
# Context Manager in Python

## Introduction {#introduction}

External resources are often used in programming, and in those cases the programmer must ensure that the descriptor to those resources was closed. If not closed properly, it could run in the background (waste CPU/memory) and cause the program and/or computer to be slow.
In this project a SQLite client was built as a context manager to ensure resource is closed when program terminates, even in the case of an error being raised.

## Background {#background}

One can open a file using `file = open(file_path, 'r')`, run some analysis and then close the file `file.close()`. This way works, but could be wasteful if an error is raised. If it is just a small file, it might not waste much memory/CPU, but for other tools, such as selenium driver, it could be bad.

Best practice is to utilize with clause:
```python
with open(file_path, 'r') as file:
	#run some code
```
This is the context manager functionality which ensures that the connection is closed when program is terminated, including cases when an error is raised. There are different ways to create a context manager that works in with clause, one of them is shown below.

## Class Methods {#methods}

There are two methods that transform a class into context manager: `__enter__` and `__exit__`. The return value from `__enter__` is assigned to the target in the `with` statement, and `__exit__` method is invoked in the end (that's where connection is closed).

First, lets define a new class called `SQLiteClient`:
```python
'''SQLite client'''
import sqlite3
from typing import Tuple, Any, List, Optional
import re


class SQLiteClient():
	def __new__(cls,
				db_name: str = "",
				timeout:int = 10):
		"""
		Ensure parameters are valid prior to memory allocation.

		Parameters
		----------
		db_name: str.
			Name of SQLite database. Can also be a path to database
			if it is in another directory.

		timeout: int.
			Number of seconds before raising OperationalError.
		"""
		if not db_name:
			raise ValueError("Database Name is Missing!")

		if not isinstance(db_name, str):
			raise TypeError("Database name must be a string!")

		if not isinstance(timeout, int):
			raise TypeError("Timeout must be an integer!")

		if  timeout < 0 :
			raise ValueError("Timeout must be greater than 0")

		return super().__new__(cls)

	def __init__(self, db_name: str = "",  timeout: int = 10):
		"""
		This method is executed after memory allocation
		(after execution of __new__ method, and given all
		parameters are valid), and initialize attributes.

		Parameters
		----------
		db_name: str.
			Name of SQLite database. Can also be a path to database
			if it is in another directory.

		timeout: int.
			Number of seconds before raising OperationalError.
		"""
		# Ensure .db file extension exists.
		self.name = db_name
		if db_name[-3:] != ".db":
			self.name += ".db"
		self.timeout = timeout
		self.__conn = None
```
Note that the `__new__` method was utilized in order to check wether arguments are valid. If invalid, then an error is raised and memory will not be allocated. Otherwise, memory is allocated for the new object and `__init__` method is invoked to initialize attributes.

At this point an object can be created by simply running `SQLiteClient(arguments)`. Adding the code below transform the client to a context manager:
```python
	def __enter__(self):
		"""
		Provides context manager functionality which
		is triggered by 'with' clause and creates connection to database.
		For example:
		`with SQLiteClient(name, 10) as client:
			#code.....
		`
		"""
		self.__conn = sqlite3.connect(self.name,
									timeout=self.timeout)

		return self

	def __exit__(self, exc_type, exc_val, exc_tb):
		"""
		Part of the context manager functionality.
		This method ensures that connection to
		database is always closed at the end of execution,
		even if an error is raised.
		"""
		self.__conn.close()
		self.__conn = None
```
Note that `__enter__` establish connection to sqlite database using `sqlite3.connect()` method and `__exit__` ensures connection is closed by executing `self.conn.close()` method.

