Data Types & Conversion

Discussion prompt

A distance sensor's reading and an LED's color both reach your program as data - but not the same KIND of data.

That is stated as though it always holds. Do one of two things: produce a case where it fails, or say precisely what rules such a case out. "It just does" is not on the menu.

Hint: Hunt at the extremes first — zero, one, negative, empty, equal. If every extreme survives, the reason they survive is the proof.

Discussion prompt

Explain A fourth type you'll meet: bool by analogy to something with no Python in it at all — a queue, a recipe, a map, a bank balance, whatever fits. Then say where your analogy breaks.

Hint: An analogy that never breaks is not an analogy, it is the same idea wearing a hat. Find the seam — that is the part that is actually new.

Predict first

When you are unsure what you are holding, ask Python directly:

Commit before you compute: what does Check the type with type() come out to? A rough magnitude and the right form is enough — the point is to have something concrete to be wrong about.

Correct: Read the output

Why: A prediction you can defend turns the computation into a check rather than a leap of faith — and an answer that contradicts it is caught on the spot. Verified by execution - each line prints the class name of the value's type.

Comparison matrix

From Check the type with type(): refill the prints column from what you know. The rest of the table is as it appeared.

Discussion prompt

Explain Every value wears a label to a student a year behind you. No notation, no jargon they have not met — and it still has to be true.

Hint: If your explanation needs a symbol they have never seen, you are describing the notation rather than the idea.

Sort into buckets

Every line below is part of the definition of string (str) or of floating-point — one or the other, never both. Put each where it belongs.

string (str)

A sequence of characters wrapped in quotes.; "50" is the two characters 5 and 0 - it is text, not the number 50, even though it looks like a number to you.

floating-point

How computers store decimal numbers.; Because the storage is approximate, 0.1 + 0.2 is not exactly 0.3.; For money, work in whole cents (ints) to stay exact.

b1

A sequence of characters wrapped in quotes. "50" is the two characters 5 and 0 - it is text, not the number 50, even though it looks like a number to you.

b2

How computers store decimal numbers. Because the storage is approximate, 0.1 + 0.2 is not exactly 0.3. For money, work in whole cents (ints) to stay exact.

Predict first

The table runs: 10 / 2 | 5.0 | float · 10 / 4 | 2.5 | float

In Where floats sneak in, given the rows so far: what is the next one — the row where expression is 0.1 + 0.2?

Correct: 0.1 + 0.2 | 0.30000000000000004 | float (approximate!)

Why: The relationship between the columns, not the individual numbers, is what generates the next row. The / operator produces a float every time, so the answer carries a decimal point even though it divides evenly.

Comparison matrix

From Where floats sneak in: every row here is a choice with a cost. Fill the value column, then say which row you would actually pick and what you give up for it.

Put in order

Put the moves of Splitting seconds into minutes into the order they have to happen.

1. Line 2: 125 // 60 is 2 - how many whole minutes fit
2. Line 3: 125 % 60 is 5 - the leftover seconds
3. Line 4: print joins everything with spaces

Why: These are the moves of the worked example in the order it makes them, and each one is set up by the one before it. Floor division keeps only the whole number of 60s inside 125.

Annotate

Walk the callouts on Splitting seconds into minutes. Each one is a place this is easy to get subtly wrong.

• Floor division keeps only the whole number of 60s inside 125.
• Modulo returns what division left behind: 125 - (2 * 60) = 5.
• Verified by execution: 2 min 5 sec. Note print() converts each piece to text for you - + would not.

Step through it

Step through Proof: input() returns a str one row at a time. One of these columns never changes — find it, and say why it cannot.

1. Step 1: the camper types is 12
2. Step 2: the camper types is hello
3. Step 3: the camper types is 3.5

Predict first

A student writes this, and it looks reasonable:

Goal: add the numbers 3 and 4.

It is wrong. Say what breaks — and say it before you turn the page.

Correct: + between two strings concatenates - it glues the characters into "34".

Convert the text to numbers first, then add.

Why: + between two strings concatenates - it glues the characters into "34". It never adds them as numbers.

Discussion prompt

Converting a whole-number reading — before any calculation: what is the plan? Name the moves in order, in plain English, without doing the arithmetic.

Hint: It starts with: Line 1: reading is the TEXT "50"

Draw it

Draw what Converting a whole-number reading just did — the shape of it, not the line-by-line working. One picture, labels only where you need them. Then check it against the steps: anything you could not draw is a step you followed rather than understood.

Predict first

A temperature sensor sends "3.5". Double it.

Commit before you compute: what does Converting a decimal reading come out to? A rough magnitude and the right form is enough — the point is to have something concrete to be wrong about.

Correct: Trace each value

Why: A prediction you can defend turns the computation into a check rather than a leap of faith — and an answer that contradicts it is caught on the spot. Verified by execution: 3.5 * 2 is 7.0, printed as 7.0 because a float stays a float.

Predict first

A student writes this, and it looks reasonable:

You have the decimal text "3.5" and reach for int().

It is wrong. Say what breaks — and say it before you turn the page.

