3. Introduction to Python

• Notebook of this section (open in a separate tab)

• Video of this section (31 minutes)

3.1. Overview

• Variables are mainly integers, real numbers, Boolean variables, characters, lists, dictionaries, etc., each of which defines operations and comparison operators.

• The elements of a list start from the zero th element.

• A partial list can be extracted from a list in slice notation.

• In for and if statements, processing blocks are distinguished by a colon and indentation. An error occurs if the indentation is not aligned in a block.

• Functions and classes are defined in the same way, using a colon and indentation.

• Classes allow variables to be used in multiple functions (class functions). This also makes it easier to avoid variable name collisions.

3.2. Python Basics

Version check

!python --version

Python 3.7.12

Arithmetic calculations

Arithmetic calculations are intuitive in Python: one instruction per line.

1 + 2

3

4 * 5

20

2 / 7

0.2857142857142857

3 ** 4 # 3 to the fourth power

81

3 ** 0.5 # 0.5 power of 3 = root 3

1.7320508075688772

Data types

Each variable in Python has a data type, which can be displayed using the type() function. Data types include integer (int), floating-point (float), and string (str).

type(10)

int

type(1.3)

float

type('hello')

str

list

Data type is list

a = [10, 31, 2.4,  6]
print(a)
type(a)

[10, 31, 2.4, 6]

list

length of lists

len(a)

4

Lists differ from vectors in the rules of operation.

[1, 2] + [3, 4]

[1, 2, 3, 4]

[1,'ABC'] * 3

[1, 'ABC', 1, 'ABC', 1, 'ABC']

The first element in the list is the zeroth.

print(a[0])
print(a[1])
print(a[2])
print(a[3])

10
31
2.4
6

print(a[-1])

6

To extract a partial list, the notation Slicing can be used. The first number is zero, the second is one, and so on.

a=[0,1,2,3,4,5]
print(a[:]) # from the first to the last, that is, a itself
print(a[1:3]) # from the first to the third "one before
print(a[2:]) # from the second to the last

[0, 1, 2, 3, 4, 5]
[1, 2]
[2, 3, 4, 5]

Dictionary

A dictionary is a set of keyed (labeled) values:

{key0: value0,key1: value1,... , key N-1: value N-1}

The data type is dict.

alice = {'height': 160, 'weight': 50}
alice

{'height': 160, 'weight': 50}

type(alice)

dict

alice['height']

160

Adds a new element to the dictionary.

alice['nationality'] = 'UK'
alice

{'height': 160, 'nationality': 'UK', 'weight': 50}

3.3. Control statements

If

If statements can be used for conditional branching.

if Condition: True
    If True, execute.
    If true, then execute.
else: Execute if False.
    Execute if False.
    Execute if False.

In Python, indentation must be aligned within the same block.

# Compare two numbers and return the larger one

a = 5
b = 3

if a > b:
    c = a
    print('a =', c ,'の方が大きい')
else:
    c = b
    print('b =', c ,'の方が大きい')

c

a = 5 の方が大きい

5

for

You can iterate using `for'' statements. `` for i in range(10): 「process」 ` the "process" is executed from`\ i=0\ ``toi=9`.

# Add 1 to 10

s = 0
for i in range(10):
    s += i+1

s

55

where += is the operation of adding the right-hand side to the original value.

It can also be written as follows.

# Add 1 to 10

s = 0
for i in range(1,11):
    s += i

s

55

Function

Define a new function using def.

def function name(argument):
    Calculation rule
    Calculation rule
    Calculation rule
    return Return value.

Example 1 (using if statement)

Function to output the maximum of three numbers.

def max3(a,b,c): # a, b, c are arguments
    x = a
    if b > x:
        x = b
    if c > x:
        x = c
    return x

m = max3(3,7,-2)
m

7

Example 2 (using for statement)

Function that returns \(1 + 2 + \cdots + n\) for a given number \(n\).

where += is the operation of adding the right-hand side to the original value.

def f(n):
    y = 0
    for i in range(n): #i from 0 to n-1
        y += i + 1
    return y

f(10)

55

3.4. Classes

Definition of Classes

Classes allow you to define your own data types and functions for those data types.

• Class: A class has its own data type, class variables and methods.

• Class variables: various variables that objects of a class have. In this course, they are sometimes called parameters for the sake of explanation.

• Methods: functions for the class.

• Instance: An object (variable) with the data type of the class.

Variables of objects of a class are called by instance.variable

Functions of a class are called by 　instance.method() or class.method(instance).

class Person:
    def __init__(self, name): # Constructor
        self.name = name

    def hello(self, keisho):    # hello() method
        print('Hello '+ self.name +' ' + keisho + '!')

    def bye(self, keisho):    # bye() method
        print('Good bye '+ self.name +' ' + keisho + '!')

where

def __init__(self, name): # Constructor
      self.name = name

is the constructor, which is the default definition. `self `` is a temporary instance name used in the class definition.

Instance Creation

abc = Person('Sato')

Checking class variables

abc.name

'Sato'

Using a method: There are two ways to write it.

abc.hello('san')

# Same if written as follows
Person.hello(abc, 'san')

Hello Sato san!
Hello Sato san!

abc.bye('san')

Good bye Sato san!

As shown in this example, class functions can be used without specifying the class variable abc.name.