Python Programming I Module Two Lesson Two Notes

Variables, Data Types, and Errors

In this lesson, you will be introduced to variables. Variables are one of the important building blocks in programming. You will remember the concept of variables from math class, but some of the programming-related details will be new.

In Python, like math, variables are used to store values. The type of data stored is called the data type of the variable.

Instruction

Variable and Data Type Notes

What is a Variable?

A variable is a named place in computer memory that holds a value. It is a named container. The user (who runs the program) or the programmer (coder) can supply the variable's value.

A variable can only hold one value at any given time. For example, item_price, grade, and last_name are variables.

A string is a common type of data used in programming. It consists of a sequence of one or more characters.

A string(str) is a sequence of characters.

Character examples:

A string in Python is contained in quotation marks (single (') or double (") quotes), but they must come in pairs—both double quotes or both single quotes, not one of each.

String examples:

"ABC"
'Joana Dias'
'I have 2 pet cats.'
"item #3 cost $3.29 USD"

A string can contain any character, including letters, spaces, punctuation, number digits, and formatting. In a string with digits ("123"), the digits are text images rather than representations of numeric values.

A numeric variable value is not placed in quotation marks. Anything in quotation marks is a string.

int

Integers are whole numbers and their opposite numbers, such as -2, -1, 0, 1, 2, and 3.

The keyword to specify an integer is int.

float

Decimal numbers such as 3.33, 0.01, 9.9999, and 3.0 are called floating numbers.

The keyword to specify a floating number is float.

Other data types, such as bool (boolean - true or false) will be introduced later in the course.

What to do with a variable? Once a variable is assigned a value, a Python program can refer to the variable by its name in code. For example, last_name can be used like the string value ("DiCaprio") it stores. Variables can be used anywhere a value can.

Variables can be initialized with different data types, such as string, int, and float.

We can reassign a variable by changing the contents to any data type.

To give a variable a value, we use an assignment statement.

1  first_name = "Marvin"
2  send_message = value

Variables are case-sensitive! Num_1 and num_1 are not the same variable!

In Python, we do not assign a variable a data type. The interpreter decides on the type based on the value entered.

A data type tells the interpreter the type of data to expect and tells the program that operations can be performed.

Python's type() function returns the data type of Python objects.

A best practice is to use type() in concert with the print() function.

Example: print(type("What am I?"))
Output: class 'str' because 'What am I?' is a string.

When does using print(type()) reveal the following?

class 'int'
class 'float'
Class 'str'

Important! There are different styles for creating variables. This course uses lowercase descriptive names connected by underscores.

Examples:

first_name
current_year
stock_symbol

Descriptive or friendly names reduce the need for comments in the code and make it easier to share code or read code written long ago.

Errors

So your program is written, but something else is needed. What’s next? It would be awesome if the first time we wrote a program, it worked perfectly. Unfortunately, this is not likely to happen.

You will run into errors. Errors are a normal part of the programming process. Figuring out the issue and fixing it is called debugging the program.

Let's talk about different types of errors, why they happen, and how to troubleshoot them.

Type Errors

The line print("my number is " + 123) causes a TypeError message to appear.

Notice 123 is not enclosed in quotes. This means 123 is an int. When adding to the string "my number is " the compiler is expecting another string but encounters a number.

Python only allows combining like data types.

Syntax Error

Syntax errors are the most basic of errors and occur when you violate the code formatting rules of Python. Most syntax errors are simple typos involving misspellings or bad punctuation. That is the first thing you should look for when your program doesn't run because syntax errors are almost always fatal.

Order matters print"(Uhh Ohh)" would generate a syntax error.

Parentheses inside quotations will be seen as part of a string and not as a parenthesis.

Name Errors

A name error occurs when Python looks for something that it can't find because it doesn't exist. This may be because you have tried to use a variable that isn't defined anywhere in your program. Assigning the variable value will solve that issue.

Sometimes typos cause this type of error, too. Such as, Print("this is a mistake") would generate a name error because Print with a capital P is not defined.

If you get a name error, look for capitalization and spelling errors to solve that issue.

Troubleshooting Errors

Python will let you know what kind of error occurs.

Syntax error:: Look for a grammatical error - typos, incorrect indentation, or incorrect arguments.
Type error:: Look for mismatched data types.
Name error:: Look for a variable that hasn't been defined or a function name that has been misspelled or miscapitalized.

Remember, good programmers ensure that their programs can handle all user input. The next lesson is all about input!

Reference List

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