Additional functionality can be added to allow executing CRUD (Create, Read, Update and Delete) operations:
```python
	def __exec(self,
		       query: str,
		       parameters: Tuple[Any,...],
		       rows: int = 0) -> List[Tuple[Any,...]]:
		"""
		Private method (starts with __) - Only accessible
		by methods defined withini the SQLiteClient class scope.

		Execute a sql query containing question marks `?`
		and replace them with parameters (based on their order).
		Rows variable is used to distinguish between CIUD (CREATE,
		INSERT, UPDATE, DELETE) operations and SELECT statements
		(fetchone, fetchall, fetchmany).

		Note: Method was set as private to prevent bad actors from
			  running dangerous sql statements (like sql injections)

		Parameters
		----------
		query: str.
			SQL script to execute. Note that values are represented wtih
			question mark (?). For example:
			`SELECT * FROM users WHERE name = ? and pass = ?`

		parameters: Tuple(Any).
			Values that are safely inserted into sql statement (replacing ?).
			For example: (username, pass)
			If there is only one variable, then tuple should look like that: (value,).
			If there are no variables, then tuple can be empty: ().

		rows: int.
			Set to CIUD (CREATE, INSERT, UPDATE and DELETE) when rows = 0,
			otherwise represent SELECT statement. rows = -1 returns all rows (fetchall),
			rows = 1 returns only one row (fetchone) and specific number limit
			the number of rows (fetchmany).
		"""
		cur = self.__conn.cursor()
		cur.execute(query, parameters)

		# CREATE, INSERT, UPDATE, DELETE operations
		if rows == 0:
			self.__conn.commit()
			return None

		# SELECT - fetchall
		if rows == -1:
			data_ls = cur.fetchall()

		# SELECT - fetchone
		if rows == 1:
			data_ls = [cur.fetchone()]

		# SELECT - get all rows
		if rows > 1:
			data_ls = cur.fetchmany(rows)

		return data_ls

	def is_conn(func):

		def check_connection(self, query, parameters, *args, **kwargs):

			if not self.__conn:
				raise ValueError("No connection to database")

			return func(self, query, parameters, *args, **kwargs)

		return check_connection

	def check_args(func):

		def wrapper(self, query, parameters, *args, **kwargs):

			# Type checks
			if not isinstance(query, str):
				raise TypeError("Query must be a string!")

			if not isinstance(parameters, tuple):
				raise TypeError("Parameters variable must be a tuple!")

			# Number of variables checks
			# Number of ? in query vs. length of parameters
			variables = re.findall('\?', query)
			if len(variables) > len(parameters):
				raise ValueError("Some variables are missing!")

			if len(variables) < len(parameters):
				raise ValueError("Too many values in parameters. This may raise sqlite3.ProgrammingError. Please check your parameters and try again.")

			# Value Errors unique cases for each function.
			if func.__name__ == "ciud":
				if query[:6].lower() not in ["create", "insert", "update", "delete"]:
					raise ValueError("Query must be CIUD (CREATE, INSERT UPDATE or DELETE)")

			if func.__name__ == "read":
				if query[:6].lower() not in ["select"]:
					raise ValueError("Query must start with `SELECT ....`")

				rows = None
				try:
					rows = args[0]
				except IndexError:
					if "rows" in kwargs.keys():
						rows = kwargs["rows"]
				finally:
					if rows == 0:
						raise ValueError("Cannot commit SELECT statement. Please set `rows` to a number other than 0")

			return func(self, query, parameters, *args, **kwargs)

		return wrapper

	@is_conn
	@check_args
	def ciud(self,
			 query: str,
			 parameters: Tuple[Any, ...]) -> None:
		"""
		Execute CREATE, INSERT, UPDATE or DELETE operation.

		Parameters
		----------
		query: str.
			String containing query that starts with CIUD operations, otherwise
			an error is raised. Values are marked with question mark (?)

		parameters: Tuple[Any].
			Tuple of values to replace question marks in the order they are presented.
			Note that the length of parameters must match the number of variables (?)
			within `query`, otherwise an error is raised.
		"""

		self.__exec(query, parameters)

	@is_conn
	@check_args
	def read(self,
		 	 query: str,
		 	 parameters: Tuple[Any, ...],
		 	 rows: int) -> Optional[List[str]]:
		"""
		Execute CREATE, INSERT, UPDATE or DELETE operation.

		Parameters
		----------
		query: str.
			String containing query that starts with CIUD operations, otherwise
			an error is raised. Values are marked with question mark (?)

		parameters: Tuple[Any].
			Tuple of values to replace question marks in the order they are presented.
			Note that the length of parameters must match the number of variables (?)
			within `query`, otherwise an error is raised.

		rows: int.
			Must be an integer number othan than 0. rows = -1 returns all rows (fetchall),
			rows = 1 returns only one row (fetchone) and specific number limit
			the number of rows (fetchmany).
		"""
		data_ls = self.__exec(query, parameters, rows)
		return data_ls
```
Final code appears in the bottom of this page.

