## An Interactive Sudoku Solver in Python – Part 1: The Single Cell

I love playing Sudoku. It’s just therapeutic to me, but I do get stuck on those harder level Sudoku puzzles that require a trial-and-error approach. I like to mark the possibilities in the corners of a cell, and I run out of space rather quickly on those harder level puzzles.

Anyway, solving the harder level puzzles is not the point of this exercise in writing a Sudoku solver. I just thought it would be fun to write a solver, and more importantly, I wanted to learn more about using Tkinter.

To begin with, here is the Github repository so you can refer to the code:

https://github.com/tanyanghan/sudoku

When you look at a Sudoku grid, the single cell is the most fundamental element. It can hold a number between 1 to 9. I start with defining a class that represents the single cell. In the first commit of `sudoku_simply.py`, you will see a class definition for the `Sudoku_Cell`:

```# This class defines a single cell on a Sudoku grid
class Sudoku_Cell:
def __init__(self, value=0):
if not value:
self.possible_values = [1,2,3,4,5,6,7,8,9]
self.cells_need_updating = False
else:
self.possible_values = [value]
self.cells_need_updating = True
```

The class has two data members. The data member `possible_values` is the list of numbers that the individual cell could contain, and if only one value is left in the list, that value is the final value of the cell.

The other data member `cells_need_updating` is a Boolean used to indicate if other cells in the row, or column, or 3×3 grid area, needs to be updated when this cell has arrived at its final value. This is important for how the solver works.

Whenever a cell has arrived at its final value, the other cells that are on the same row, or column, or 3×3 grid area, would need to remove this cell’s final value from their list of `possible_values`. This is achieved by calling the class method `remove_possible_value()`. The code below is from the initial commit of `sudoku_simply.py`:

```    def remove_possible_value(self,value):
if len(self.possible_values) == 1:
return

try:
self.possible_values.remove(value)
except ValueError:
pass
else:
# value removed, check if we have arrived at an answer
if len(self.possible_values) == 1:
# cell value has been determined, flag that we need to update
#other cells
self.cells_need_updating = True
```

In this method, we do nothing and return if the cell has already arrived at its final value. Otherwise, we try to remove the `value` that is passed into the method from the list of `possible_values`. If the `value` was not in the list of `possible_values`, we quietly exit the function. If we do succeed in removing the `value` from the list, we check if we have arrived at the final value, and if so, we set `cells_need_updating` to `True`.

The function `other_cells_need_updating()` is to check if `cells_need_updating` Boolean is set, and the function `get_value`() is to read the final value, or return zero if the cell has not arrived at its final value.

With the `Sudoku_Cell` class more or less defined, we’ll next look at the Sudoku grid as a whole in the next post.

## Something fun to play over Zoom (or other video conferencing tool that has screen sharing)

A simple text based word guessing game that can be screen-shared over Zoom. Can you guess the famous movie quote above? (Answer below).

As almost the whole world is under some form of stay-at-home order during the COVID-19 pandemic, a lot of our social interaction with colleagues and friends have moved online to video conferencing, such as with Zoom.

For me and my church growth group, we have been using Zoom over the past 5 or 6 weeks. This past Friday evening, I was tasked with conducting an ice-breaker, and I only remembered that I needed to come up with something a couple of hours before the growth group session.

I came up with an idea of a word guessing game like the Wheel of Fortune, as it can be played by a big group and could be fun for a wide age-range of people. We have some pre-teen kids in our growth group.

The execution of the game would have been challenging with a pen and paper or white board, so I thought I’d quickly mash up a simple Python script that will take a list of phrases and replace it all with hyphens as a placeholder for each letter, and fill in the appropriate letter as someone guesses it.

Half an hour later, I had a simple and functioning game, and chose a couple test bible verses to play-test with my wife in the other room. The game went down really well in our growth group, and everyone had a good time playing it. Since then, I found some movie quotes and played the same game again using the movie quotes with my old high-school friends.

I have cleaned up the code, and made it load the list of phrases from a JSON text file and uploaded it to my Github for anyone wanting to try it: https://github.com/tanyanghan/wheel_of_fortune

To increase legibility over the screen-share, I recommend increasing the font size in Terminal via Preferences->Text->Font Change. I used a font size of 20.

