diff --git a/solver.py b/solver.py
index 45119f2..8c3c9e8 100644
--- a/solver.py
+++ b/solver.py
@@ -11,10 +11,20 @@ PATH = pathlib.Path(__file__).parent
 INPUT_PATH = PATH / "input"
 
 class Tests(list):
+    """
+    Test cases class. A test case is a tuple of the form
+    (puzzle_input, partI_result, partII_result).
+    """
+
     def add(self, test_input, partI=None, partII=None):
+        """Add a test case"""
         self.append((str(test_input), partI, partII))
 
 def testf(tests, f, part):
+    """
+    Tests a function f on test cases tests and compares the result
+    to test_case[part]. The test is not run if test_case[part] == None.
+    """
     tests = [(t[0], t[part]) for t in tests if t[part] != None]
     if tests == []:
         return "No tests"
@@ -30,8 +40,16 @@ def testf(tests, f, part):
 
 
 class Solver:
+    """
+    Solver class. A solver is a module with functions partI and partII,
+    an optional preprocessing function preproc, and an optional variable
+    tests of type Tests.
+    """
+
     def __init__(self, mod):
+        """Initilizes the solver instance of module mod."""
         def unpack(name, default):
+            """Returns mod.name if it exists and default otherwise."""
             return getattr(mod, name) if name in dir(mod) else default
 
         dn_pattern = re.compile("\d\d?")
@@ -41,6 +59,11 @@ class Solver:
         self.partII = getattr(mod, 'partII')
 
     def format_result(f):
+        """
+        Formatting decorator. Formats and prints a tuple representing
+        the results of partI and partII.
+        """
+
         def aux(*args):
             solver = args[0]
             ptI, ptII = f(*args)
@@ -50,6 +73,7 @@ class Solver:
 
     @format_result
     def solve(self, puzzle_input):
+        """Solves puzzle input and prints the results."""
         puzzle_input = puzzle_input.rstrip()
         if self.preproc != NotImplemented:
             puzzle_input = self.preproc(puzzle_input)
@@ -59,6 +83,7 @@ class Solver:
 
     @format_result
     def test(self):
+        """Tests the solver and prints the results."""
         tests = self.tests
         if self.preproc != NotImplemented:
             tests = [(self.preproc(str(t)), s1, s2)
@@ -70,6 +95,7 @@ class Solver:
 
 
 def read_input(mod_name):
+    """Reads input of module with mod_name."""
     name = mod_name + ".txt" 
     with open(INPUT_PATH / name) as f:
         r = f.read()
@@ -93,6 +119,7 @@ parser.add_argument('-t', '--test',
                     help="run tests")
 
 def parse_day_arg(s):
+    """Parses day argument. Returns a list of days."""
     split = s.split('-')
     if len(split) == 1:
         return [int(split[0])]