Variables, Data Types, Inputs, and Outputs

What does Python do with anything after the pound (#) sign?

It ignores it

Benefits of writing comments in Python

• Understand the code that was written at a later time

• Helps others to read the code

• Temporarily “comment out” code instead of deleting it

Variable

A name that refers to a value in the computer's memory

What does the assignment operator (=) do?

The name on the left gets the value on the right

How should “=” be read in Python?

• “Gets”

• Compute the right side first and then attach the name to the result

What must happen before you can use a variable?

You must assign a value before you use it

What happens when you reassign a variable?

Reassigning replaces the old value

If count = 10, what does count = count + 1 do?

The right side is computed first as 11, then count gets 11

What does count =+ 1 mean?

It is shorthand for count = count + 1

Other shorthand assignment operators introduced

• -=

• *=

• /=

What naming rules does Python enforce for variables?

• Start with a letter or underscore

• No spaces

• No $, #, or @ symbols

• Cannot start with numbers

• Variable names are case-sensitive

• Python keywords cannot be used

What naming conventions were introduced for variables?

• snake_case (name_of_variable)

• CamelCase (Name of Variable)

Why is pet_name better than thenameofpet?

It’s shorter, easier to scan, and harder to mistype

What data type is used for whole numbers (integers)?

Int — used for counting and when fractions cannot occur

What data type is used for fractional numbers (decimals)?

float — used for measurements and money

True or False: You do not declare the data type of a variable in Python. Python decides it from the value you assign.

True

String

A sequence of characters (letters, digits, symbols) treated as one unit

What is the data type for a string?

str

How must string literals be written?

They must be wrapped in matching quotes, either ‘ ‘ or “ “

Concatenation

Joining two or more strings with +

What are the two jobs of the + symbol?

• Adds numbers

• Joins strings

Which one happens depends on the type

What does the type() function do?

It tells you what Python thinks a value is

Why is year = “2023” a string rather than a number?

The quotes decide, not the characters inside

When code behaves strangely, what is almost always the fastest first move?

Check type()

What do int(), float(), and str() do?

They build a new value of the requested type

What is the result of “5” + “3” and why?

“53” because the two values are strings and are joined together

What is the result of “5” * 3

“555” — the string is repeated three times

Why would int(year) + 1 work if year = “2023”?

int(year) converts the string to an integer first, then Python can do the math

When can conversion between types fail?

When the value cannot be converted to the requested type. For example, float(“abc”) raises a ValueError because the text has to look like a number

What data type does everything typed by a user arrive as?

A string (str)

//

Integer Division

**

Exponent

%

Remainder

Order of Precedence for Python Math

() —> ** —> *, /, //. % —> +, -

True or False: / always produces a float, even when the division is exact

True

True or False: Use / if you want a whole number

False

4 / 2

2.0

4 / / 2

2

Floats are ___________, not exact decimals

Approximations

Dividing by zero creates what type of error?

A ZeroDivisionError

What are Python’s comparison operators?

>, <, <=, >=, ==, !=

What data type results from a comparison?

bool

What are the two values of bool?

• True

• False

When an expression contains both calculations and comparisons, which happens first?

Calculations

== vs. =

== asks a question while = gives an instruction