Correct: int() only parses whole-number text like "3".

Use float() for decimal text. If you then want a whole number, chop it with int() after.

Why: int() only parses whole-number text like "3". The ".5" makes it refuse, and the program stops.

Discussion prompt

The rule this trap just fixed:

Use float() for decimal text. If you then want a whole number, chop it with int() after.

Now break it on purpose. Build a case that violates it and follow the consequences until something visibly fails. Where does the failure first show up — and would you have noticed it if you had not been looking?

Hint: The dangerous rules are the ones whose violation still produces an answer. If yours fails loudly, try to find one that fails quietly.

Put in order

Put the moves of Building a message with str() into the order they have to happen.

1. Line 1: score is the number 100 (an int)
2. Line 2: str(score) makes "100", then + joins
3. Trace each value

Why: These are the moves of the worked example in the order it makes them, and each one is set up by the one before it. You cannot join an int directly onto "Score: " with + - the types do not match.

Discussion prompt

Explain Next level: f-strings to a student a year behind you. No notation, no jargon they have not met — and it still has to be true.

Hint: If your explanation needs a symbol they have never seen, you are describing the notation rather than the idea.

Fill in the blanks

From The f-string version — one line has had its right-hand side removed. Put it back.

name = "Robo"
score = 100
print(f"___ scored ___")

Why: score is what everything below it consumes, so the wrong expression here fails later and somewhere else. {name} becomes Robo and {score} becomes 100 - automatically converted, no str() call.

Comparison matrix

From The f-string version: refill the fills in with column from what you know. The rest of the table is as it appeared.

Predict first

A student writes this, and it looks reasonable:

A sensor reading "5" plus the number 5.

It is wrong. Say what breaks — and say it before you turn the page.

Correct: One side is text, the other a number.

Decide what you want - math, or joined text - and convert one side to match.

Why: One side is text, the other a number. Python refuses to guess and stops. (5 + "5" fails the same way, with the message worded the other direction.)

Match the pairs

Match each term to the definition this lesson gave it — not the one you would guess from the word.

• t1. string (str)
• t2. floating-point
• t3. modulo (%)
• t4. truncate

• d1. A sequence of characters wrapped in quotes. "50" is the two characters 5 and 0 - it is text, not the number 50, even though it looks like a number to you.
• d2. How computers store decimal numbers. Because the storage is approximate, 0.1 + 0.2 is not exactly 0.3. For money, work in whole cents (ints) to stay exact.
• d3. The remainder after division. 125 % 60 is 5. Camp use: turn 125 total seconds into 2 minutes (125 // 60) and 5 seconds (125 % 60).
• d4. To cut off the decimal part and keep only the whole-number part. int(3.9) truncates to 3 - it ignores how close 3.9 is to 4.

Why: These are the working definitions of string (str), floating-point, modulo (%), truncate as Data Types & Conversion uses them. Pairing them correctly is the test of whether you could state each one with the slide switched off.

Predict first

A student writes this, and it looks reasonable:

You want to round 3.9 to the nearest whole number.

It is wrong. Say what breaks — and say it before you turn the page.

Correct: Because 3.9 is almost 4, people expect 4.

To round to the nearest whole number, use round().

Why: Because 3.9 is almost 4, people expect 4. But int() only chops the decimal, leaving 3.

Sort into buckets

Some of these hold up and some are the exact mistakes this lesson is built to prevent. Sort them.

Holds up

A distance sensor's reading and an LED's color both reach your program as data - but not the same KIND of data.; A bool is secretly a number: True counts as 1 and False counts as 0. You will lean on this when a robot decides whether to act. We come back to it at the end.; Picture each value sitting in a box with a label saying what kind of thing it is. The label decides which tools work on it.

Breaks

Goal: add the numbers 3 and 4.; You have the decimal text "3.5" and reach for int().

sound

These are stated as this lesson states them — each one survives the edge cases Data Types & Conversion puts it through.

flawed

Each of these is lifted from a trap in this deck: reasonable-sounding, and wrong in a way that only shows up once you rely on it.

Discussion prompt

Explain round(): to the nearest - mostly by analogy to something with no Python in it at all — a queue, a recipe, a map, a bank balance, whatever fits. Then say where your analogy breaks.

Hint: An analogy that never breaks is not an analogy, it is the same idea wearing a hat. Find the seam — that is the part that is actually new.

Draw it

Draw what round() in action just did — the shape of it, not the line-by-line working. One picture, labels only where you need them. Then check it against the steps: anything you could not draw is a step you followed rather than understood.

Discussion prompt

bool(x) answers is this value truthy? Zero, an empty string, and nothing-at-all are falsy; everything else is truthy.

That is stated as though it always holds. Do one of two things: produce a case where it fails, or say precisely what rules such a case out. "It just does" is not on the menu.

Hint: Hunt at the extremes first — zero, one, negative, empty, equal. If every extreme survives, the reason they survive is the proof.

