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Exercises/ex9_1.md

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+\[ [Index](index.md) | [Exercise 8.6](ex8_6.md) | [Exercise 9.2](ex9_2.md) \]
+
+# Exercise 9.1
+
+*Objectives:*
+
+- A review of module basics
+
+This exercise is about some of the more tricky details of library modules. 
+Start this exercise by creating a very simple library module:
+
+```python
+# simplemod.py
+
+x = 42        # A global variable
+
+# A simple function
+def foo():   
+    print('x is', x)
+
+# A simple class
+class Spam:
+    def yow(self):
+        print('Yow!')
+
+# A scripting statement
+print('Loaded simplemod')
+```
+    
+##  (a) Module Loading and System Path
+
+Try importing the module you just created:
+
+```python
+>>> import simplemod
+Loaded simplemod
+>>> simplemod.foo()
+x is 42
+>>>
+```
+    
+If this failed with an `ImportError`, your path setting is
+flaky.  Look at the value of `sys.path` and fix it.
+
+```python
+>>> import sys
+>>> sys.path
+... look at the result ...
+>>>
+```
+    
+## (b) Repeated Module Loading
+    
+Make sure you understand that modules are only loaded
+once.  Try a repeated import and notice how you do not see
+the output from the `print` function:
+
+```python
+>>> import simplemod
+>>>
+```
+
+Try changing the value of `x` and see that a repeated import
+has no effect.
+
+```python
+>>> simplemod.x
+42
+>>> simplemod.x = 13
+>>> simplemod.x
+13
+>>> import simplemod
+>>> simplemod.x
+13
+>>>
+```
+
+Use `importlib.reload()` if you want to force a module to reload.
+
+```python
+>>> import importlib
+>>> importlib.reload(simplemod)
+Loaded simplemod
+<module 'simplemod' from 'simplemod.py'>
+>>> simplemod.x
+42
+>>>
+```
+    
+`sys.modules` is a dictionary of all loaded modules.  Take
+a look at it, delete your module, and try a repeated import.
+
+```python
+>>> sys.modules
+... look at output ...
+>>> sys.modules['simplemod']
+<module 'simplemod' from 'simplemod.py'>
+>>> del sys.modules['simplemod']
+>>> import simplemod
+Loaded simplemod
+>>>
+```
+
+## (c) from module import
+
+Restart Python and import a selected symbol from a module.
+
+```python
+>>> ############### [ RESTART ] ###############
+>>> from simplemod import foo
+Loaded simplemod
+>>> foo()
+x is 42
+>>>
+```
+
+Notice how this loaded the entire module (observe the output of
+the print function and how the `x` variable is used).
+
+When you use `from`, the module object itself is not
+visible.  For example:
+
+```python
+>>> simplemod.foo()
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+NameError: name 'simplemod' is not defined
+>>> 
+```
+
+Make sure you understand that when you export things from a module,
+they are simply name references.  For example, try this and explain:
+
+```python
+>>> from simplemod import x,foo
+>>> x
+42
+>>> foo()
+x is 42
+>>> x = 13
+>>> foo()
+x is 42                   # !! Please explain
+>>> x
+13
+>>>
+```
+
+## (d) Broken reload()
+
+Create an instance:
+
+```python
+>>> import simplemod
+>>> s = simplemod.Spam()
+>>> s.yow()
+Yow!
+>>>
+```
+
+Now, go to the `simplemod.py` file and change the implementation of `Spam.yow()` to the
+following:
+
+```python
+# simplemod.py
+...
+
+class Spam:
+    def yow(self):
+        print('More Yow!')
+```
+
+Now, watch what happens on a reload.  Do not restart Python for this part.
+
+```python
+>>> importlib.reload(simplemod)
+Loaded simplemod
+<module 'simplemod' from 'simplemod.py'>
+>>> s.yow()
+'Yow!'
+>>> t = simplemod.Spam()
+>>> t.yow()
+'More Yow!'
+>>>
+```
+
+Notice how you have two instances of `Spam`, but they're using different implementations
+of the `yow()` method.  Yes, actually both versions of code are loaded at the same time.
+You'll find other oddities as well.  For example:
+
+```python
+>>> s
+<simplemod.Spam object at 0x1006940b8>
+>>> isinstance(s, simplemod.Spam)
+False
+>>> isinstance(t, simplemod.Spam)
+True
+>>>
+```
+
+Bottom line:  It's probably best not to rely on reloading for anything important. 
+It might be fine if you're just trying to debug some things (as long as you're
+aware of its limitations and dangers).
+
+\[ [Solution](soln9_1.md) | [Index](index.md) | [Exercise 8.6](ex8_6.md) | [Exercise 9.2](ex9_2.md) \]
+
+----
+`>>>` 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/)