[Tutor] passing variables to functions

[Mats Wichmann]

On 11/16/20 8:46 AM, steve10brink1 at comcast.net wrote:
> Hi,
> 
> I always assumed that variables could only be passed to functions via function calls unless they are designated as global.  So I was suprised that this code works without any errors.  How does the variable 'b' get indirectly assigned in the function testVar(a)?  I expected an error.  I am using python3 version 3.5.3.
> 
> Code:
> -----------------------------------------------------------
> def testVar(a):
> print(b,a)    #b is from main and not passed thru function call, expect error
> return a
> 
> print("Test variables in function.....")
> b = 23.4
> c = testVar(b)
> print("Final: ",c)
> 
> -----------------------------------------------------------
> Output:
> 
> Test variables in function.....
> 23.4 23.4
> Final: 23.4

"That's how it works" :)

If there's not a value in the local scope (inside the function 
definition), Python picks it from the global scope.  On the other hand, 
if you were assigning to b in the function, Python would create b in the 
local scope if you did not use the "global" statement.

It might be useful to think of it as a set of dictionaries, since in 
most cases that's what it is:

 >>> b = 23.4
 >>> print(globals())
{'__name__': '__main__', '__doc__': None, '__package__': None, 
'__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': 
None, '__annotations__': {}, '__builtins__': <module 'builtins' 
(built-in)>, 'b': 23.4}
 >>> def testVar(a):
...     c = 10
...     print(locals())
...
 >>>
 >>> testVar(b)
{'a': 23.4, 'c': 10}
 >>>

The local scope in testVar got the var 'c' from the direct assignment 
and the var 'a' from the function argument; if there where a reference 
to 'b', it wouldn't find it in the locals dict so it would fall back to 
the globals, and find it from there.