Add initial version of chapter 02, part 2

README.md

@@ -62,6 +62,11 @@ Alternatively, the content can be viewed in a web browser
     - [exercises <img height="12" style="display: inline-block" src="static/link/to_nb.png">](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/develop/chapter_02_functions/01_exercises.ipynb)
       [<img height="12" style="display: inline-block" src="static/link/to_mb.png">](https://mybinder.org/v2/gh/webartifex/intro-to-python/develop?urlpath=lab/tree/chapter_02_functions/01_exercises.ipynb)
       (Volume of a Sphere)
+    - [content 2 <img height="12" style="display: inline-block" src="static/link/to_nb.png">](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/develop/chapter_02_functions/02_content.ipynb)
+      [<img height="12" style="display: inline-block" src="static/link/to_mb.png">](https://mybinder.org/v2/gh/webartifex/intro-to-python/develop?urlpath=lab/tree/chapter_02_functions/02_content.ipynb)
+      (Standard Library: `math` & `random` Modules;
+       Third-party Packages: `numpy` Library;
+       Writing one's own Modules)
 
 
 #### Videos

chapter_02_functions/sample_module.py

@@ -0,0 +1,76 @@
+"""This is a sample module.
+
+It defines three functions average(), average_evens(), and average_odds().
+The point is to show how we can put Python code in a .py file to be re-used
+in some other place.
+
+We should never forget to document the code as well, both on the module
+level (i.e., this docstring) but also in every function it defines.
+
+When imported, Python modules are executed top to bottom before the flow of
+execution returns to wherever they were imported into.
+
+An important convention is to prefix variables and functions that are not to
+be used outside the module with a single underscore "_". This way, we can
+design the code within a module in a modular fashion and only "export" what we
+want.
+
+Here, all three functions internally forward parts of their computations
+to the utility functions _round_all() and _scaled_average() that contain all
+the logic common to the three functions.
+
+While this example is stylized, it shows how Python modules are often
+designed.
+"""
+
+def _round_all(numbers):
+    """Internal utility function to round all numbers in a list."""
+    return [round(n) for n in numbers]
+
+
+def _scaled_average(numbers, scalar):
+    """Internal utility function to calculate scaled averages."""
+    average = sum(numbers) / len(numbers)
+    return scalar * average
+
+
+def average(numbers, *, scalar=1):
+    """Calculate the average of all numbers in a list.
+
+    Args:
+        numbers (list of int's/float's): numbers to be averaged;
+            if non-whole numbers are provided, they are rounded
+        scalar (float, optional): multiplies the average; defaults to 1
+
+    Returns:
+        scaled_average (float)
+    """
+    return _scaled_average(_round_all(numbers), scalar)
+
+
+def average_evens(numbers, *, scalar=1):
+    """Calculate the average of all even numbers in a list.
+
+    Args:
+        numbers (list of int's/float's): numbers to be averaged;
+            if non-whole numbers are provided, they are rounded
+        scalar (float, optional): multiplies the average; defaults to 1
+
+    Returns:
+        scaled_average (float)
+    """
+    return _scaled_average([n for n in _round_all(numbers) if n % 2 == 0], scalar)
+
+
+def average_odds(numbers, *, scalar=1):
+    """Calculate the average of all odd numbers in a list.
+
+    Args:
+        numbers (list of int's/float's): numbers to be averaged;
+            if non-whole numbers are provided, they are rounded
+        scalar (float, optional): multiplies the average; defaults to 1
+
+    Returns:
+        scaled_average (float)
+    """
+    return _scaled_average([n for n in _round_all(numbers) if n % 2 != 0], scalar)