1.3: Variables

Learning Objectives

By the end of this section you should be able to

• Assign variables and print variables.
• Explain rules for naming variables.

Assignment statement

Variables allow programs to refer to values using names rather than memory locations. Ex: age refers to a person's age, and birth refers to a person's date of birth.

A statement can set a variable to a value using the assignment operator (=). Note that this is different from the equal sign of mathematics. Ex: age = 6 or birth = "May 15". The left side of the assignment statement is a variable, and the right side is the value the variable is assigned.

Checkpoint: Assigning and using variables

Concepts in Practice \(\PageIndex{1}\): Assigning and using variables

Which line of code correctly retrieves the value of the variable, city, after the following assignment?

city = "Chicago"

1. print("In which city do you live?")
2. city = "London"
3. print("The city where you live is", city)

Answer

c. This would produce the output string, "The city where you live is Chicago" .

Concepts in Practice \(\PageIndex{2}\): Assigning and using variables

Which program stores and retrieves a variable correctly?

1. print("Total =", total)
   total = 6
   

2. total = 6
   print("Total =", total)
   

3. print("Total =", total)
   total = input()

Answer

b. These two lines of code would produce the output string, "Total = 6" .

Concepts in Practice \(\PageIndex{3}\): Assigning and using variables

Which is the assignment operator?

1. :
2. ==
3. =

Answer

c. The single = character is the assignment operator.

Concepts in Practice \(\PageIndex{4}\): Assigning and using variables

Which is a valid assignment?

1. temperature = 98.5
2. 98.5 = temperature
3. temperature - 23.2

Answer

a. An assignment statement has the variable name on the left side and the value on the right side of the assignment statement.

Variable naming rules

A variable name can consist of letters, digits, and underscores and be of any length. The name cannot start with a digit. Ex: 101class is invalid. Also, letter case matters. Ex: Total is different from total. Python's style guide recommends writing variable names in snake case, which is all lowercase with underscores in between each word, such as first_name or total_price.

A name should be short and descriptive, so words are preferred over single characters in programs for readability. Ex: A variable named count indicates the variable's purpose better than a variable named c.

Python has reserved words, known as keywords, which have special functions and cannot be used as names for variables (or other objects).

Table 1.2 Keywords

False	await	else	import	pass
None	break	except	in	raise
True	class	finally	is	return
and	continue	for	lambda	try
as	def	from	nonlocal	while
assert	del	global	not	with
asynch	elif	if	or	yield

Concepts in Practice \(\PageIndex{5}\): Valid variable names

Which can be used as a variable name?

1. median
2. class
3. import

Answer

a. median is a descriptive name for a variable that stores a median value.

Concepts in Practice \(\PageIndex{6}\): Valid variable names

Why is the name, 2nd_input, not a valid variable name?

1. contains an underscore
2. starts with a digit
3. is a keyword

Answer

b. A variable name can contain, but not start with, a digit.

Concepts in Practice \(\PageIndex{7}\): Valid variable names

Which would be a good name for a variable storing a zip code?

1. z
2. var_2
3. zip_code

Answer

c. The name clearly indicates the value being stored is a zip code.

Concepts in Practice \(\PageIndex{8}\): Valid variable names

Given the variable name, DogBreed, which improvement conforms to Python's style guide?

1. dog_breed
2. dogBreed
3. dog-breed

Answer

a. Snake case separates each word with an underscore and uses only lowercase letters.

Try It: Final score

Write a Python computer program that:

• Creates a variable, team1, assigned with the value "Liverpool".
• Creates a variable, team2, assigned with the value "Chelsea".
• Creates a variable score1, assigned with the value 4.
• Creates a variable, score2, assigned with the value 3.
• Prints team1, "versus", and team2 as a single line of output.
• Prints "Final score: ", score1, "to", score2 as a single line of output.

Interactive Code

# Type your code after this comment

Answer

team1 = "Liverpool"
score1 = 4
team2 = "Chelsea"
score2 = 3

print(team1, "versus", team2)
print("Final score:", score1, "to", score2)