## Tests

#### Test 1: Initialize an object
```python
test1 = SQLiteClient("test.db")
print(test1.name)
test1.ciud("CREATE TABLE IF NOT EXISTS users (id INTEGER PRIMARY KEY, username text, hash text)",())
```

Output:
```output
test.db
Traceback (most recent call last):
	...
ValueError: No Connection to database
```
Object was initialied and name was set to `test.db`, but there is no connection.


#### Test 2: Using with clause
```python
with SQLiteClient("another_test") as client:
	# Create table 'users' with columns: username, hash
	client.ciud("CREATE TABLE IF NOT EXISTS users (id INTEGER PRIMARY KEY, username text, hash text)",())

	# Insert values
	client.ciud("INSERT INTO users VALUES (NULL,?,?)",("Test 1", "a"))
	client.ciud("INSERT INTO users VALUES (NULL,?,?)",("Test 2", "b"))
	client.ciud("INSERT INTO users VALUES (NULL,?,?)",("Test 3", "c"))

	# Read all columns and all rows
	print(client.read("SELECT * FROM users", (), rows = -1))

	# Read all columns of the first 2 rows.
	print(client.read("SELECT * FROM users",(), rows= 2))
```
A database file (another_test.db) was created in the current working directory, and the output of the `SELECT` statements appears below

