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1.3: Variables

Last updated

Mar 30, 2025
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- 1.2: Input/Output
- 1.4: String Basics

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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

Access multimedia content

Concepts in Practice: Assigning and using variables

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

city = "Chicago"

print("In which city do you live?")

city = "London"

print("The city where you live is", city)

Which program stores and retrieves a variable correctly?

```
print("Total =", total)
total = 6
```
```
total = 6
print("Total =", total)
```

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

Which is the assignment operator?

:
==
=

Which is a valid assignment?

temperature = 98.5
98.5 = temperature
temperature - 23.2

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).

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

Table 1.2 Keywords

Concepts in Practice: Valid variable names

Which can be used as a variable name?

median

class

import

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

contains an underscore
starts with a digit
is a keyword

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

z
var_2
zip_code

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

dog_breed
dogBreed
dog-breed

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.