change examples based on the feedback

Author: HumphreyYang

lectures/python_essentials.md

@@ -326,8 +326,38 @@ out
 print(out)
 ```
 
-We can also use a `with` statement (also known as a [context manager](https://realpython.com/python-with-statement/#the-with-statement-approach)) which enables you to group operations on the file within a block which improves the clarity of your code.
+We can also use a `with` statement to ensure the files are properly acquired 
+and released.
 
+Containing the operations within the same block also improves the clarity of your code.
+
+```{note}
+This kind of block is formally referred to as a [*context*](https://realpython.com/python-with-statement/#the-with-statement-approach).
+```
+
+Let's try to convert the two examples above into a `with` statement.
+
+We change the writing example first
+```{code-cell} python3
+
+with open('newfile.txt', 'w') as f:  
+    f.write('Testing\n')         
+    f.write('Testing again')
+```
+
+Note that we do not need to call the `close()` method since the `with` block
+will ensure the stream is closed at the end of the block.
+
+With slight modifications, we can also read files using `with`
+
+```{code-cell} python3
+with open('newfile.txt', 'r') as fo:
+    out = fo.read()
+    print(out)
+```
+Now suppose that we want to read input from one file and write output to another. 
+Here's how we could accomplish this task while correctly acquiring and returning 
+resources to the operating system using `with` statements:
 
 ```{code-cell} python3
 with open("newfile.txt", "r") as f:
@@ -337,30 +367,48 @@ with open("newfile.txt", "r") as f:
             fo.write(f'Line {i}: {line} \n')
 ```
 
+The output file will be
+
 ```{code-cell} python3
-fo = open('output.txt', 'r')
-out = fo.read()
-print(out)
+with open('output.txt', 'r') as fo:
+    print(fo.read())
 ```
 
-We can write the example above under the same context to reduce redundancy
+We can simplify the example above by grouping the two `with` statements into one line
 
 ```{code-cell} python3
 with open("newfile.txt", "r") as f, open("output2.txt", "w") as fo:
         for i, line in enumerate(f):
             fo.write(f'Line {i}: {line} \n')
 ```
 
+The output file will be the same
+
 ```{code-cell} python3
-fo = open('output2.txt', 'r')
-out = fo.read()
-print(out)
+with open('output2.txt', 'r') as fo:
+    print(fo.read())
 ```
 
-```{note}
-Note that we only used `r` and `w` mode. There are [more modes](https://www.geeksforgeeks.org/reading-writing-text-files-python/) you could experiment with.
+Suppose we want to continue to write into the existing file 
+instead of overwriting it.
+
+we can switch the mode to `a` which stands for append mode
+
+```{code-cell} python3
+with open('output2.txt', 'a') as fo:
+    fo.write('\nThis is the end of the file')
+```
+
+```{code-cell} python3
+with open('output2.txt', 'r') as fo:
+    print(fo.read())
 ```
 
+```{note}
+Note that we only covered `r`, `w`, and `a` mode here, which are the most commonly used modes. 
+Python provides [a variety of modes](https://www.geeksforgeeks.org/reading-writing-text-files-python/) 
+that you could experiment with.
+```
 
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