The Python Conceptual Hierarchy

Before we get to the code, let’s first establish a clear picture of how this chapter fits into the overall Python picture. From a more concrete perspective, Python programs can be decomposed into modules, statements, expressions, and objects, as follows:

  1. Programs are composed of modules.
  2. Modules contain statements.
  3. Statements contain expressions.
  4. Expressions create and process objects

Numbers:

Intergers: no fractional part

Floating-point numbers: with fractional part.

3.14e-2 # Scientific notation

123 + 222 # Integer addition

1.5 * 4 # Floating-point multiplication

2 ** 100 # 2 to the power 100

len(str(2 ** 1000000)) # How many digits in a really BIG number?

import math
print(math.pi)
print(math.sqrt(85))

The math module contains more advanced numeric tools as functions, while the random module performs random-number generation and random selections (here, from a Python list coded in square brackets — an ordered collection of other objects to be introduced later):

import random
print(random.random())
random.choice([1, 2, 3, 4])

String:

"

'

Question: What if there's quotation mark in the string?

print('I\'m \"ok\".')  # Use escape character \

print('I\'m learning\nPython.')

print('\\\n\\')

We could use r'' to avoid escape character:

print('\\\t\\')

print(r'\\\t\\')

Instead of \n, we could use '''...''' to display multiple lines.

print('''line1
... line2
... line3''')

Boolean

either True or False.

True

False

3 > 2

3 > 5

Logical operators: and, or, not

True and True

True and False

False and False

5 > 3 and 3 > 1

True or True

True or False

False or False

5 > 3 or 1 > 3

not True

not False

not 1 > 2

We often use Boolean type in conditional statements:

if age >= 21:
    print('Yes, you can.')
else:
    print('Sorry.')

Precedence matters

General rule: arithmetic > comparison > not > and/or

Much more detailed operator precedence: https://docs.python.org/3/reference/expressions.html#operator-precedence

5 * 7 >= 3 + 5 * (7 - 1) and not False
35 >= 3 + 5 * 6 and not False
35 >= 3 + 30 and not False
35 >= 33 and not False
True and not False
True and True
True

Nonetype

None

The sole value of the type NoneType. None is frequently used to represent the absence of a value, as when default arguments are not passed to a function.

Exercises

1. Below is a transcript of a session with the Python shell. For each expression being evaluated, provide the type and the value the expression returns. I encourage you to answer them directly since this will help reinforce your understanding of basic Python expressions.

a = 3
a + 2

a = a + 1.0
a

a = 3
b

a = 3
a == 5.0
a 

b = 10
c = b > 9
c 

5/2 == 5/2.0

2. For each of the following expressions, indicate the value returned. I encourage you to answer them directly since this will help reinforce your understanding of basic Python expressions.

3.0 - 1.0 != 5.0 - 3.0

3 > 4 or (2 < 3 and 9 > 10)

4 > 5 or 3 < 4 and 9 > 8

not(4 > 3 and 100 > 6)

3. The time module provides a function, also named time, that returns the current Greenwich Mean Time in “the epoch”, which is an arbitrary time used as a reference point. On UNIX systems, the epoch is 1 January 1970.

import time

print(time.time())

Write a script that reads the current time and converts it to a time of day in hours, minutes, and seconds, plus the number of days since the epoch. You can only use time module.