Python Internals for Developers: Practice Python 3.x Fundamentals, Including Data Structures, Asymptotic Analysis, and Data Types

CHAPTER 1

Python: A Quick Recap

Introduction

Python is one of many programming languages which exist in the software world to help developers solve real-time problems. Programming language is a formal language like English, Hindi, Spanish, and so on. Just like English or any other language used by humans to communicate with each other, the programming language is used for communication between humans and computers.

Many programming languages exist for human-computer communication. Now you must be wondering why we need so many programming languages for the same task, that is, communication. The following are some of the reasons why so many languages exist:

Some languages are easier to read and maintain than others.

All languages have different performance for different applications.

Different languages are suitable for solving different problems. For example, HTML can be used for developing websites and C language is best for embedded software development.

Languages differ in support of libraries to support common functions.

Some languages have fewer vulnerabilities than others, thus making them more secure. For example, Java is more secure than C.

Thus, developers choose the programming language based on the problem to be solved, the infrastructure to be used, and the type of software to be developed.

Structure

In this chapter, we will discuss the following topics:

Understanding Python

Variables and expressions

Variables

Variable names

Operators

Arithmetic operators

Comparison operators

Logical operators

Identity operators

Membership operators

Bitwise operators

Assignment operators

Operator precedence

Expressions

Comments

Conditional statements

The if statement

The if…else statement

The if…elif…else statement

The nested conditional statements

Loops and iterations

The while loop

The for loop

The break statement

The continue statement

Functions

Pre-defined functions

Objectives

By the end of this chapter, you will be able to install Python in your systems. You will be able to understand Python syntax and write basic programs in Python. You will get basic knowledge of interactive and script mod in Python. You will get an idea of how to write and run Python programs in both modes and also be able to understand the concept of functions in Python and how they are implemented.

Understanding Python

Python is a high-level language like C++, Java, and so on. High-level languages are more understandable by humans than low-level languages. High-level languages hide register, memory details from the developer. The computer understands low-level languages such as machine/assembly language. High-level languages convert solutions written in high-level languages to low-level languages.

Figure 1.1: Compiler takes source code and converts it into machine language code (executable file)

Python is an interpreted language. Table 1.1 shows the difference between Interpreted languages and Compiled languages.

Table 1.1: Difference between Interpreted and Compiled languages.

The following figure shows how the interpreter takes source code and execute the output

Figure 1.2: Interpreter takes source code and executes directly

Python can be downloaded and installed by following the instructions available at https://www.python.org/downloads/. Python code can be written in interactive mode or script mode. To run the Python code in interactive mode, open command prompt and then type py. If Python is successfully installed in your system, the command prompt will display as shown in Example 1.1.

Example 1.1: Run Python programs

py

Figure 1.3: Python prompt

Type the following commands in the command prompt:

x=3

print(x)

Figure 1.4: Interactive Mode

The preceding commands are being run in an interactive mode and output 3is printed. To exit the interactive mode the quit() or exit() functions can be used:

exit()

Figure 1.5: Exiting Interactive Mode

To run your Python program in script mode, open any text editor or IDE and type the instructions that you want to execute, and save the file with the.py extension. To run the preceding instructions using script mode, type the following lines in the text editor or IDE:

x = 3

print(x)

Save the file with any name and ensure that the extension is.py, say, for example, test.py. Change the directory to the folder on the terminal where the .py file has been saved. In our case, the file is saved on the desktop, to change the directory, run the following command:

cd desktop

Figure 1.6: Change Directory

Run the program using the following command:

pytest.py

Figure 1.7: Executing Script

The script executes and prints the output 3 in command prompt, same as interactive mode. Script mode has the following benefits over interactive mode:

Scripts can be reused. They can be run again as many times as needed without the need to type all the instructions.

In case any error is encountered in the script, only the error code needs to be modified. In the case of interactive mode, correct instructions need to be typed again.

Thus, it is better to use the script mode for developing programs.

Variables and expressions

This section explains what variables are and how they are declared and defined in Python. The following subsections give details about operators and expressions in Python.

Variables

Variables in programming languages are names given by the developers to store values while developing programs. Take a look at the following example:

x = 3

Here, x is a variable name given to store the value 3. The preceding instruction is an assignment statement. It stores the value 3 in computer memory and names that memory as variable x as shown in figure 1.8.

Figure 1.8: Computer memory portion. Value 3 is saved in one portion of memory and named as variable x

Naming a computer memory helps you to extract the value from the computer memory as and when required. The following instruction extracts the value of x stored in computer memory and prints the value on output:

print(x)

Example 1.2: Assigning values to different variables

pi = 3.14

greeting = hello

i = 0

isFound = True

Variable names

Python language has some pre-defined rules for defining the variable names. These rules are considered while writing the syntax of the Python programming language. The following rules have been defined for naming variables:

Variable names can contain both letters and numbers.

