PyMyth: Global variables are evil... WRONG!

Terry Reedy tjreedy at
Wed Nov 13 02:20:09 CET 2013

On 11/11/2013 11:46 PM, Rick Johnson wrote:

> No, Python modules can be poked, prodded, and violated by
> any pervert who can spell the word "import".

Or by clever programmers.

> Attribute values can be reassigned and state can be
> externally manipulated

Perhaps for good reasons.

> resulting in all types of undefined behaviors

Not necessarily. Manipulation can also eliminate undefined behaviors.

Suppose I want to test a Idle function that uses a tkinter.messagebox 
class to communicate with users. Perhaps the module has

from tkinter.messagebox import askretrycancel

The behavior of askretrycancel(*args) is undefined insofar as it depends 
on the arbitrary actions of a human user (it may even hang forever). In 
an automated test, there is no user, and it is difficult to simulate the 
eyes and fingers of one while leaving askretrycancel as it is. 
Monkeypatching with a mock solves this. Simplifying a bit, and leaving 
out details, I have done something like this.

from mock_tkinter import mbox  # mocks all messageboxes
mock_arc = mbox()
import mod
mod.askretrycancel = mock_arc

This makes mod.askretrycancel deterministic in that I can preload a 
response into mock_arc before calling the function or method in mod. 
(This simulates a user's fingers.) The mock also saves the values sent 
to it to see if they are what they should be. (This simulates a user's 

Terry Jan Reedy

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