Episode 8, functions : about reading a global variable inside a function (#942)

* First proposal

* Remove the global trap explanation from the section about variable scope, and add a sentence in the dedicated challenge.

* Remove dummy sentence extract.

* Move the debate about the local/global trap in the challenge.

* Second review

* Wrap line to keep under 100 characters

Co-authored-by: Lauren Ko <[email protected]>

Author: chavid

_episodes/08-func.md

@@ -18,8 +18,9 @@ keypoints:
 - "Call a function using `function_name(value)`."
 - "Numbers are stored as integers or floating-point numbers."
 - "Variables defined within a function can only be seen and used within the body of the function."
-- "If a variable is not defined within the function it is used,
-   Python looks for a definition before the function call"
+- "Variables created outside of any function are called global variables."
+- "Within a function, we can access global variables."
+- "Variables created within a function override global variables if their names match."
 - "Use `help(thing)` to view help for something."
 - "Put docstrings in functions to provide help for that function."
 - "Specify default values for parameters when defining a function using `name=value`
@@ -177,6 +178,29 @@ temperature in Kelvin was: 373.15
 ~~~
 {: .output}
 
+The variable `temp_kelvin`, being defined outside any function,
+is said to be [global]({{ page.root }}/reference.html#global-variable).
+
+Inside a function, one can read the value of such global variables:
+~~~
+def print_temperatures():
+  print('temperature in Fahrenheit was:', temp_fahr)
+  print('temperature in Kelvin was:', temp_kelvin)
+
+temp_fahr = 212.0
+temp_kelvin = fahr_to_kelvin(temp_fahr)
+
+print_temperatures()
+~~~
+{: .language-python}
+
+~~~
+temperature in Fahrenheit was: 212.0
+temperature in Kelvin was: 373.15
+~~~
+{: .output}
+
+
 ## Tidying up
 
 Now that we know how to wrap bits of code up in functions,
@@ -872,6 +896,9 @@ readable code!
 > > `k`. The function does not return any values 
 > > and does not alter `k` outside of its local copy. 
 > > Therefore the original value of `k` remains unchanged.
+> > Beware that a local `k` is created because `f2k` internal statements
+> > *affect* a new value to it. If `k` was only `read`, it would simply retreive the
+> > global `k` value.
 > {: .solution}
 {: .challenge}
 

reference.md

@@ -111,6 +111,10 @@ function
 function call
 :   A use of a function in another piece of software.
 
+global variable
+:   A variable defined outside of a function. It can be used in global statements,
+    and read inside functions.
+
 heat map
 :   A graphical representation of two-dimensional data in which colors,
     ranging on a scale of hue or intensity, represent the data values.