Remember, expressions are just values combined with operators, and they always evaluate down to a single value. A data type is a category for values, and every value belongs to exactly one data type. The most common data types in Python are listed in Table 1-2. The values -2 and 30, for example, are said to be integer values. The integer (or int) data type indicates values that are whole numbers. Numbers with a decimal point, such as 3.14, are called floating-point numbers (or floats). Note that even though the value 42 is an integer, the value 42.0 would be a floating-point number.

Table 1-2. Common Data Types

Data type	Examples
Integers	`-2`, `-1`, `0`, `1`, `2`, `3`, `4`, `5`
Floating-point numbers	`-1.25`, `-1.0`, -`-0.5`, `0.0`, `0.5`, `1.0`, `1.25`
Strings	`'a'`, `'aa'`, `'aaa'`, `'Hello!'`, `'11 cats'`

Python programs can also have text values called strings, or strs (pronounced “stirs”). Always surround your string in single quote (') characters (as in 'Hello' or 'Goodbye cruel world!') so Python knows where the string begins and ends. You can even have a string with no characters in it, '', called a blank string. We will cover strings in more detail in later weeks.

If you ever see the error message SyntaxError: EOL while scanning string literal, you probably forgot the final single quote character at the end of the string, such as in this example:

>>> 'Hello world!
SyntaxError: EOL while scanning string literal

String Concatenation and Replication

The meaning of an operator may change based on the data types of the values next to it. For example, + is the addition operator when it operates on two integers or floating-point values. However, when + is used on two string values, it joins the strings as the string concatenation operator. Enter the following into the interactive shell:

>>> 'Alice' + 'Bob'
'AliceBob'

The expression evaluates down to a single, new string value that combines the text of the two strings. However, if you try to use the + operator on a string and an integer value, Python will not know how to handle this, and it will display an error message.

>>> 'Alice' + 42
Traceback (most recent call last):
  File "<pyshell#26>", line 1, in <module>
    'Alice' + 42
TypeError: Can't convert 'int' object to str implicitly

The error message Can't convert 'int' object to str implicitly means that Python thought you were trying to concatenate an integer to the string 'Alice'. Your code will have to explicitly convert the integer to a string because Python cannot do this automatically. (We'll cover how to do this later on)

The * operator is used for multiplication when it operates on two integer or floating-point values. But when the * operator is used on one string value and one integer value, it becomes the string replication operator. Enter a string multiplied by a number into the interactive shell to see this in action.

>>> 'Alice' * 5
'AliceAliceAliceAliceAlice'

The expression evaluates down to a single string value that repeats the original a number of times equal to the integer value. String replication is a useful trick, but it’s not used as often as string concatenation.

The * operator can be used with only two numeric values (for multiplication) or one string value and one integer value (for string replication). Otherwise, Python will just display an error message.

>>> 'Alice' * 'Bob'
Traceback (most recent call last):
  File "<pyshell#32>", line 1, in <module>
    'Alice' * 'Bob'
TypeError: can't multiply sequence by non-int of type 'str'
>>> 'Alice' * 5.0
Traceback (most recent call last):
  File "<pyshell#33>", line 1, in <module>
    'Alice' * 5.0
TypeError: can't multiply sequence by non-int of type 'float'

It makes sense that Python wouldn’t understand these expressions: You can’t multiply two words, and it’s hard to replicate an arbitrary string a fractional number of times.

Storing Values in Variables

A variable is like a box in the computer’s memory where you can store a single value. If you want to use the result of an evaluated expression later in your program, you can save it inside a variable.

Assignment Statements

You’ll store values in variables with an assignment statement. An assignment statement consists of a variable name, an equal sign (called the assignment operator), and the value to be stored. If you enter the assignment statement spam = 42, then a variable named spam will have the integer value 42 stored in it.

Think of a variable as a labeled box that a value is placed in, as in Figure 1-2.

spam = 42 is like telling the program, “The variable spam now has the integer value 42 in it.”

Figure 1-2. spam = 42 is like telling the program, “The variable spam now has the integer value 42 in it.”

For example, enter the following into the interactive shell:

➊ >>> spam = 40
   >>> spam
   40
   >>> eggs = 2
➋ >>> spam + eggs
   42
   >>> spam + eggs + spam
   82
➌ >>> spam = spam + 2
   >>> spam
   42

A variable is initialized (or created) the first time a value is stored in it ➊. After that, you can use it in expressions with other variables and values ➋. When a variable is assigned a new value ➌, the old value is forgotten, which is why spam evaluated to 42 instead of 40 at the end of the example. This is called overwriting the variable. Enter the following code into the interactive shell to try overwriting a string:

>>> spam = 'Hello'
>>> spam
'Hello'
>>> spam = 'Goodbye'
>>> spam
'Goodbye'

Just like the box in Figure 1-3, the spam variable in this example stores 'Hello' until you replace it with 'Goodbye'.

When a new value is assigned to a variable, the old one is forgotten.

Figure 1-3. When a new value is assigned to a variable, the old one is forgotten.

Variable Names

Table 1-3 has examples of legal variable names. You can name a variable anything as long as it obeys the following three rules:

It can be only one word.
It can use only letters, numbers, and the underscore (_) character.
It can’t begin with a number.

Table 1-3. Valid and Invalid Variable Names

Valid variable names	Invalid variable names
`balance`	`current-balance` (hyphens are not allowed)
`currentBalance`	`current balance` (spaces are not allowed)
`current_balance`	`4account` (can’t begin with a number)
`_spam`	`42` (can’t begin with a number)
`SPAM`	`total_$um` (special characters like `$` are not allowed)
`account4`	`'hello'` (special characters like `'` are not allowed)

Variable names are case-sensitive, meaning that spam, SPAM, Spam, and sPaM are four different variables. It is a Python convention to start your variables with a lowercase letter.

This book uses camelcase for variable names instead of underscores; that is, variables lookLikeThis instead of looking_like_this. Some experienced programmers may point out that the official Python code style, PEP 8, says that underscores should be used. I unapologetically prefer camelcase and point to “A Foolish Consistency Is the Hobgoblin of Little Minds” in PEP 8 itself:

Consistency with the style guide is important. But most importantly: know when to be inconsistent—sometimes the style guide just doesn’t apply. When in doubt, use your best judgment.”

A good variable name describes the data it contains. Imagine that you moved to a new house and labeled all of your moving boxes as Stuff. You’d never find anything! The variable names, spam, eggs, and bacon are used as generic names for the examples in this course and in much of Python’s documentation (inspired by the Monty Python “Spam” sketch), but in your programs, a descriptive name will help make your code more readable.

What does the variable bacon contain after the following code runs? (Hint: the answer isn't 21)

bacon = 20
bacon + 1

What should the following two expressions evaluate to?

'spam' + 'spamspam'
'spam' * 3

Lectures on Python are licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
Based on the book, Automate the Boring Stuff by Al Sweigart.