Output:
```output
[(1, 'Test 1', 'a'), (2, 'Test 2', 'b'), (3, 'Test 3', 'c')]
[(1, 'Test 1', 'a'), (2, 'Test 2', 'b')]
```



```python {"title": "Final Code"}
'''SQLite client'''
import sqlite3
from typing import Tuple, Any, List, Optional
import re


class SQLiteClient():
	def __new__(cls,
				db_name: str = "",
				timeout:int = 10):
		"""
		Ensure parameters are valid prior to memory allocation.

		Parameters
		----------
		db_name: str.
			Name of SQLite database. Can also be a path to database
			if it is in another directory.

		timeout: int.
			Number of seconds before raising OperationalError.
		"""
		if not db_name:
			raise ValueError("Database Name is Missing!")

		if not isinstance(db_name, str):
			raise TypeError("Database name must be a string!")

		if not isinstance(timeout, int):
			raise TypeError("Timeout must be an integer!")

		if  timeout < 0 :
			raise ValueError("Timeout must be greater than 0")

		return super().__new__(cls)

	def __init__(self, db_name: str = "",  timeout: int = 10):
		"""
		This method is executed after memory allocation
		(after execution of __new__ method, and given all
		parameters are valid), and initialize attributes.

		Parameters
		----------
		db_name: str.
			Name of SQLite database. Can also be a path to database
			if it is in another directory.

		timeout: int.
			Number of seconds before raising OperationalError.
		"""
		# Ensure .db file extension exists.
		self.name = db_name
		if db_name[-3:] != ".db":
			self.name += ".db"
		self.timeout = timeout
		self.__conn = None

	def __enter__(self):
		"""
		Provides context manager functionality which
		is triggered by 'with' clause and creates connection to database.
		For example:
		`with SQLiteClient(name, 10) as client:
			#code.....
		`
		"""
		self.__conn = sqlite3.connect(self.name,
									timeout=self.timeout)

		return self

	def __exit__(self, exc_type, exc_val, exc_tb):
		"""
		Part of the context manager functionality.
		This method ensures that connection to
		database is always closed at the end of execution,
		even if an error is raised.
		"""
		self.__conn.close()
		self.__conn = None

	def __exec(self,
		       query: str,
		       parameters: Tuple[Any,...],
		       rows: int = 0) -> List[Tuple[Any,...]]:
		"""
		Private method (starts with __) - Only accessible
		by methods defined withini the SQLiteClient class scope.

		Execute a sql query containing question marks `?`
		and replace them with parameters (based on their order).
		Rows variable is used to distinguish between CIUD (CREATE,
		INSERT, UPDATE, DELETE) operations and SELECT statements
		(fetchone, fetchall, fetchmany).

		Note: Method was set as private to prevent bad actors from
			  running dangerous sql statements (like sql injections)

		Parameters
		----------
		query: str.
			SQL script to execute. Note that values are represented wtih
			question mark (?). For example:
			`SELECT * FROM users WHERE name = ? and pass = ?`

		parameters: Tuple(Any).
			Values that are safely inserted into sql statement (replacing ?).
			For example: (username, pass)
			If there is only one variable, then tuple should look like that: (value,).
			If there are no variables, then tuple can be empty: ().

		rows: int.
			Set to CIUD (CREATE, INSERT, UPDATE and DELETE) when rows = 0,
			otherwise represent SELECT statement. rows = -1 returns all rows (fetchall),
			rows = 1 returns only one row (fetchone) and specific number limit
			the number of rows (fetchmany).
		"""
		cur = self.__conn.cursor()
		cur.execute(query, parameters)

		# CREATE, INSERT, UPDATE, DELETE operations
		if rows == 0:
			self.__conn.commit()
			return None

		# SELECT - fetchall
		if rows == -1:
			data_ls = cur.fetchall()

		# SELECT - fetchone
		if rows == 1:
			data_ls = [cur.fetchone()]

		# SELECT - get all rows
		if rows > 1:
			data_ls = cur.fetchmany(rows)

		return data_ls

	def is_conn(func):

		def check_connection(self, query, parameters, *args, **kwargs):

			if not self.__conn:
				raise ValueError("No connection to database")

			return func(self, query, parameters, *args, **kwargs)

		return check_connection

	def check_args(func):

		def wrapper(self, query, parameters, *args, **kwargs):

			# Type checks
			if not isinstance(query, str):
				raise TypeError("Query must be a string!")

			if not isinstance(parameters, tuple):
				raise TypeError("Parameters variable must be a tuple!")

			# Number of variables checks
			# Number of ? in query vs. length of parameters
			variables = re.findall('\?', query)
			if len(variables) > len(parameters):
				raise ValueError("Some variables are missing!")

			if len(variables) < len(parameters):
				raise ValueError("Too many values in parameters. This may raise sqlite3.ProgrammingError. Please check your parameters and try again.")

			# Value Errors unique cases for each function.
			if func.__name__ == "ciud":
				if query[:6].lower() not in ["create", "insert", "update", "delete"]:
					raise ValueError("Query must be CIUD (CREATE, INSERT UPDATE or DELETE)")

			if func.__name__ == "read":
				if query[:6].lower() not in ["select"]:
					raise ValueError("Query must start with `SELECT ....`")

				rows = None
				try:
					rows = args[0]
				except IndexError:
					if "rows" in kwargs.keys():
						rows = kwargs["rows"]
				finally:
					if rows == 0:
						raise ValueError("Cannot commit SELECT statement. Please set `rows` to a number other than 0")

			return func(self, query, parameters, *args, **kwargs)

		return wrapper

	@is_conn
	@check_args
	def ciud(self,
			 query: str,
			 parameters: Tuple[Any, ...]) -> None:
		"""
		Execute CREATE, INSERT, UPDATE or DELETE operation.

		Parameters
		----------
		query: str.
			String containing query that starts with CIUD operations, otherwise
			an error is raised. Values are marked with question mark (?)

		parameters: Tuple[Any].
			Tuple of values to replace question marks in the order they are presented.
			Note that the length of parameters must match the number of variables (?)
			within `query`, otherwise an error is raised.
		"""

		self.__exec(query, parameters)

	@is_conn
	@check_args
	def read(self,
		 	 query: str,
		 	 parameters: Tuple[Any, ...],
		 	 rows: int) -> Optional[List[str]]:
		"""
		Execute CREATE, INSERT, UPDATE or DELETE operation.

		Parameters
		----------
		query: str.
			String containing query that starts with CIUD operations, otherwise
			an error is raised. Values are marked with question mark (?)

		parameters: Tuple[Any].
			Tuple of values to replace question marks in the order they are presented.
			Note that the length of parameters must match the number of variables (?)
			within `query`, otherwise an error is raised.

		rows: int.
			Must be an integer number othan than 0. rows = -1 returns all rows (fetchall),
			rows = 1 returns only one row (fetchone) and specific number limit
			the number of rows (fetchmany).
		"""
		data_ls = self.__exec(query, parameters, rows)
		return data_ls
```


