Correct IDLE usage (was Reason for not allowing import twice but allowing reload())

[Terry Reedy]

On 3/2/2016 10:22 AM, Rustom Mody wrote:
> On Tuesday, March 1, 2016 at 12:23:02 PM UTC+5:30, Terry Reedy wrote:
>> On 2/29/2016 7:42 AM, Rustom Mody wrote:
>>
>>> Is import needed at all when trying out in Idle?
>> ...
>>> So it does appear that
>>> 1. import not necessary with(in) idle
>>> 2. However import and f5 (ie is run as main) are different
>>>
>>> May some idle experts elaborate on this? Whats the idle idiom of import-ing?
>>
>> Rustom, since I know that you are not a rank beginner, I have trouble
>> understanding what you are asking.
>
> Heh!
> I know some things; dont know many things
>
>> F5 when editing foo.py is equivalent
>> to running "python -i foo.py" on a command line while 'in' the directory
>> containing foo.py.  In both cases, foo.py is run as a main module, with
>> __name__ == '__main__'.  The difference is that F5 runs foo.py under
>> IDLE supervision, with results going into and interactive inputs coming
>> from IDLE shell instead of the console interpreter.
>>
>> Imports are used in a module to access objects within the imported module.
>
> Let me try to explain again
>
> There is import and import.
> There is the formal meaning of the import keyword in python -- call it import-f
> There is the informal expectation and need of programmers to 'pull something
> into python' -- call it import-i

What do you mean by import-i that is different module import?  Keyboard 
input with input()? File input with file.read(), etcetera? Running a 
file with 'python filename' or 'python -m filename'?

> That there is some cognitive dissonance between import-f and import-i is seen
> in the OP's question itself;

The OP's question is faulty.  Repeat imports *are* allowed.

> also Chris' "I dont believe the language should be changed"
>
> So the question is around:
> What is the best practice for doing import-i in python?

Since I don't know what you mean by import-i, I cannot answer.

> As the OP finds import-f works once and fails thereafter

This is false.  Import is a name binding statement.  If a module is not 
builtin, the first attempt to bind a name to a particular module has to 
create the module and cache it in sys.modules.  If it is builtin, the 
first attempt caches the module.  Subsequent attempts reuse the cached 
module.  Here are three imports that involve the same module.  All three 
run, none fail.

 >>> import itertools as it
 >>> import itertools as it2
 >>> from itertools import count
 >>> it, it2, count
(<module 'itertools' (built-in)>, <module 'itertools' (built-in)>, 
<class 'itertools.count'>)

If you want to create and cache a new module of the same name, perhaps 
after editing a file, delete the cache entry before importing again. 
Reload does this for you. What it does not attempt to do is to change or 
delete all the old bindings and references.

If one does plan to reload a module, then one should only access the 
module *and its contents* via one name and keep track of any other 
references to its contents, such as from instances to classes in the 
module.  This may be easy for a simple module of functions, but in the 
general case, can be be difficult to impossible.

> In idle one can get the desired result of import-i with F5
> Is that right?

Having no idea what import-i is, I cannot answer.  I explained F5 in my 
previous answer.

> Also in general is there good usecases for import-f at that interpreter prompt

As I said before, the purpose of an import is to access the contents of 
module 2 from within module 1, where module1 can be the main module, 
including in interactive mode.  A beginner experimenting with core 
python might proceed for hours without an import.  Anyone experimenting 
with a module must import the module first.  I usually import one or 
more modules in a given interactive session.

 > in idle?

Whether the prompt is in a console text window or IDLE gui window is 
irrelevant.  The IDLE Shell gui window imitates interactive python in a 
text console.  It submits each statement entered to Python for execution 
and displays the stdout and stderr results.

> I think not but not sure of it

Here is an example use of import for learning tkinter.

 >>> import tkinter as tk

As expected, nothing visible happens.

 >>> root = tk.Tk()

A default master windows is created *and displayed*.

 >>> b = tk.Button(root, text = 'hello world')

Nothing visible happens.
 >>> b
<tkinter.Button object .2092827275560>

But an instance of the class was created.

 >>> b.grid()

Button appears. Master window shrinks to fit.

 >>> t = tk.Toplevel(root)

A new toplevel window appears.

-- 
Terry Jan Reedy