Then, in the centre of your Zoom meeting controls at the bottom, click on `Share Screen` and then select the `Terminal` window where the Python code is running, and off you go. You conduct the game asking people to guess a letter and then moving on to the next person if they cannot guess the phrase.

As for the answer to the famous movie quote in the opening image at the top of this blog post…

Keep safe, everyone!

## Using logging.config.dictConfig with QueueHandler and Timing Measurements Posting to Slack

Following on from the previous post, I have made a quick timing measurement posting 4 messages to Slack from three locations:

Location Time Taken
(seconds)
San Francisco, California 2.70512
Kuala Lumpur, Malaysia 5.56670
Suzhou, China 12.67123

As you can see, if you’re not using the asynchronous `QueueListener` then your program could be stuck for over 12 seconds waiting on the communication with Slack.

Here’s an example JSON dictionary config file to use `QueueHandler` in logging:

```{
"version": 1,
"disable_existing_loggers": false,
"formatters": {
"simple": {
"format": "%(asctime)s - %(levelname)s - %(message)s"
}
},

"handlers": {
"consoleHandler": {
"class": "logging.StreamHandler",
"level": "DEBUG",
"formatter": "simple",
"stream": "ext://sys.stdout"
},
"slack-queue": {
"class":"SlackQueue.sqHandler",
"level":"DEBUG",
"formatter": "simple",
"queue": "ext://Global.slack_queue"
}
},

"root": {
"level": "DEBUG",
"handlers": ["consoleHandler", "slack-queue"]
}
}

```

Of note here is the `ext://` tag that allows you to pass in references to an external object into the logging `dictConfig`. I have put the instantiation of the queue object in a file named `Global.py`.

```import Queue as queue

#Instantiate a queue for the handler/listener
slack_queue = queue.Queue(-1)
```

This queue object is required by both the `QueueHandler` which is instantiated by `dictConfig`, and also the `QueueListener` which I have instantiated in my main program, `test_program_timing.py`.

```import logging
import logging.config
import json
import atexit
import SlackQueue
import Global
import time

time_program_start = time.time()
time_program_end = 0.0

def print_timing():
time_slack_queue_finish = time.time()
print ("Time taken by main program: %.5f"
%(time_program_end - time_program_start))
print ("Extra time taken by sending Slack messages: %.5f"
%(time_slack_queue_finish - time_program_end))

#Load the logging config json file.
with open('Logger.json','r') as f:

#Instantiate the queue listener with the same queue above, and also set
#the settings for communicating with Slack.
#You will need to get your own Incoming Webhook URL from Slack's Custom
#Integration setup.
SlackListener = SlackQueue.sqListener(
Global.slack_queue,
logging_url="https://hooks.slack.com/services/<your_webhook_url>",
channel="#your_channel",
icon_emoji = ":penguin:"
)

#Start the listener
SlackListener.start()

#Print out the timing measurement right at the end.
atexit.register(print_timing)

#Register the listener stop method to run upon exit in order to allow the
#queue to be emptied before exiting.
#If you miss this step, the queue might not be emptied before the program
#exits and you won't receive all the messages on Slack.
atexit.register(SlackListener.stop)

#Begin logging
logging.info("Test info message")

#You can temporarily change the Slack settings via the "extra" parameter.
#The settings in extra will only apply to this one log message.
logging.debug("Another test message",extra= {
"channel":"@someone",
"icon_emoji":":coffee:",
})

#You can put the <!channel> tag in the message to send an announcement.
msg="Some sort of warning!"
logging.warning("<!channel>: %s"%msg)

logging.error("Gasp!")

time_program_end = time.time()

```

I registered the function `print_timing( )` with `atexit.register( )` before registering the `SlackListener.stop` function so that the timing print out will happen right at the end after the queue has been cleared. This is because `atexit` uses a “last in, first out” ordering when it comes to executing registered functions.

You can find all the code on GitHub

## Asynchronous Posting to Slack from Python Logging QueueHandler

We had a problem at work where the regular blocking call to request post was getting in the way at physical locations that didn’t have ideal latency times connecting to the Slack server, like from within China. What takes a second in sunny California was suddenly 8 to 12 seconds from behind the Great Firewall.