Variable name can’t contain any special character except underscore (_).

Variable name can either start from a letter or underscore.

Python is case-sensitive. Thus, "name, Name and NAME" are three different variable names. But it is better to avoid defining similar variable names in the same file for better program readability.

Some developers define variable names in camel case for better understanding. Example: bookName, studentName, and so onIt is advisable to keep meaningful variable names such as count or name instead of c or n.

Python has some reserved words like if, else, and so on. These reserved words can’t be used as variable names. Reserved words for Python are given in table 1.2:

Table 1.2: Python reserved words

Using reserved words as variables causes syntax error as shown in figure 1.9:

Example 1.3: Assigning values to reserved variables

elif = 1

nonlocal = 1

raise = 1

Figure 1.9: Reserved words usage as variable name causes a syntax error

Operators

Like any other programming language, Python also has operators which are used to perform various operations on different values and variables. Values and variables on which an operation is performed are known as operands. Python has the following type of operators:

Arithmetic operators

Comparison operators

Logical operators

Identity operators

Membership operators

Bitwise operators

Assignment operators

Arithmetic operators

Arithmetic operators perform mathematical operations on operands such as addition, subtraction, and so on. Arithmetic operators are binary operators. Binary operators operate on two operands. Table 1.3 describes the various Python arithmetic operators:

Table 1.3: Python arithmetic operators

Example 1.4 shows few arithmetic operators and their output.

Example 1.4: Arithmetic operators

2+3

//Output

5

3-2

//Output

1

2*3

//Output

6

5/2

//Output

2.5

23%5

//Output

3

2**4

//Output

16

5//2

//Output

2

The preceding example shows different outputs obtained while using arithmetic operators. As shown in the preceding example, the modulus operator (%) is the same as the mathematical remainder operator which outputs the remainder when 23 is divided by 5 (which is 3). The exponentiation operator (**) is the same as the mathematical power operator which outputs the value of 24 (which is 16). The floor division operator (//) divides the operand’s and the output’s floor value of division like in the preceding example, 5/2=2.5 but 5//2 outputs floor value of 2.5 which is 2.

The addition and multiplication operators in Python can be used for strings too. Python addition operator adds two strings and merges them. Python multiplication operator can take one string and one integer as input. Example 1.5 gives examples of addition and multiplication operators on a string.

Example 1.5: Addition and Multiplication operators on a String

hello +world"

//Output

‘hello world’

abc+def

//Output

‘abcdef’

a*3

//Output

‘aaa’

hello*2

//Output

‘hellohello’

Comparison operators

Comparison operators compare values of operands and outputs true or false based on comparison output. Comparison operators are also binary operators. Table 1.4 describes the various Python comparison operators:

Table 1.4: Python comparison operators

Example 1.6: Comparison operators

2==3

//Output

False

john==John

//Output

False

john!=John

//Output

True

2!=3

//Output

True

2<3

//Output

True

‘a’<’b’

//Output

True

‘a’>’b’

//Output

False

5>2

//Output

True

2<=2

//Output

True

5>=2

//Output

True

The preceding example shows different outputs obtained while using comparison operators. Comparison operators can also be used to compare strings like in the preceding example comparison of "john with John".

Logical operators

Logical operators are used in making decisions for conditions in a program. Logical operators perform operations on True/False operands. The output of the logical operator is either true or false. Table 1.5 describes the various Python logical operators:

Table 1.5: Python logical operators

Note: Unary operators operate on a single operand. The not operator requires only one operand as shown in Example 1.7.

Example 1.7: Logical operators

TrueandTrue

//Output

True

TrueandFalse

//Output

False

TrueorFalse

//Output

True

FalseorFalse

//Output

False

not(True)

//Output

False

not(False)

//Output

True

Identity operators

Identity operators are like comparison operators. They are used to compare the objects and gives output true if both objects are exactly the same and are stored in the same memory. Identity operators are binary operators. Table 1.6 describes various Python identity operators:

Table 1.6: Python identity operators

Example 1.8: Identity operators

name = John

test = name

nameis test

//Output

True

a = 20

b = 20

ais not b

//Output

False

Membership operators

Membership operators check if the left operand is a part of the right operand. Membership operators are binary operators. Table 1.7 describes the various Python membership operators:

Table 1.7: Python membership operators

Example 1.9: Membership operators

’e’ in ‘hello’

//Output

True

‘z’ in ‘hello’

//Output

False

‘z’ not in ‘hello’

//Output

True

‘e’ not in ‘hello’

//Output

False

Bitwise operators

Bitwise operators operate on each bit of number. Bitwise operator converts operands to a binary number, performs operations on each bit, converts the binary output to a decimal number, and outputs decimal number. Table 1.8 describes the various Python membership operators:

Table 1.8: Python bitwise operators

Example 1.10: Bitwise operators

3&6

//Output

2

The preceding example shows the output of the Bitwise AND operator. The binary conversion of 3 is 011 and that of 6 is 110. Bitwise AND operator will take each bit of both the binary numbers and perform the AND operation as follows:

3&6 => 011 & 110 => 010 => 2

The decimal conversion of 010 is 2.