Context Manager


# SSH Connection

## Introduction {#introduction}


In this project, SSH connection was enabled in [aCloud](/Projects/EE/acloud), and it was configured to authenticate with keys (public and private) for enhanced security. This webpage is a step-by-step guide to enable and configure SSH in a server.


***


## Enable SSH on Raspbian {#enable}

Raspbian comes with SSH installed, so enabling it is through settings:
```shell
sudo raspi-config
```

Configuration window will open in the same terminal, there scroll down to __Interface Options (No. 3)__
and press Enter. This will open the options for interface such as VNC, SSH etc. Scroll down to SSH and follow instructions to enable it.

At this stage, you should be able to access the server by running the following command:
```shell
ssh user@IP_ADDRESS
```
If you login for the first time, type "yes" for the message that appears. It will also ask for password, so make sure to enter the password of that user you are trying to log into (password is hidden so while typing nothing appears on screen).
To enhance security, keys can be utilized for login instead. There are a few steps:


- On local computer (not the server), run the following command to generate both public and private keys:
  ```shell
  ssh-keygen -t rsa
  ```
  or to make a larger key (more secure):
  ```shell
  ssh-keygen -t rsa 4096
  ```
  For more information, visit [ssh-keygen manual page](https://linux.die.net/man/1/ssh-keygen).

- It will ask for a directory, press enter for the default directory (~/.ssh/id_rsa).
  Also, it will ask for passphrase which is another layer of security.
  You can leave it empty and press Enter if you don't want to add password.

- Copy **public** key (id_rsa.pub) to the server by running the following command:
  ```shell
  ssh-copy-id username@IP_ADDRESS
  ```
  You will need to authneticate with password, so make sure it matches the password of "username".

- At this point login without password should work, although, for me it didn't work until password authentication was disabled
  on the server. So, in this step, make sure that the server is connected with HDMI to a screen, just in case you lose SSH connection and can't login. Open terminal and run:
  ```shell
  sudo nano /etc/ssh/sshd_config
  ```
  It will open the configuration file with write premissions. Make sure the following settings are applied:
  ```shell
  PasswordAuthentication no
  ChallengeResponseAuthentication no
  PubkeyAuthentication yes
  AuthorizedKeysFile  .ssh/authorized_keys
  ```
  Now try SSH again, it shouldn't ask for password, and use keys instead. For more information on those settings, visit [ssh-config manual page](https://linux.die.net/man/5/ssh_config).
  SSH session can be limited to 5 minutes (300 seconds) without activity by adding the following line to /etc/ssh/sshd_config:
  ```shell
  ClientAliveInterval 300
  ```
  For more information, vist [session timeouts](https://serverauth.com/posts/how-to-configure-ssh-session-timeouts).


