False) -> None:
 
         self.assembled = array.array("H", assembled)
 
-        if build_debuginfo:
-            self.debuginfo = (linemap, text_program)
-        else:
-            self.debuginfo = None
+        self.debuginfo = (linemap, text_program) if build_debuginfo else None
 
-    def print_c_program(self, name, qualifier="const"):
+    def print_c_program(self, name: str, qualifier: str = "const") -> None:
         """Print the program into a C program snippet"""
         if self.debuginfo is None:
-            linemap = None
-            program_lines = None
+            linemap = []
+            program_lines = []
         else:
             linemap = self.debuginfo[0][:]  # Use a copy since we destroy it
             program_lines = self.debuginfo[1].split("\n")
@@ -306,7 +307,7 @@ def print_c_program(self, name, qualifier="const"):
         print()
 
 
-def assemble(program_text: str) -> array.array:
+def assemble(program_text: str) -> MutableSequence[int]:
     """Converts pioasm text to encoded instruction bytes
 
     In new code, prefer to use the `Program` class so that the extra arguments
diff --git a/tests/pytest_helpers.py b/tests/pytest_helpers.py
index 678e9b7..72ac7cf 100644
--- a/tests/pytest_helpers.py
+++ b/tests/pytest_helpers.py
@@ -6,17 +6,22 @@
 Pytest helper functions
 """
 
+try:
+    from typing import Any, List, Optional, Type
+except ImportError:
+    pass
+
 import pytest
 
 import adafruit_pioasm
 
 
-def nice_opcode(opcode):
-    opcode = f"{opcode:016b}"
-    return opcode[:3] + "_" + opcode[3:8] + "_" + opcode[8:]
+def nice_opcode(opcode: int) -> str:
+    nice = f"{opcode:016b}"
+    return nice[:3] + "_" + nice[3:8] + "_" + nice[8:]
 
 
-def assert_assembles_to(source, expected):
+def assert_assembles_to(source: str, expected: List[int]) -> None:
     actual = adafruit_pioasm.assemble(source)
     expected_bin = [nice_opcode(x) for x in expected]
     actual_bin = [nice_opcode(x) for x in actual]
@@ -25,7 +30,9 @@ def assert_assembles_to(source, expected):
     ), f"Assembling {source!r}: Expected {expected_bin}, got {actual_bin}"
 
 
-def assert_assembly_fails(source, match=None, errtype=RuntimeError):
+def assert_assembly_fails(
+    source: str, match: Optional[str] = None, errtype: Type[Exception] = RuntimeError
+) -> None:
     with pytest.raises(errtype, match=match):
         adafruit_pioasm.assemble(source)
     # if match:
@@ -36,6 +43,6 @@ def assert_assembly_fails(source, match=None, errtype=RuntimeError):
     #        adafruit_pioasm.assemble(source)
 
 
-def assert_pio_kwargs(source, **kw):
+def assert_pio_kwargs(source: str, **kw: Any) -> None:
     program = adafruit_pioasm.Program(source)
     assert kw == program.pio_kwargs
diff --git a/tests/test_in.py b/tests/test_in.py
index d2b76dd..9c7eeb7 100644
--- a/tests/test_in.py
+++ b/tests/test_in.py
@@ -57,13 +57,13 @@ def test_in_delay_with_sideset() -> None:
     assert_assembles_to("\n".join(source), [0b010_10_101_000_10001])
 
 
-def test_in_bad_source():
+def test_in_bad_source() -> None:
     assert_assembly_fails(
         "in bad, 17", match="Invalid in source 'bad'", errtype=ValueError
     )
 
 
-def test_in_bad_bitcount():
+def test_in_bad_bitcount() -> None:
     assert_assembly_fails(
         "in pins, 0", match="Count out of range", errtype=RuntimeError
     )
diff --git a/tests/test_mov.py b/tests/test_mov.py
index b6015cb..52492d7 100644
--- a/tests/test_mov.py
+++ b/tests/test_mov.py
@@ -9,21 +9,21 @@
 from pytest_helpers import assert_assembles_to, assert_assembly_fails
 
 
-def test_mov_non_happy():
+def test_mov_non_happy() -> None:
     # non happy path
     assert_assembly_fails(
         "mov x, blah", match="Invalid mov source 'blah'", errtype=ValueError
     )
 
 
-def test_mov_invert():
+def test_mov_invert() -> None:
     # test moving and inverting
     assert_assembles_to("mov x, ~ x", [0b101_00000_001_01_001])
     assert_assembles_to("mov x, ~x", [0b101_00000_001_01_001])
     assert_assembles_to("mov x, !x", [0b101_00000_001_01_001])
 
 
-def test_mov_reverse():
+def test_mov_reverse() -> None:
     # test moving and reversing bits
     assert_assembles_to("mov x, :: x", [0b101_00000_001_10_001])
     assert_assembles_to("mov x, ::x", [0b101_00000_001_10_001])
diff --git a/tests/test_nop.py b/tests/test_nop.py
index 7fd7bca..68bd3c5 100644
--- a/tests/test_nop.py
+++ b/tests/test_nop.py
@@ -9,11 +9,11 @@
 from pytest_helpers import assert_assembles_to, assert_assembly_fails, assert_pio_kwargs
 
 
-def test_nonsense():
+def test_nonsense() -> None:
     assert_assembly_fails("nope")
 