3|6

//Output

7

The preceding example shows the output of the Bitwise OR operator. The Bitwise OR operator will take each bit of both the binary numbers and perform the OR operation as follows:

3|6 => 011 | 110 => 111 => 7

The decimal conversion of 111 is 7. The following example shows the output of the Bitwise XOR operator:

3^6

//Output

5

The Bitwise XOR operator will take each bit of both the binary numbers and perform the XOR operation as follows:

3^6 => 011 ^ 110 => 101 => 5

The decimal conversion of 101 is 5. The following example shows the output of the Bitwise NOT operator:

~3

//Output

-4

The Bitwise NOT operator will take each bit of the binary number and reverses the bits. NOT reverses the sign bit of binary number too. Thus, the positive number becomes negative and the negative number becomes positive. The digned binary number of 3 is 0011.

~3 => ~0011 => 1100 => -4

The decimal conversion of 1100 is -4.

Note: In the case of a signed integer, the leftmost bit is the sign bit. For a positive number, the leftmost bit is 0 and for a negative number, the leftmost bit is 1. For 3, the signed binary representation is 0011. The first 0 represents that it is a positive number. Similarly, for -4, a signed binary representation is 1100, the first 1 represents that it is a negative number.

3<<1

//Output

6

3<<2

//Output

12

The preceding examples show the output of the left shift operator. It works as follows:

3<<1 => 0011<<1 => 0110 => 6 (shifted all bits by 1 position to the left)

3<<2 => 0011<<2 => 1100 => 12 (shifted all bits by 2 positions to the left)

The decimal conversion of 0110 is 6 and that of 1100 is 12.

6>>1

//Output

3

6>>2

//Output

1

The preceding examples show the output of right shift operator. Right shift operator works as shown as follows:

6>>1 => 0110>>1 => 0011 => 3 (shifted all bits by 1 position to the right)

6>>2 => 0110>>2 => 0001 => 1 (shifted all bits by 2 positions to the right)

The decimal conversion of 0011 is 3 and that of 0001 is 1.

Assignment operators

Assignment operators are used to assigning values to a variable. Some assignment operators are compound operators which are a combination of simple assignment and arithmetic/bitwise operators. Table 1.9 describes the various Python assignment operators:

Table 1.9: Python assignment operators

Example 1.11: Assignment operators

x = 5

print(x)

//Output

5

x+=2

print(x)

//Output

7

x-=5

print(x)

//Output

2

x*=3

print(x)

//Output

6

x/=2

print(x)

//Output

3.0

x%=2

print(x)

//Output

1.0

x=2

x**=3

print(x)

//Output

8

x//=3

print(x)

//Output

2

x&=3

print(x)

//Output

2

x|=3

print(x)

//Output

3

x^=6

print(x)

//Output

5

x<<=2

print(x)

//Output

20

x>>=2

print(x)

//Output

5

The preceding examples show that the compound assignment operators perform the operation on the left and right operand and then assign the result to the left operand.

Operator precedence

Just like mathematical operations have precedence, Python language also has defined precedence for operators. Operator precedence means the order in which the operations will be performed if single instruction has more than one operator. Python operator precedence is shown in Table 1.10 from higher precedence to lower precedence. Operators in the same precedence are executed from left to right (except for exponentiation, which is executed from right to left).

Table 1.10: Python operator precedence

Example 1.12: Order precedence

6-3+2*4/2

//Output

7.0

((6-3)+2)*4/2

//Output

10.0

The preceding example shows the order precedence for arithmetic operators with and without parenthesis. The first example, 6-3+2*4/2, is executed as follows:

6-3+2*4/2 => 6-3+8/2 => 6-3+4.0 => 3+4.0 => 7.0

The second example, ((6-3) +2) *4/2, is executed as follows:

((6-3) +2) *4/2 => (3+2) *4/2 => 5*4/2 => 20/2 => 10.0

Expressions

An expression is a combination of values, variables, and operators. A value or variable alone can also be considered as an expression. All the instructions shown in all the examples are expressions. Some other examples of expressions are shown in example 1.13.

Example 1.13: Expressions

X

5

X=5

X+=2

2+3

False

X=True

Comments

Comments help in the documentation of the solution being developed. As the program gets bigger and complex, comments help developers to understand the code. Comments start with # in a Python program. The Python interpreter ignores any statement starting with #.

Example 1.14: Comments

x=10

if(x%5==0): #Checks if x is divisible by 5

print (x, is divisible by5)

//Output

10 is