***


## Shorter SSH Command {#short-ssh}

One can create an alias, so to avoid writing user@IP_ADDRESS everytime we would like to connect through SSH.
To do so, in your local machine (not the server), open terminal and run the following command:
```shell
sudo nano ~/.ssh/config
```
Then, add the following settings:
```shell
Host ALIAS_NAME
    Hostname SERVER_IP_ADDRESS
    User SERVER_USERNAME
    IdentityFile ~/.ssh/id_rsa
```
**Note:** if you decided to rename the key (other than id_rsa), then it should be updated in IdentityFile as well.
Restart SSH service, then run the following command to connect to server:
```shell
ssh ALIAS_NAME
```


***

## Email Notification (Optional) {#email}

This step is crucial for monitoring SSH sessions and enhance security. The sessions are automatically logged in the system log file (/var/log/auth.log), and that's where I get the information from. Email notifications are sent everytime a connection is established, so it allows a quick response in case that someone penetrated the server. This step has 3 files: rc, and two .sh files (bash programs). The first file (rc) is being executed after every successful SSH connection. The other files create an email template (.txt file), and send the email. Additionaly, you will need an email service that allows third party apps (app password in Gmail). I also tried Yahoo, but their third party service is down.

- Open terminal in the .ssh directory, or open terminal and run the following Command
  ```shell
  cd ~/.ssh
  ```

- Create a new .sh file (email_temp.sh):
  ```shell
  sudo nano email_temp.sh
  ```
  This is the template of the email. You can change it according to your needs.
  ```shell
  echo "From: ssh-connection &lt;${SENDER_EMAIL}&gt;\n"\
          "To: avivfaraj &lt;${RECEIVER_EMAIL}&gt;\n"\
          "Subject: ssh connection established\n\n"\
          $1 	> ~/.ssh/mail.txt
  ```
  This file will create the email_template (mail.txt) inside the .ssh directory. Note that $1 means first parameter sent while executing file.
  For example:
  ```shell
  sh email_temp.sh "This is just a test"
  ```
  This code will execute the file and the content will be "This is just a test"

- Create a new .sh file (send_email.sh):
  ```shell
  sudo nano send_email.sh
  ```
  This code will send the email using the curl command:
  ```shell {"filename": "send_email.sh"}
  curl -s -o /dev/null --ssl-reqd \
        --url "smtps://smtp.gmail.com:465" \
        --user "${SENDER_EMAIL}:${EMAIL_PASSWORD}" \
        --mail-from "${SENDER_EMAIL}" \
        --mail-rcpt "${RECEIVER_EMAIL}" \
        --upload-file ~/.ssh/mail.txt
  ```
  `SENDER_EMAIL`, `RECEIVER_EMAIL` and `EMAIL_PASSWORD` are all environment variables and should be defined prior to execution.
  The -s -o /dev/null options are utilized for silent execution meaning there will be no output in terminal.

- Create rc file that will execute the above .sh programs and send an email for every connection:
  ```shell
  sudo nano rc
  ```
  In the file write the following code:
  ```shell
  details=$(sudo grep "sshd\[[0-9]*\]: Accepted"  /var/log/auth.log | sed 's/ssh2.*/ /g' | tail -1)
  echo "$details" >> ~/.ssh/history.log<br/>
  sh ~/.ssh/email_temp.sh "$details"
  sh ~/.ssh/send_email.sh<br/>
  ```
  The first line (details=$...) reads the system log file, and filters the file using regular expression.
  Then, the key part in the notification is removed using sed command. Finally, the last occurence is saved using tail -1.
  The second line appends connection information to history.log file (will be created if not exists).
  This is optional, but I wanted to have a history of sessions just in case I would like to investigate an anomaly.
  Third line executes email_temp.sh and creates the .txt file (email content), which is sent by the fouth line.


***


## Test {#test}

To test it, I connected through SSH and checked my email:

![email test {600x125} {caption: Email Received}](ssh/email_notification.png)

Then, I tried to connect from a different computer to test authentication without keys. Since password authentication was disabled, I expect SSH login to be denied.

![Access test  {450x50} {caption: Access Denied}](ssh/access_denied.png)

I also tried to copy-paste both public and private authentication keys from the authorized computer to another computer, and login was denied, so server is secured.


SSH Connection


[iframe {caption: "Karp-Rabin Algorithm"}](/karp-rabin-algorithm/karp_rabin_algorithm_review.pdf)



Karp Rabin Algorithm


# Python Decorators

## Introduction {#introduction}

Decorator is a nested function that receives another function as an argument by using the @ symbol. Decorators can be utilized to print, debug, time and run checks on values as shown in this page.