 
-def test_nop():
+def test_nop() -> None:
     assert_assembles_to("nop", [0b101_00000_010_00_010])
     assert_assembles_to("nop\nnop", [0b101_00000_010_00_010, 0b101_00000_010_00_010])
     assert_assembles_to("nop [1]", [0b101_00001_010_00_010])
@@ -22,7 +22,7 @@ def test_nop():
     assert_assembles_to(".side_set 1\nnop side 1 [15]", [0b101_11111_010_00_010])
 
 
-def test_sideset_opt():
+def test_sideset_opt() -> None:
     assert_assembles_to(".side_set 1 opt\nnop side 1", [0b101_11000_010_00_010])
     assert_assembles_to(".side_set 1 opt\nnop side 0", [0b101_10000_010_00_010])
     assert_assembles_to(".side_set 1 opt\nnop side 0 [1]", [0b101_10001_010_00_010])
@@ -32,14 +32,14 @@ def test_sideset_opt():
     assert_assembles_to(".side_set 1 opt\nnop side 0 [7]", [0b101_10111_010_00_010])
 
 
-def test_set():
+def test_set() -> None:
     # non happy path
     assert_assembly_fails(
         "set isr, 1", match="Invalid set destination 'isr'", errtype=ValueError
     )
 
 
-def test_jmp():
+def test_jmp() -> None:
     assert_assembles_to("l:\njmp l", [0b000_00000_000_00000])
     assert_assembles_to("l:\njmp 7", [0b000_00000_000_00111])
     assert_assembles_to("jmp l\nl:", [0b000_00000_000_00001])
@@ -56,7 +56,7 @@ def test_jmp():
     )
 
 
-def test_wait():
+def test_wait() -> None:
     assert_assembles_to("wait 0 gpio 0", [0b001_00000_0_00_00000])
     assert_assembles_to("wait 0 gpio 1", [0b001_00000_0_00_00001])
     assert_assembles_to("wait 1 gpio 2", [0b001_00000_1_00_00010])
@@ -69,7 +69,7 @@ def test_wait():
     assert_assembles_to("wait 0 irq 1 rel", [0b001_00000_0_10_10001])
 
 
-def test_limits():
+def test_limits() -> None:
     assert_assembly_fails(".side_set 1\nnop side 2")
     assert_assembly_fails(".side_set 1\nnop side 2 [1]")
     assert_assembly_fails("nop [32]")
@@ -77,7 +77,7 @@ def test_limits():
     assert_assembly_fails(".side_set 1 opt\nnop side 0 [8]")
 
 
-def test_cls():
+def test_cls() -> None:
     assert_pio_kwargs("", sideset_enable=False)
     assert_pio_kwargs(".side_set 1", sideset_pin_count=1, sideset_enable=False)
     assert_pio_kwargs(".side_set 3 opt", sideset_pin_count=3, sideset_enable=True)
diff --git a/tests/test_out.py b/tests/test_out.py
index 5956fce..67bc978 100644
--- a/tests/test_out.py
+++ b/tests/test_out.py
@@ -59,13 +59,13 @@ def test_out_delay_with_sideset() -> None:
     assert_assembles_to("\n".join(source), [0b011_10_101_000_10001])
 
 
-def test_out_bad_destination():
+def test_out_bad_destination() -> None:
     assert_assembly_fails(
         "out bad, 17", match="Invalid out destination 'bad'", errtype=ValueError
     )
 
 
-def test_out_bad_bitcount():
+def test_out_bad_bitcount() -> None:
     assert_assembly_fails(
         "out pins, 0", match="Count out of range", errtype=RuntimeError
     )
diff --git a/tests/test_pseudo.py b/tests/test_pseudo.py
index 72e817c..66d93c2 100644
--- a/tests/test_pseudo.py
+++ b/tests/test_pseudo.py
@@ -9,5 +9,5 @@
 from pytest_helpers import assert_pio_kwargs
 
 
-def test_offset():
+def test_offset() -> None:
     assert_pio_kwargs(".origin 7", offset=7, sideset_enable=False)
diff --git a/tests/test_radix.py b/tests/test_radix.py
index f3cfb9e..19775f4 100644
--- a/tests/test_radix.py
+++ b/tests/test_radix.py
@@ -9,13 +9,13 @@
 from pytest_helpers import assert_assembles_to
 
 
-def test_octal():
+def test_octal() -> None:
     assert_assembles_to(".side_set 0o1\nset x, 0o11", [0b111_00000_001_01001])
 
 
-def test_binary():
+def test_binary() -> None:
     assert_assembles_to(".side_set 0b101\nnop side 0b10001", [0b101_10001_010_00_010])
 
 
-def test_hex():
+def test_hex() -> None:
     assert_assembles_to(".side_set 0x0\nnop [0x10]", [0b101_10000_010_00_010])
diff --git a/tests/test_wrap.py b/tests/test_wrap.py
index b866a26..05eed73 100644
--- a/tests/test_wrap.py
+++ b/tests/test_wrap.py
@@ -9,7 +9,7 @@
 from pytest_helpers import assert_assembly_fails, assert_pio_kwargs
 
 
-def test_wrap():
+def test_wrap() -> None:
     assert_assembly_fails(".wrap")
     assert_pio_kwargs(
         "nop\n.wrap_target\nnop\nnop\n.wrap",