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Exercises/ex8_4.md

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+\[ [Index](index.md) | [Exercise 8.3](ex8_3.md) | [Exercise 8.5](ex8_5.md) \]
+
+# Exercise 8.4
+
+*Objectives:*
+
+- Managing what happens at the `yield` statements
+
+*Files Modified:* `follow.py`, `cofollow.py`
+
+## (a) Closing a Generator
+
+A common question concerning generators is their lifetime and garbage
+collection. For example, the `follow()` generator runs forever in
+an infinite `while` loop. What happens if the iteration loop that's
+driving it stops?  Also, is there anyway to prematurely terminate the 
+generator?
+
+Modify the `follow()` function so that all of the code is enclosed in
+a `try-except` block like this:
+
+```python
+def follow(filename):
+    try:
+        with open(filename,'r') as f:
+            f.seek(0,os.SEEK_END)
+            while True:
+                 line = f.readline()
+                 if line == '':
+                     time.sleep(0.1)    # Sleep briefly to avoid busy wait
+                     continue
+                 yield line
+    except GeneratorExit:
+        print('Following Done')
+```
+
+Now, try a few experiments:
+
+```python
+>>> from follow import follow
+>>> # Experiment: Garbage collection of a running generator
+>>> f = follow('Data/stocklog.csv')
+>>> next(f)
+'"MO",70.29,"6/11/2007","09:30.09",-0.01,70.25,70.30,70.29,365314\n'
+>>> del f
+Following Done
+>>> # Experiment: Closing a generator
+>>> f = follow('Data/stocklog.csv')
+>>> for line in f:
+        print(line,end='')
+        if 'IBM' in line:
+            f.close()
+
+"VZ",42.91,"6/11/2007","09:34.28",-0.16,42.95,42.91,42.78,210151
+"HPQ",45.76,"6/11/2007","09:34.29",0.06,45.80,45.76,45.59,257169
+"GM",31.45,"6/11/2007","09:34.31",0.45,31.00,31.50,31.45,582429
+...
+"IBM",102.86,"6/11/2007","09:34.44",-0.21,102.87,102.86,102.77,147550
+Following Done
+>>> for line in f:
+        print(line, end='')    # No output: generator is done
+
+>>> 
+```
+
+In these experiments you can see that a `GeneratorExit` exception is
+raised when a generator is garbage-collected or explicitly closed via
+its `close()` method. 
+
+One additional area of exploration is whether or not you can resume
+iteration on a generator if you break out of a for-loop.  For example,
+try this:
+
+```python
+>>> f = follow('Data/stocklog.csv')
+>>> for line in f:
+        print(line,end='')
+        if 'IBM' in line:
+            break
+
+"CAT",78.36,"6/11/2007","09:37.19",-0.16,78.32,78.36,77.99,237714
+"VZ",42.99,"6/11/2007","09:37.20",-0.08,42.95,42.99,42.78,268459
+...
+"IBM",102.91,"6/11/2007","09:37.31",-0.16,102.87,102.91,102.77,190859
+>>> # Resume iteration
+>>> for line in f:
+        print(line,end='')
+        if 'IBM' in line:
+            break
+
+"AA",39.58,"6/11/2007","09:39.28",-0.08,39.67,39.58,39.31,243159
+"HPQ",45.94,"6/11/2007","09:39.29",0.24,45.80,45.94,45.59,408919
+...
+"IBM",102.95,"6/11/2007","09:39.44",-0.12,102.87,102.95,102.77,225350
+>>> del f
+Following Done
+>>>
+```
+
+In general, you can break out of running iteration and resume it later
+if you need to.  You just need to make sure the generator object isn't
+forcefully closed or garbage collected somehow.
+
+## (b) Raising Exceptions
+
+In the file `cofollow.py`, you created a coroutine `printer()`.  Modify the
+code to catch and report exceptions like this:
+
+```python
+# cofollow.py
+...
+@consumer
+def printer():
+    while True:
+        try:
+            item = yield
+            print(item)
+        except Exception as e:
+            print('ERROR: %r' % e)
+```
+
+Now, try an experiment:
+
+```python
+>>> from cofollow import printer
+>>> p = printer()
+>>> p.send('hello')
+hello
+>>> p.send(42)
+42
+>>> p.throw(ValueError('It failed'))
+ERROR: ValueError('It failed',)
+>>> try:
+        int('n/a')
+    except ValueError as e:
+        p.throw(e)
+
+ERROR: ValueError("invalid literal for int() with base 10: 'n/a'",)
+>>> 
+```
+
+Notice how the running generator is not terminated by the exception. This
+is merely allowing the `yield` statement to signal an error instead of
+receiving a value.
+
+\[ [Solution](soln8_4.md) | [Index](index.md) | [Exercise 8.3](ex8_3.md) | [Exercise 8.5](ex8_5.md) \]
+
+----
+`>>>` Advanced Python Mastery  
+`...` A course by [dabeaz](https://www.dabeaz.com)  
+`...` Copyright 2007-2023  
+
+![](https://i.creativecommons.org/l/by-sa/4.0/88x31.png). This work is licensed under a [Creative Commons Attribution-ShareAlike 4.0 International License](http://creativecommons.org/licenses/by-sa/4.0/)