Introduction to Python Programming

Introduction

The source chapter assumes that you have already used IPython as a calculator. Now the focus changes: instead of only evaluating expressions, you start writing actual Python statements and small scripts.

Variables and Assignment Statements

A variable is a name that refers to a value/object. You create or change a variable with the assignment symbol =.

Creating variables

x = 7
y = 3
x + y

The statements x = 7 and y = 3 store values for later use.

Calculations in an assignment

total = x + y
total

Python evaluates the right-hand side first and then assigns the result to the name on the left.

Variable names

Python variable names may contain letters, digits and underscores, but they cannot begin with a digit. Python is case-sensitive, so number and Number are different names.

student_name = "Aman"
age2 = 20
# 2age = 20   # invalid: starts with a digit

Types

Every Python value has a type. The built-in type() function lets you inspect it.

type(7)
type(10.5)

Arithmetic

Python provides familiar arithmetic operators plus operators that are especially useful in programming.

Exponentiation and square roots

2 ** 10
9 ** (1 / 2)

A square root can be expressed as raising a number to the power 1/2.

True division vs. floor division

7 / 4
7 // 4
-13 / 4
-13 // 4

Remainder operator

17 % 5
7.5 % 3.5

The remainder operator is commonly useful when checking divisibility, such as finding whether a number is even.

8 % 2
9 % 2

Straight-line form

Programming expressions are written in a horizontal, straight-line form. For example, a fraction is written using / or //.

Parentheses

10 * (5 + 3)
10 * 5 + 3

Parentheses can force part of an expression to be evaluated first and can make the intended calculation clearer.

Operator precedence

2 + 3 * 4
(2 + 3) * 4

Exceptions and tracebacks

Some operations cannot be completed. For example, division by zero raises ZeroDivisionError.

123 / 0

Using a variable that has not been defined can raise NameError.

z + 7

Function print and Single-/Double-Quoted Strings

The built-in print() function displays text or values.

print('Welcome to Python!')
print("Welcome to Python!")

A string is a sequence of characters. You can normally delimit it with single quotes or double quotes.

Printing several values

print('Welcome', 'to', 'Python!')

When multiple arguments are passed to print, Python separates them with spaces by default.

Escape sequences

print('Welcome\nto\nPython!')

Long strings

A long string can be continued onto another source line using the continuation character \. The source chapter also notes that parentheses are the preferred way to break long code lines when possible.

print('This is a long message that is '
      'split into two strings.')

Printing an expression

print('Sum is', 7 + 3)

Triple-Quoted Strings

Triple-quoted strings use either ''' or """. The source chapter recommends three double quotes in its style guidance.

message = """This is a string
that spans two lines."""
print(message)

Internally, a multiline string contains newline characters where the source lines were separated.

Including both kinds of quotes

print("""Display "hi" and 'bye' in quotes""")

Getting Input from the User

The built-in input() function displays a prompt and waits for the user to type something. The important point is that input() always returns a string.

name = input("What's your name? ")
print(name)

Why numeric input can surprise beginners

value1 = input('Enter first number: ')
value2 = input('Enter second number: ')
print(value1 + value2)

If the user enters 7 and 3, the values are strings, so + concatenates them.

This is string concatenation: the second string is placed after the first string.

Convert input to an integer

value = int(input('Enter an integer: '))
another_value = int(input('Enter another integer: '))
print(value + another_value)

Convert to floating-point

price = float(input('Enter price: '))

Use float() when the input may contain a decimal value.

Conversion errors

If text cannot be converted to the requested numeric type, Python raises an exception such as ValueError.

int('hello')

Decision Making: The if Statement and Comparison Operators

A condition is a Boolean expression that evaluates to either True or False.

7 > 4
7 < 4

Comparison operators

Basic if syntax

if number1 == number2:
    print('The numbers are equal')

The structure is:

Comparing two integers

number1 = int(input('Enter first integer: '))
number2 = int(input('Enter second integer: '))

if number1 == number2:
    print(number1, 'is equal to', number2)

if number1 != number2:
    print(number1, 'is not equal to', number2)

if number1 < number2:
    print(number1, 'is less than', number2)

if number1 > number2:
    print(number1, 'is greater than', number2)

if number1 <= number2:
    print(number1, 'is less than or equal to', number2)

if number1 >= number2:
    print(number1, 'is greater than or equal to', number2)

Comments, docstrings and whitespace

A comment starts with # and is ignored by Python.

# This comment explains the next statement
age = 20

The source chapter also introduces a script-level docstring to describe a program's purpose:

"""Compare two integers using if statements."""

Blank lines and spaces make code easier to read. Indentation is different: indentation is part of Python's syntax for an if suite.

Chaining comparisons

Python allows comparisons such as:

x = 3
1 <= x <= 5

This is a convenient way to ask whether a value lies within a range.

Splitting a long statement

Long statements can be split across lines. The source chapter notes that using parentheses to break long lines is preferred when practical.

print('Enter two integers, and I will tell you',
      'the relationships they satisfy.')

Objects and Dynamic Typing

Python values such as 7, 4.1 and 'dog' are objects. Every object has a type and a value.

Variables refer to objects

When you assign an object to a variable, the variable name becomes associated with that object.

x = 7
x + 10

Dynamic typing

Python uses dynamic typing. The type is associated with the object a variable currently refers to; the same variable name can later refer to an object of another type.

x = 7
print(type(x))

x = 4.1
print(type(x))

x = 'dog'
print(type(x))

Garbage collection

Python creates objects in memory and automatically removes objects that are no longer needed. This automatic memory cleanup is called garbage collection.

Operator precedence so far

Intro to Data Science: Basic Descriptive Statistics

Data science often begins by getting to know the data. Descriptive statistics provide simple ways to summarize a collection of values.

Finding the minimum manually

The chapter demonstrates a simple algorithm for three values:

1. Assume the first value is the smallest.
2. Compare the second value with the current minimum. Replace the minimum if needed.
3. Compare the third value with the current minimum. Replace it if needed.
4. Display the final minimum.

number1 = int(input('Enter first integer: '))
number2 = int(input('Enter second integer: '))
number3 = int(input('Enter third integer: '))

minimum = number1

if number2 < minimum:
    minimum = number2

if number3 < minimum:
    minimum = number3

print('Minimum value is', minimum)

Using built-in min() and max()

Python already provides built-in functions for common tasks.

min(36, 27, 12)
max(36, 27, 12)

Range

For the values 36, 27, 12, the minimum is 12 and the maximum is 36, so the range extends from 12 through 36.

Reduction

The source introduces reduction as a functional-style programming idea: many values are reduced to one result. min() and max() are examples. Other reductions encountered later include sum, average, variance and standard deviation.

Wrap-Up

This chapter builds the first practical layer of Python programming. You move from calculator-style expressions to small programs that can store, display, receive, compare and summarize data.

Quick Memory Map

If you remember one idea: Python programs are built by combining values, names, expressions and statements. Once you understand those building blocks, larger programs become combinations of the same ideas.

Common Beginner Mistakes

Revision Questions

Practice

1. Create variables for your name, age and height, then print them.
2. Calculate the result of 8 * (10 - 3) / 2 and explain the evaluation order.
3. Write examples showing the difference between /, // and %.
4. Ask the user for two integers and print their sum.
5. Ask the user for a number and use if to determine whether it is greater than 100.
6. Write a program that compares two integers using all six comparison operators.
7. Create a triple-quoted multiline message and print it.
8. Use type() to inspect an integer, float and string.
9. Write a program that finds the minimum of three user-entered integers.
10. Use min() and max() on a collection of values and describe the resulting range.