Python Programming/Variables

This lesson introduces variables, expressions, and statements.

Objectives and Skills

Objectives and skills for this lesson include:

• Language Basics
  • Language elements (constants, numbers and strings)
  • Strings types (single quotes, double quotes and triple quotes)
  • Escape Sequence, string concatenation and format method
  • Variables naming, types and objects
  • Indentation, logical and physical lines
• Operators and Expressions
  • Operators and Expressions
  • Evaluation Order and Associativity
• Input Output
  • User input

Readings

1. Wikipedia: Variable (computer science)
2. Wikipedia: Data type
3. Wikipedia: Expression (computer science)
4. Wikipedia: Statement (computer science)
5. Wikipedia: Order of operations
6. Python for Everyone: Variables, expressions, and statements

Multimedia

1. YouTube: Python for Informatics: Chapter 2 - Expressions
2. YouTube: Python Numbers
3. YouTube: Python3 Input & Output
4. Youtube: Python 3 Programming Tutorial: Variables
5. Youtube: Setting up Pycharm and getting started

Examples

Data Types

Built-in Python data types include integer (int), floating point (float), string (str), and Boolean (bool) data types.

value = 1 + 1
print(value)          # Displays 2
print(type(value))    # Displays <type 'int'>

value = 0.1 + 0.1
print(value)          # Displays 0.2 
print(type(value))    # Displays <type 'float'>

value = '1' + '1'
print(value)          # Displays 11
print(type(value))    # Displays <type 'str'>

value = True
print(value)          # Displays True
print(type(value))    # Displays <type 'bool'>

Type Conversion

An object’s type is accessed by the built-in function type().

value = 1.9
print(value)          # Displays 1.9
print(type(value))    # Displays <type 'float'>
value = int(value)
print(value)          # Displays 1
print(type(value))    # Displays <type 'int'>

value = 1
print(value)          # Displays 1
print(type(value))    # Displays <type 'int'>
value = float(value)  
print(value)          # Displays 1.0
print(type(value))    # Displays <type 'float'>

value = 1
print(value)          # Displays 1
print(type(value))    # Displays <type 'int'>
value = str(value)  
print(value)          # Displays 1
print(type(value))    # Displays <type 'str'>

Quotes

String literals are written in a variety of ways, including single quotes, double quotes, and triple quotes. Triple quoted strings may span multiple lines. The backslash (\) character is used to escape characters that otherwise have a special meaning, such as newline, backslash itself, or the quote character.

value = 'single quotes'
print(value)

value = "double quotes"
print(value)

value = \
'''triple quotes
    span multiple lines'''
print(value)

value = '"nested quotes"'
print(value)

value = "'nested quotes'"
print(value)

value = "\"escape character quotes\nand multiple lines\""
print(value)

Numeric Operations

All numeric types (except complex) support the following operations, sorted by ascending priority.

a = 3
b = 2

print(a + b)     # 5
print(a - b)     # 1
print(a * b)     # 6
print(a / b)     # 1.5
print(a // b)    # 1
print(a % b)     # 1
print(-a)        # -3
print(a ** b)    # 9

Assignment Operations

An assignment statement evaluates the expression and assigns the result to the target. Augmented assignment is the combination, in a single statement, of an operation and an assignment statement.

a = 3
b = 2

a += b    # a = 5
a -= b    # a = 3
a *= b    # a = 6
a /= b    # a = 1.5
a //= b   # a = 1
a %= b    # a = 1.0
a **= b   # a = 9

Input Function

Python 2: If the prompt argument is present, it is written to standard output without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), which is then parsed and evaluated as a Python expression.

Python 3: If the prompt argument is present, it is written to standard output without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that.

input([prompt])

#Python 2
value = input("Enter a numeric value: ")
print("You entered " + str(value))

value = input('Enter a string value in "quotes": ')
print("You entered " + value)

#Python 3
value = input("Enter a value: ")
print("You entered a string value " + value)

Activities

Tutorials

1. Complete one or more of the following tutorials:
   • LearnPython
     • Variables and Types
     • Basic Operators
   • TutorialsPoint
     • Basic Syntax
     • Variable Types
     • Basic Operators
   • Codecademy
     • Strings and Console Output
   • SoloLearn
     • Basic Concepts
   • Wikiversity
     • Python/Numbers
   • Wikibooks
     • Wikibooks: A Beginner's Python Tutorial/Variables, Scripts

Practice

1. Experiment with different numeric operations to ensure you understand how they work. Then review either MathsIsFun: Order of Operations or Teachoo: What is BODMAS?. Create a Python program that demonstrates the order of operations.
2. Create a Python program to prompt the user for hours and rate per hour to compute gross pay (hours * rate).
3. Review MathsIsFun: Conversion of Temperature. Create a Python program that asks the user for a Fahrenheit temperature and then calculate and display the corresponding Celsius temperature or ask the user for a Celsius temperature and then calculate and display the corresponding Fahrenheit temperature.
4. Create a Python program that asks the user how old they are in years, and then calculate and display their approximate age in months, days, hours, and seconds.
5. Review MathsIsFun: Area of Plane Shapes. Create a Python program that asks the user for the dimensions of different shapes and then calculate and display the area of the shapes.

