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Variables

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

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Concepts in Practice

Assigning and using variables

1.
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)
2.
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()
3.
Which is the assignment operator?
  1. :
  2. ==
  3. =
4.
Which is a valid assignment?
  1. temperature = 98.5
  2. 98.5 = temperature
  3. 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

5.
Which can be used as a variable name?
  1. median
  2. class
  3. import
6.
Why is the name, 2nd_input, not a valid variable name?
  1. contains an underscore
  2. starts with a digit
  3. is a keyword
7.
Which would be a good name for a variable storing a zip code?
  1. z
  2. var_2
  3. zip_code
8.
Given the variable name, DogBreed, which improvement conforms to Python's style guide?
  1. dog_breed
  2. dogBreed
  3. 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.
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