minte9
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Logging

p255 Logging is a great way to understand what's happening in your program. 
 
"""Logging
Set basic configuration to add logging to your program.
Factorial: n! = n(n-1)!
"""
import logging
logging.basicConfig(level=logging.DEBUG, 
    format='%(asctime)s - %(levelname)s - %(message)s'
)

def factorial(n):
    if (n == 0): return 1
    logging.debug('factorial(%s)' % n)
    n = n * factorial(n-1)
    logging.debug('n is ' + str(n))
    return n

factorial(4)

"""
2021-12-10 16:04:03,046 - DEBUG - factorial(4)
2021-12-10 16:04:03,046 - DEBUG - factorial(3)
2021-12-10 16:04:03,047 - DEBUG - factorial(2)
2021-12-10 16:04:03,047 - DEBUG - factorial(1)
2021-12-10 16:04:03,047 - DEBUG - n is 1
2021-12-10 16:04:03,047 - DEBUG - n is 2
2021-12-10 16:04:03,047 - DEBUG - n is 6
2021-12-10 16:04:03,047 - DEBUG - n is 24
"""

Debug

p259 It's bettter not to debug a code with print() function. 
 
"""Debug
If you use print() calls to debug your code, 
you'll spend a lot of time to remove thems, once you are done.
With logging module you can easily switch between ...
showing or hidding messages.
"""
import logging
logging.basicConfig(level=logging.DEBUG, 
    format='%(asctime)s - %(levelname)s - %(message)s'
)

def factorial(n):
    if (n < 0):
        logging.critical('n is not positive!')
        return None
    if (n == 0): return 1
    n = n * factorial(n-1)
    logging.debug('n is ' + str(n))
    return n


"""Logging messages enabled:
"""
assert factorial(-1) == None
assert factorial(4)  == 24
    # 2021-12-10 16:31:44,977 - CRITICAL - n is not positive!
    # 2021-12-10 16:18:59,379 - DEBUG - n is 1
    # 2021-12-10 16:18:59,379 - DEBUG - n is 2
    # 2021-12-10 16:18:59,379 - DEBUG - n is 6
    # 2021-12-10 16:18:59,379 - DEBUG - n is 24


"""Logging messages disabled:
Only level > ERROR messages are displayed
"""
logging.disable(logging.ERROR)
assert factorial(-1) == None
assert factorial(0)  == 1
assert factorial(3)  == 6
assert factorial(4)  == 24
    # 2021-12-10 16:34:58,048 - CRITICAL - n is not positive!

File

Insteed of displaying messages to the screen, you can write them to a file. 
 
"""Logging messages to file:
Screen messages are helpful, but they can make the output ...
hard to read.
Writting logging messages to a file keeps your screen clean.
"""
import logging, pathlib
DIR = pathlib.Path(__file__).resolve().parent

logging.basicConfig(
    level=logging.DEBUG, 
    filename=DIR/'logs/file1.log', 
    format='%(asctime)s - %(levelname)s - %(message)s'
)

def factorial(n):
    if (n < 0):
        logging.critical(f'Number {n} is not positive!')
        return None
    if (n == 0): return 1
    n = n * factorial(n-1)
    logging.debug('n is ' + str(n))
    return n

assert factorial(-1)  == None
assert factorial(4)   == 24
assert factorial(-11) == None


""" logs/file1.log
--------------------------------------------------------------
2021-12-10 17:09:40,229 - CRITICAL - Number -1 is not positive!
2021-12-10 17:09:40,229 - DEBUG - n is 1
2021-12-10 17:09:40,229 - DEBUG - n is 2
2021-12-10 17:09:40,229 - DEBUG - n is 6
2021-12-10 17:09:40,229 - DEBUG - n is 24
2021-12-10 17:09:40,229 - CRITICAL - Number -11 is not positive!
2021-12-10 17:09:59,231 - CRITICAL - Number -1 is not positive!
2021-12-10 17:09:59,231 - DEBUG - n is 1
2021-12-10 17:09:59,231 - DEBUG - n is 2
2021-12-10 17:09:59,231 - DEBUG - n is 6
2021-12-10 17:09:59,232 - DEBUG - n is 24
2021-12-10 17:09:59,232 - CRITICAL - Number -11 is not positive!
"""

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