Games

1. Play CodeCombat.

Lesson Summary

• Built-in Python data types include integer (int()), floating point (float()), string (str()), and Boolean (bool()).
• An object’s type is accessed by the built-in function type().
• String literals are written in a variety of ways, including single quotes, double quotes, and triple quotes. Triple quoted strings may span multiple lines.
• The backslash (\) character is used to escape characters that otherwise have a special meaning, such as newline, backslash itself, or the quote character.
• Numeric operators include +, -, *, /, //, %, and **.
• Assignment operators include +=, -=, *=, /=, //=, %=, and **=.
• The Python 2 input() function reads a line from input, converts it to a string (stripping a trailing newline), which is then parsed and evaluated as a Python expression.
• The Python 3 input() function reads a line from input, converts it to a string (stripping a trailing newline), and returns that.

Key Terms

assignment

A statement that assigns a value to a variable.

concatenate

To join two operands end to end.

comment

Information in a program that is meant for other programmers (or anyone reading the source code) and has no effect on the execution of the program.

escape character

A character which invokes an alternative interpretation on subsequent characters in a character sequence.

evaluate

To simplify an expression by performing the operations in order to yield a single value.

expression

A combination of variables, operators, and values that represents a single result value.

floating point

A type that represents numbers with fractional parts.

floor division

The operation that divides two numbers and chops off the fractional part.

integer

A type that represents whole numbers.

keyword

A reserved word that is used by the compiler to parse a program; you cannot use keywords like if, def, and while as variable names.

mnemonic

A memory aid. We often give variables mnemonic names to help us remember what is stored in the variable.

modulus operator

An operator, denoted with a percent sign (%), that works on integers and yields the remainder when one number is divided by another.

operand

One of the values on which an operator operates.

operator

A special symbol that represents a simple computation like addition, multiplication, or string concatenation.

rules of precedence

The set of rules governing the order in which expressions involving multiple operators and operands are evaluated.

statement

A section of code that represents a command or action. So far, the statements we have seen are assignments and print statements.

string

A type that represents sequences of characters.

type

A category of values. The types we have seen so far are integers (type int), floating-point numbers (type float), and strings (type str).

value

One of the basic units of data, like a number or string, that a program manipulates.

variable

A name that refers to a value.

Review Questions

1. Built-in Python data types include _____, _____, _____, and _____.

   Built-in Python data types include integer (int()), floating point (float()), string (str()), and Boolean (bool()).
2. An object’s type is accessed by the built-in function _____.

   An object’s type is accessed by the built-in function type().
3. String literals are written in a variety of ways, including _____, _____, and _____. _____ may span multiple lines.

   String literals are written in a variety of ways, including single quotes, double quotes, and triple quotes. Triple quoted strings may span multiple lines.
4. The _____ character is used to escape characters that otherwise have a special meaning, such as _____.

   The backslash (\) character is used to escape characters that otherwise have a special meaning, such as newline, backslash itself, or the quote character.
5. Numeric operators include _____.

   Numeric operators include +, -, *, /, //, %, and **.
6. Assignment operators include _____.

   {{{2}}}
7. The Python 2 input() function _____.

   The Python 2 input() function reads a line from input, converts it to a string (stripping a trailing newline), which is then parsed and evaluated as a Python expression.
8. The Python 3 input() function _____.

   The Python 3 input() function reads a line from input, converts it to a string (stripping a trailing newline), and returns that.

Assessments

• Flashcards: Quizlet: Python Variables
• Flashcards: Quizlet: Python Variable Types
• Flashcards: Quizlet: Python Variables, Expressions, and Statements
• Quiz: Quizlet: Python Variables
• Quiz: Quizlet: Python Variable Types
• Quiz: Quizlet: Python Variable, Expressions, and Statements

See Also

• Python.org: An Informal Introduction to Python
• Python.org: Built-in Types
• Python.org: Expressions
• Python.org: Data model
• Python.org: Simple statements
• SoloLearn: Python
• Open Book Project: Python 3 - Variables, expressions and statements
• Digital Ocean: How to use Variables in Python 3
• PythonBasics: variables

References

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