python-doc/Docs/Basic/09-standard-libs.md

09 – Python Standard Library

This document introduces some of the most commonly used Python standard library modules:

• datetime
• math
• random
• decimal

These modules come bundled with Python and require no external installation.

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1. Date and Time with datetime

The datetime module provides classes for working with dates and times.

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Working with Dates

Code

import datetime

date_1 = datetime.date(2026, 1, 1)

print(date_1.year)
print(date_1.month)
print(date_1.day)

print(date_1.weekday)
print(date_1.ctime)

Explanation

• datetime.date(year, month, day) creates a date object.
• .year, .month, .day access individual components.

Important Note

date_1.weekday()

• Returns the day of the week as an integer:

  • Monday = 0
  • Sunday = 6

date_1.ctime()

• Returns a human-readable string representation of the date.

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Working with Time

Code

time_1 = datetime.time(12, 12)

print(time_1.hour)
print(time_1.min)

Explanation

• datetime.time(hour, minute) creates a time object.
• .hour returns the hour.
• .minute returns the minute.

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Working with Date and Time Together

Code

abbas_birth = datetime.datetime(2026, 1, 1, 12, 12)
today = datetime.date.today()
now = datetime.datetime.now()

diff_time = now - abbas_birth

Explanation

• datetime.datetime includes both date and time.
• date.today() returns today’s date.
• datetime.now() returns the current date and time.
• Subtracting two datetime objects returns a timedelta.

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2. Mathematical Operations with math

The math module provides advanced mathematical functions and constants.

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Mathematical Constants

import math

print(math.pi)
print(math.e)
print(math.inf)

• math.pi: π constant
• math.e: Euler’s number
• math.inf: infinity

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Power and Rounding

print(math.pow(2, 3))

print(round(4.2))
print(round(4.8))

• math.pow(a, b) returns a raised to the power of b.
• round() rounds to the nearest integer.

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Floor and Ceil

print(math.floor(4.2))
print(math.floor(4.9))

print(math.ceil(4.2))
print(math.ceil(4.9))

• floor: rounds down
• ceil: rounds up

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Logarithms

print(math.log(100, 10))

• Returns the logarithm of 100 with base 10.

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3. Random Values with random

The random module is used to generate pseudo-random values.

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Random Numbers

import random

print(random.randint(1, 6))
print(random.random())

• randint(a, b): random integer between a and b (inclusive)
• random(): random float between 0 and 1

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Random Selection

number_list = list(range(15))
print(random.choice(number_list))

char_list = ['a', 'm', 's']
print(random.choice(char_list))

• choice() selects a random element from a sequence.

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Shuffling

random.shuffle(number_list)
print(number_list)

• shuffle() randomly rearranges the list in place.

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4. Decimal Precision with decimal

The decimal module provides precise decimal arithmetic, avoiding floating-point errors.

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Decimal Context

import decimal

print(decimal.getcontext())

• Shows current precision and rounding settings.

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Float vs Decimal

print(decimal.Decimal(0.1))
print(decimal.Decimal('0.1'))

• Passing a float carries floating-point error.
• Passing a string preserves exact value.

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Precision Comparison

print(0.1 + 0.2 == 0.3)

Returns False due to floating-point precision issues.

print(decimal.Decimal(0.1) + decimal.Decimal(0.2) == decimal.Decimal(0.3))

Still False because the floats are imprecise.

print(decimal.Decimal('0.1') + decimal.Decimal('0.2') == decimal.Decimal('0.3'))

Returns True because strings preserve precision.

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Summary

• datetime handles dates and times
• math provides mathematical constants and functions
• random generates pseudo-random values
• decimal solves floating-point precision problems
• Always use strings when creating Decimal values