---

## Background {#background}

In Python, functions can be nested:
```python
def welcome_msg():
	def inner():
		return "Welcome"
	return inner
```
It could also get a function (and its parameters) as arguments:
```python
def welcome_msg(func):
	def inner(*args, **kwargs):

		# Print function name
		print(f"Welcome from {func.__name__}")

		# Execute function
		return func(*args, **kwargs)
	return inner
```
It can be used as follows:
```python
@welcome_msg
def hello_world():
	print("Hello World")

hello_world()
```
In this case the output is:
```output
Welcome from hello_world
Hello World
```
The decorator (@welcome_msg) was executed first receiving `hello_world` as an argument. This decorator only prints a welcome message: `Welcome from {func.__name__}` where `func.__name__` is the name of the decorated function, so it is `hello_world`. Then, the function is executed and one can see that `Hello World` was printed. This functionality will be used in the next examples to show debugging, timing and running validation checks.

---

## Timing

One can utilize decorators to print the time before and after function execution as can be seen below:
```python
from datetime import datetime
DATETIME_FORMAT = '%Y-%m-%d %H:%M:%S'

def timeit(func):
	def inner(*args, **kwargs):

		# Print 'DATE HOUR:MINUTE:SECOND' before execution
		print(f"Started Execution: {datetime.now().strftime(DATETIME_FORMAT)}")

		# Execute function
		var = func(*args, **kwargs)

		# Print `DATE HOUR:MINUTE:SECOND' after execution
		print(f"Finished Execution: {datetime.now().strftime(DATETIME_FORMAT)}")

		return var
	return inner
```

For example, a program that count to `num` (with 1 second delay between each count).
```python
import time

@timeit
def counter(num, delay = 1):
	for i in range(num):
		print(i+1)
		time.sleep(delay)

counter(5)
```
Output:
```output
Started Execution: 2024-02-04 11:12:17
1
2
3
4
5
Finished Execution: 2024-02-04 11:12:22
```

One can calculate execution time instead of printing time before and after execution. Consider the following `timeit2` decorator:
```python
def timeit2(func):
	def inner(*args, **kwargs):
		start = datetime.now()
		# Execute function
		var = func(*args, **kwargs)

		end = datetime.now()

		# Calculate execution time
		duration = end-start

		args_str = ','.join([str(i) for i in args])
		print(f"{func.__name__}({args_str}) Execution Time: {str(duration).split('.')[0]}")

		return var
	return inner

@timeit2
def counter(num, delay = 1):
	for i in range(num):
		print(i+1)
		time.sleep(delay)

counter(10)
```
Output:
```output
1
2
3
4
5
6
7
8
9
10
counter(10) Execution Time: 0:00:10
```

---

## Debugging

Decorators can be used to print the values of arguments passed to a function and the returned value:
```python
import inspect

def debug(func):
	def inner(*args, **kwargs):

		# Print function's signature for reference.
		# Parameters with default values are not accessible
		# via either *args or kwargs, so this line is meant for those
		# parameters to ensure they are taken into account in debugging.
		print(f'Signature: {func.__name__}{inspect.signature(func)}')

		# Convert *args and **kwargs into strings
		arguments = ",".join([str(i) for i in args])
		keyworded_args = ",".join([key + "=" + str(value) for key,value in kwargs.items()])

		# Print Execution signature.
		print(f'Execution: {func.__name__}({arguments}{"," + keyworded_args if keyworded_args else ""})')

		# Execute function
		var = func(*args, **kwargs)

		# Print results
		print(f'Returned Value: {str(var)}')
		
		return var

	return inner

@debug
def summation(upper, lower = 1):
	return sum([i for i in range(lower, upper + 1)])

summation(3)
```
Summation was implemented and was debugged using the `@debug` decorator which prints the function's signature, values of arguments at the time of execution, and the returned value:
```output
Signature: summation(upper, lower=1)
Execution: summation(3)
Returned Value: 6
```