The solution is to integrate Slack into Python logging and use the logging `QueueHandler` to make it asynchronous.

Here’s my implementation of an extension to the `QueueHandler` and `QueueListener` classes:

```from logging import Handler
import requests
import json
#QueueHandler and QueueListener is part of standard Python logging module
#from Python 3.2 onwards; but if you're using Python 2.x, you can just
#copy the QueueHandler and QueueListener code and save it into your own
#queueHandler.py.
try:
#check if they're part of standard Python logging module
from logging.handlers import QueueHandler
from logging.handlers import QueueListener
except ImportError:
#if not, use your own copy
from queueHandler import QueueHandler
from queueHandler import QueueListener

class sqHandler(QueueHandler):
def __init__(self, queue=None):
QueueHandler.__init__(self,queue)

def prepare(self, record):
"""
Override the method to allow the formatter to work.
"""
record.msg = self.format(record)
record.args = None
record.exc_info = None
return record

class sqListener(QueueListener,Handler):
def __init__(self, queue=None, logging_url="", channel="", username="",
icon_emoji = ""):
QueueListener.__init__(self,queue)
Handler.__init__(self)
"""
logging_url, channel, username, icon_emoji can all be overridden
by the extra dictionary parameter of a logging record
For example:
logging.info('Test messate',extra={'channel':'@someone',
'icon_emoji':':penguin:'})
"""
self.logging_url = logging_url
"channel": channel,
"icon_emoji": icon_emoji
}

def handle(self, record):
"""
Override the QueueListener.handle method with the Handler.handle
method
"""
Handler.handle(self, record)

def emit(self, record):
#make a copy of the default settings
new_logging_url = self.logging_url

#override default settings if necessary
if hasattr(record,'logging_url'):
new_logging_url = record.logging_url

if hasattr(record,key):

msg = self.format(record)

#post the request

```

And here’s a usage example:

```import logging
import Queue as queue
import atexit
import sys
import SlackQueue

#Set up the root logger
logging.basicConfig(stream=sys.stdout,
level=logging.DEBUG,
format='%(asctime)s - %(levelname)s - %(message)s')
root = logging.getLogger()

#Instantiate a queue for the handler/listener
slack_queue = queue.Queue(-1)

#Instantiate a queue handler using the queue above
SlackHandler = SlackQueue.sqHandler(slack_queue)
SlackHandler.setLevel("DEBUG")

#Set a formatter
simple = logging.Formatter('%(asctime)s - %(levelname)s - %(message)s')
SlackHandler.setFormatter(simple)

#Add the queue handler to the root logger

#Instantiate the queue listener with the same queue above, and also set
#the settings for communicating with Slack.
#You will need to get your own Incoming Webhook URL from Slack's Custom
#Integration setup.
SlackListener = SlackQueue.sqListener(
slack_queue,
logging_url="https://hooks.slack.com/services/<your_webhook_url>",
channel="#your_channel",
icon_emoji = ":penguin:"
)

#Start the listener
SlackListener.start()

#Register the listener stop method to run upon exit in order to allow the
#queue to be emptied before exiting.
#If you miss this step, the queue might not be emptied before the program
#exits and you won't receive all the messages on Slack.
atexit.register(SlackListener.stop)

#Begin logging
logging.info("Test info message")

#You can temporarily change the Slack settings via the "extra" parameter.
#The settings in extra will only apply to this one log message.
logging.debug("Another test message",extra= {
"channel":"@someone",
"icon_emoji":":coffee:",
})

#You can put the <!channel> tag in the message to send an announcement.
msg="Some sort of warning!"
logging.warning("<!channel>: %s"%msg)

logging.error("Gasp!")

```

You can use the `extra` parameter in logging to alter the Slack settings for a particular message. It can be useful if you decide that a particular logging message should ping a certain person directly, or post to a different channel from the default channel.

You can find all the code on GitHub.

If you’re using Python 2.x and need the source of queueHandler.py, check out Vinay Sajip’s blog post, specifically his whole script example.