Difference between revisions of "Russ Python Tips and Techniques"
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Revision as of 09:37, 10 February 2017
These are Tips and Techniques I have found useful in my Python programing User:Russ_hensel. I would be interested in your feedback. Let me know if you think the tip is useful/wrong/something everyone knows. Most of this material relates to applications with a GUI. Just starting Feb 2017 check history to see if progress is being made.
Parameters for Applications
There are various ways of storing start up parameters for applications. In the old days almost all windows programs had a "ini" file. Linux had/has "config" files. In both cases these are text files that are easily edited with a simple text editor. More recently xml files ( which i tend to hate ) have come into vogue. Finally some applications give you a gui for editing configuration values and then save them any way they want, perhaps in the "registry".
Using a gui is the easiest way for the user in particular the naive user. They are also a lot of work to implement and maintain. So I favor a flat, non-xml file. But what file? For Python my answere is a python.py file that has pretty much one class, Parameters, that is implemented as a singleton ( in what ever way you want to implement singletons ). These are easy to implement, easy to maintain, and very flexible. They can also do clever things like detecting what operating system they are running on and make automatic adjustments. There are no issues of converting type into strings for the file. Comments are easily included. Loading can print the progress of initialization.
I will put a brief sample here soon so you can see what a file looks like.
code comming here
I use the logging class that is part of standard python. I provide a button to view and edit the file. Why invent your own method. Here is some code:
When an application ( here I am thinking about GUI applications ) starts there can be a considerable delay as all the modules are imported. Typically a restart is done after an exception or when parameters are changed. So I include a restart button on the GUI to quickly restart the application. In my case applications are usually started by constructing some application class. So what I do is break up the __init__ method into 2 parts, only the second needs to be run to restart the application. Here is some sample code:
def __init__(self ): """ create the application, its GUI is called self.gui which is created in restart """ ...... more code ...... self.restart( ) # continue in restart # -------------------------------------------------------- def restart(self ): """ use to restart the app without ending it parameters will be reloaded and the gui rebuilt call from a button on the GUI args: zip ret: zip ... all sided effects """ self.no_restarts += 1 # if we want to keep track if not( self.gui is None ): # determines if a restart or the start # !! # need to shut down other thread self.gui.root.destroy() reload( parameters ) self.logger = None .... and so on ......
Threading with Tkinker
Tkinker wants to own the processing of the application. If you make a button start a long running operation then the GUI blocks and becomes unresponsive. So you need some sort of threading. Here are some tips.
Polling withing Tkinker
You can poll in the Tkinter event loop and call any function ( no arguments I think ) using a Tkinter function. Use this to run something short. You can only run it once this way.
self.gui.root.after( self.parameters.gt_delta_t, self.polling ) # self.gui.root = Tkinter root, .1 = .1 sec, self.polling = name of the function # ( note no () which is a function call
To overcome the fact that it only runs once, ask for it again at the end of the function.
def polling( self, ): ..... do something quick self.gui.root.after( .1, self.polling )
If you function throws an exception it will stop running. Try Except works for me:
def polling( self, ): try: ..... do something quick except Exception, ex_arg: .... I log the error finally: self.gui.root.after( .1, self.polling )
Use real threading. I inherit from threading.Thread and implement a override of the run method. Use threading mechanisms to "message" back and fourth to the gui, Tkinter, thread. Seems to work fine.
Calling across Threads
If you have objects in 2 different threads calling across the threads can be very dangerous. I have worked out a method that I find useful/safe/easy. It does make some assumptions.
- Both objects/threads have something resembling polling where checks can be made at a convient time to see if the other thread wants attention.
Step 1, Set Up Queus
self.queue_to_helper = Queue.Queue( 20 ) # send from tkinker mainloop to helper here self.queue_fr_helper = Queue.Queue( 20 ) # send back from helper thread to tkinker mainloop here