---

## Validation Checks

Ensures that all arguments are valid (type, range, etc.) and prevents execution of the decorated function by raising an error. It can enhance debugging since the source of the error is known given error messages are clear. I have been using this technique for database client to ensure that the program is connected to database.
```python
def validate(func):
	def inner(*args):
		invalid_arg = None
		for arg in args:
			if not isinstance(arg, int):
				raise TypeError("Both Upper and Lower bounds of summation must be integers!")
	return inner

@validate
def summation(upper, lower = 1):
	return sum([i for i in range(lower, upper + 1)])

summation(3.5)
```
output:
```output
TypeError: Both Upper and Lower bounds of summation must be integers!
```

---

## Example - SQLiteClient

Validation checks were used in [Context Manager](/Projects/CS/context-manager) page where I created two decorators: `is_conn` (line 141 in Final Code section) to check connection, and `check_args` (line 152 in Final Code section) to validate arguments.
They can be merged into one decorator, but I wanted to keep the separate for convenience.



Python Decorators


[iframe {caption: "Hierholzer  Algorithm"}](/hierholzer/Hierholzer_algorithm_review.pdf)


Hierholzer Algorithm



# todocom (Todo Comments)


## Introduction {#introduction}

CLI program that retrieves all TODO comments from file(s) and prints them in terminal/shell.
It was created in order to automatically update a list of TODO tasks by simply adding "TODO:" comments in the code [Comments Format](#format)
It also enables prioritization of tasks by using "TODO soon:" or "TODO urgent" and assign tasks to a specific teammate.

---

## Usage {#usage}

Add TODO comments according to the format, open terminal and run the following command:
```shell
todo [folder/file]
```

[gif {title: "General TODO Command"}](https://user-images.githubusercontent.com/73610201/211216011-27e057b0-0420-4d90-8950-999f75583566.gif)

---

## Types {#types}

This command will print out all TODO comments that were found in the code, sorted by their prioritization: urgent, soon and regular.
__Urgent__ tasks will be printed in RED, __soon__ in CYAN and __regular__ comments in WHITE to make it easier to read. There is also an option to filter comments by their priotization:

```shell
# Prints urgent TODOs
todo -u [folder/file]
```

[gif {title: "Urgent TODO command"}](https://user-images.githubusercontent.com/73610201/211216002-c00860d3-7a61-425f-8cb2-939de85c01ec.gif)

or:
```shell
# Prints soon TODOs
todo -s [folder/file]
```
[gif {title: "Soon TODO command"}](https://user-images.githubusercontent.com/73610201/211216007-f4eabb81-76d0-42c5-9334-0f13857e809b.gif)


Comments can also be assigned to a user by adding "Todo @username" comment:
```shell
# Prints Assigned TODOs
todo -a [USERNAME] [folder/file]
```

[gif {title: "Assigned TODO command"}](https://user-images.githubusercontent.com/73610201/211216263-ca453589-e490-49b3-a839-65315366f34f.gif)

Finally, there is an option to save the list in a text file (stores as regular text without colors):
```shell
# Store results in a txt file
todo -o [path/to/sample.txt] [folder/file]
```

---

## Setup {#install}

```shell
pip install todocom
```

---

## Comments Format {#format}

There are two types of comments: single line and multi-line. Currently, multi-line comments (docstrings) are only supported in Python, but single line should work for most programming languages.

Format is flexible and can be lower-case, upper-case or a combination of both. Below are several examples:
```list
                        - TODO:       - ToD0:
                        - To-D0:      - to-do:
                        - TODo:       - TOD0:
```

In __Urgent__ and __soon__ comments the TODO part is flexible as shown above, but must be followed by either __urgent__ or __soon__ in lower-case:
```list
                    - TO-DO soon:       - tODo soon:
                    - ToD0 urgent:      - T0-D0 urgent:
```


Computer Science Projects

Context Manager

SSH Connection

Karp Rabin Algorithm

Python Decorators

Hierholzer Algorithm

TODO Comment (TODOCOM)