Fix broken links

Exercises/README.md

@@ -13,18 +13,18 @@ This course requires the use of Python 3.6 or newer.  If you are
 using Python 2, most of the material still applies, but you will
 have to make minor code modifications here and there. 
 
-- link:PythonMastery.pdf[`PythonMastery.pdf`] is a PDF that contains
+- [`PythonMastery.pdf`](../PythonMastery.pdf) is a PDF that contains
 all of the presentation slides.
 
-- The link:Exercises/index.html[`Exercises/`] folder is where you
+- The [`Exercises/`](index.md) folder is where you
 find all the class exercises.
 
-- The `Data/` folder is where you find data files, scripts, and
+- The [`Data/`](../Data/) folder is where you find data files, scripts, and
 other files used by the exercises.
 
-- The `Solutions/` folder contains complete solution code for
+- The [`Solutions/`](../Solutions/) folder contains complete solution code for
 various exercises.  Each problem has its own directory.  For example,
-the solution to exercise 3.2 can be found in the `Solution/3_2/` directory.
+the solution to exercise 3.2 can be found in the [`Solutions/3_2/`](../Solutions/3_2/) directory.
 
 Every attempt has been made to make sure exercises work.  However, it's 
 possible that you will find typos or minor mistakes. If you find any

Exercises/ex2_3.md

@@ -310,7 +310,7 @@ you do need to make sure you don't go overboard with the syntax.
 
 ## (f) Saving a lot of memory
 
-In link:ex2_1.html[Exercise 2.1] you wrote a function
+In [Exercise 2.1](ex2_1.md) you wrote a function
 `read_rides_as_dicts()` that read the CTA bus data into a list of
 dictionaries.  Using it requires a lot of memory. For example,
 let's find the day on which the route 22 bus had the greatest

Exercises/ex3_1.md

@@ -8,7 +8,7 @@
 
 *Files Modified:* `stock.py`
 
-In link:ex1_5.html[Exercise 1.5], you defined a simple class
+In [Exercise 1.5](ex1_5.md), you defined a simple class
 `Stock` for representing a holding of stock.  In this exercise,
 we're simply going to add a few features to that class as well as
 write some utility functions.
@@ -49,7 +49,7 @@ reads a file of portfolio data into a list of `Stock` objects. Here's how it sho
 ```
 
 You already wrote a similar function as part of
-link:ex2_3.html[Exercise 2.3].  Design discussion: Should
+[Exercise 2.3](ex2_3.md).  Design discussion: Should
 `read_portfolio()` be a separate function or part of the class
 definition?
 

Exercises/ex3_2.md

@@ -66,7 +66,7 @@ price 490.1
 
 ## (c) Table Output
 
-In link:ex3_1.html[Exercise 3.1], you wrote a function `print_portfolio()`
+In [Exercise 3.1](ex3_1.md), you wrote a function `print_portfolio()`
 that made a nicely formatted table.   That function was custom tailored
 to a list of `Stock` objects.  However, it can be completely generalized
 to work with any list of objects using the technique in part (b).

Exercises/ex3_5.md

@@ -13,7 +13,7 @@ CSV, and HTML.
 
 One major use of classes in Python is in writing code that be
 extended/adapted in various ways.  To illustrate, in
-link:ex3_2.html[Exercise 3.2] you created a function `print_table()`
+[Exercise 3.2](ex3_2.md) you created a function `print_table()`
 that made tables.  You used this to make output from the `portfolio`
 list. For example:
 

Exercises/ex3_6.md

@@ -98,7 +98,7 @@ Make this change and try comparing two objects again.
 
 ## (c) A Context Manager
 
-In link:ex3_5.html[Exercise 3.5], you made it possible for users to make
+In [Exercise 3.5](ex3_5.md), you made it possible for users to make
 nicely formatted tables.  For example:
 
 ```python

Exercises/ex3_7.md

@@ -9,7 +9,7 @@
 
 *Files Modified:* `tableformat.py`
 
-In link:ex3_5.html[Exercise 3.5], we modified the `tableformat.py` file to have a `TableFormatter`
+In [Exercise 3.5](ex3_5.md), we modified the `tableformat.py` file to have a `TableFormatter`
 class and to use various subclasses for different output formats.  In this exercise, we extend that
 code a bit more.
 

Exercises/ex3_8.md

@@ -10,7 +10,7 @@
 
 ## (a) The Trouble with Column Formatting
 
-If you go all the way back to link:ex3_1.txt[Exercise 3.1], you 
+If you go all the way back to [Exercise 3.1](ex3_1.md), you 
 wrote a function `print_portfolio()` that produced a table like this:
 
 ```python
@@ -204,7 +204,7 @@ different ways.
 Using mixins can be a useful tool for framework builders for reducing 
 the amount of code that needs to be written.  However, forcing users
 to remember how to properly compose classes and use multiple inheritance can
-fry their brains.  In link:ex3_5.html[Exercise 3.5], you wrote a 
+fry their brains.  In [Exercise 3.5](ex3_5.md), you wrote a 
 function `create_formatter()` that made it easier to create a custom
 formatter.  Take that function and extend it to understand a few optional
 arguments related to the mixin classes. For example:

Exercises/ex4_2.md

@@ -123,7 +123,7 @@ the end of the MRO since it is the parent to all of the classes being composed t
 
 ## (b) Build a Value Checker
 
-In link:ex3_4.html[Exercise 3.4], you added some properties to the `Stock` class that
+In [Exercise 3.4](ex3_4.md), you added some properties to the `Stock` class that
 checked attributes for different types and values (e.g., shares had to be a positive
 integer).  Let's play with that idea a bit.  Start by creating a file `validate.py` and
 defining the following base class:

Exercises/ex6_1.md

@@ -16,7 +16,7 @@ sane way.   Before doing anything, copy your work in `stock.py` to a new file
 `orig_stock.py`.  
 
 We're going to recreate the `Stock` class from scratch using some new techniques.
-Make sure you have your unit tests from link:ex5_4.html[Exercise 5.4] handy. You'll want those.
+Make sure you have your unit tests from [Exercise 5.4](ex5_4.md) handy. You'll want those.
 
 If you define a function, you probably already know that it can be
 called using a mix of positional or keyword arguments.  For example:

Exercises/ex6_3.md

@@ -63,7 +63,7 @@ mappingproxy(OrderedDict([('x', <Parameter "x">), ('y', <Parameter "y">)]))
 
 ## (c) Putting it Together
 
-In link:ex6_1.html[Exercise 6.1], you created a class `Structure`
+In [Exercise 6.1](ex6_1.md), you created a class `Structure`
 that defined a generalized `__init__()`, `__setattr__()`, and `__repr__()`
 method.   That class required a user to define a `_fields` class
 variable like this:
@@ -75,7 +75,7 @@ class Stock(Structure):
 
 The problem with this class is that the `__init__()` function didn't
 have a useful argument signature for the purposes of help and
-keyword argument passing.  In link:ex6_2.html[Exercise 6.2], you 
+keyword argument passing.  In [Exercise 6.2](ex6_2.md), you 
 did a sneaky trick involving a special `self._init()` function. For example:
 
 ```python

Exercises/ex6_4.md

@@ -60,7 +60,7 @@ There are no weird hacks involving a special `_init()` method or stack frames.
 
 ## (b) Creating an `__init__()` function
 
-In link:ex6_3.txt[Exercise 6.3], you wrote code that inspected the
+In [Exercise 6.3](ex6_3.md), you wrote code that inspected the
 signature of the `__init__()` method to set the attribute names
 in a `_fields` class variable.   For example:
 
@@ -108,7 +108,7 @@ methods on the `Structure` class--that approach was kind of weird.
 
 ## (c) Named Tuples
 
-In link:ex2_1.html[Exercise 2.1], you experimented with `namedtuple` objects
+In [Exercise 2.1](ex2_1.md), you experimented with `namedtuple` objects
 in the `collections` module.  Just to refresh your memory, here is how
 they worked:
 

Exercises/ex7_3.md

@@ -14,7 +14,7 @@ developed over the last few days. Hang on to your hat.
 
 ## (a) Descriptors Revisited
 
-In link:ex4_3.html[Exercise 4.3] you defined some descriptors that
+In [Exercise 4.3](ex4_3.md) you defined some descriptors that
 allowed a user to define classes with type-checked attributes like
 this:
 
@@ -29,7 +29,7 @@ class Stock:
 ```
 
 Modify your `Stock` class so that it includes the above descriptors
-and now looks like this (see link:ex6_4.html[Exercise 6.4]):
+and now looks like this (see [Exercise 6.4](ex6_4.md)):
 
 ```python
 # stock.py

Exercises/ex7_6.md

@@ -11,7 +11,7 @@
 
 ## (a) The Final Frontier
 
-In link:ex7_3.html[Exercise 7.3], we made it possible to define type-checked structures as follows:
+In [Exercise 7.3](ex7_3.md), we made it possible to define type-checked structures as follows:
 
 ```python
 from validate import String, PositiveInteger, PositiveFloat

Exercises/ex8_3.md

@@ -13,7 +13,7 @@
 For this exercise the `stocksim.py` program should still be
 running in the background.  
 
-In link:ex8_2.html[Exercise 8.2] you wrote some code that used
+In [Exercise 8.2](ex8_2.md) you wrote some code that used
 generators to set up a processing pipeline.  A key aspect of that
 program was the idea of data flowing between generator functions.  A
 very similar kind of dataflow can be set up using coroutines.  The
@@ -24,7 +24,7 @@ processing elements as opposed to pulling data out with a for-loop.
 
 Getting started with coroutines can be a little tricky.  Here is an
 example program that performs the same task as
-link:ex8_2.html[Exercise 8.2], but with coroutines.  Take this program
+[Exercise 8.2](ex8_2.md), but with coroutines.  Take this program
 and copy it into a file called `cofollow.py`.
 
 ```python
@@ -71,7 +71,7 @@ Run this program and make sure produces output..   Make sure you understand how
 ## (b) Build some pipeline components
 
 In a file `coticker.py`, build a series of pipeline components that carry out the same tasks as
-the `ticker.py` program in link:ex8_2.html[Exercise 8.2].  Here is the implementation of the
+the `ticker.py` program in [Exercise 8.2](ex8_2.md).  Here is the implementation of the
 various pieces.
 
 ```python

Exercises/ex8_6.md

@@ -19,7 +19,7 @@ clean-up code that relies on generators.
 
 ## (a) Example: Receiving messages
 
-In link:ex8_3.html[Exercise 8.3], we looked at the definitions of coroutines.
+In [Exercise 8.3](ex8_3.md), we looked at the definitions of coroutines.
 Coroutines were functions that you sent data to.  For example:
 
 ```python
@@ -80,7 +80,7 @@ is a bit more descriptive--it indicates that the function will yield until
 it receives a message of a given type. 
 
 Now, modify all of the coroutines in `coticker.py` to use the new `receive()`
-function and make sure the code from link:ex8_3.html[Exercise 8.3] still
+function and make sure the code from [Exercise 8.3](ex8_3.md) still
 works.
 
 ## (b) Wrapping a Socket