Predict first

The table runs: bool(0) | False | zero is the only falsy number · bool(5) | True | any non-zero number is truthy · bool("") | False | an empty string is falsy

In Truthy and falsy values, given the rows so far: what is the next one — the row where expression is bool("0")?

Correct: bool("0") | True | non-empty string - even "0"

Why: The relationship between the columns, not the individual numbers, is what generates the next row. bool(0) is False; any non-zero number, like 5, is True.

Sort into buckets

Sort these by value, from memory, without looking back at Truthy and falsy values. Telling them apart on the spot is the skill; the table is only where the answer happens to be written down.

False

bool(0); bool("")

True

bool(5); bool("0")

g1

value is "False" for bool(0), bool("") — that is what the table on "Truthy and falsy values" records, and it is the single property separating this group from the rest.

g2

value is "True" for bool(5), bool("0") — that is what the table on "Truthy and falsy values" records, and it is the single property separating this group from the rest.

Discussion prompt

Build it: the area calculator — before any calculation: what is the plan? Name the moves in order, in plain English, without doing the arithmetic.

Hint: It starts with: Lines 1-2: the measurements arrive as TEXT

Step through it

Step through Build it: the area calculator one row at a time. What is driving the change, and what would the row after the last one be?

1. Step 1: line is 1
2. Step 2: line is 2
3. Step 3: line is 3
4. Step 4: line is 4

Put in order

Put the moves of Build it bigger: temperature converter into the order they have to happen.

1. Line 1: input() gives text, int() wraps it into a number
2. Line 2: the formula - and / makes it a float
3. Line 3: an f-string drops both values into text

Why: These are the moves of the worked example in the order it makes them, and each one is set up by the one before it. If the camper types 100, input() returns "100"; int("100") makes it 100.

Annotate

Walk the callouts on Build it bigger: temperature converter. Each one is a place this is easy to get subtly wrong.

• If the camper types 100, input() returns "100"; int("100") makes it 100. The int() is what lets the next line do math.
• 100 * 9 / 5 + 32 is 212.0. Because / is used, fahrenheit is a float (note the .0).
• Verified by execution (typing 100): prints 100C is 212.0F. No str() calls needed - the braces convert for you.

Discussion prompt

Outside this lesson: where does Data Types & Conversion actually turn up? Name one concrete situation — a job, a piece of software someone ships, a decision somebody has to make — and say which part of The conversion recipe is doing the work in it.

Hint: Vague is the failure mode here. "Engineering" is not a situation; "deciding whether this build is fast enough to ship" is.

Check your understanding

What does this program display?

• A. 7
• B. 34 (correct)
• C. 12
• D. TypeError

Answer: B

Why: Both operands are strings, so + concatenates them into the text "34"; print shows it without the quotes. To add them as numbers you would convert first: int("3") + int("4") is 7. Verified by execution: this prints 34.

Why A tempts people

Treated the strings as numbers. + between two strings joins them; it never adds. You would need int() on each side to get 7.

Why C tempts people

Multiplied 3 by 4. + joins text - and even as numbers, 3 + 4 is 7, not 12.

Why D tempts people

Two strings concatenate cleanly, so there is no error. The TypeError only happens when you mix a str and a number, like "5" + 5.

Check your understanding

What does this program print?

• A. 4 (correct)
• B. 5
• C. 4.9
• D. 3

Answer: A

Why: int(3.9) truncates - it chops the decimal to 3, it does not round to 4. Then 3 + 1 is 4. Verified by execution: this prints 4.

Why B tempts people

Assumed int(3.9) rounds up to 4, then 4 + 1 is 5. int() truncates to 3 - use round(3.9) if you want 4.

Why C tempts people

Kept the decimal: 3.9 + 1 is 4.9. int() removes the decimal entirely, leaving the whole number 3.

Why D tempts people

Stopped at int(3.9) = 3 and forgot to add the 1.

Comparison matrix

From Check yourself: chop or round?: every row here is a choice with a cost. Fill the value column, then say which row you would actually pick and what you give up for it.

Check your understanding

What does type(answer) print?

• A. <class 'int'>
• B. <class 'str'> (correct)
• C. <class 'float'>
• D. <class 'input'>

Answer: B

Why: input() always returns a str, even when the camper types digits. answer is the text "12", so type(answer) is <class 'str'>. To get a number you must wrap it: int(input(...)). Verified by execution.

Why A tempts people

input() does not detect numbers - it never returns an int. You only get an int if you wrap it yourself with int(input(...)).

Why C tempts people

input() does not return a float either. Typing 12 gives the text "12", not 12.0.

Why D tempts people

There is no 'input' type. input() hands back an ordinary str.

Eliminate the wrong options

What does this program print, line by line?

3 of these 4 are wrong. Strike them one at a time, and say what rules each one out before you strike the next. The survivor is the answer.

• A. 3 then 3.5
• B. 3.5 then 3.5
• C. 3 then 3
• D. 4 then 3.5

Survives elimination: A

Why: // is floor division: 7 // 2 throws away the remainder and gives the int 3. / is true division: 7 / 2 gives the float 3.5. Verified by execution: the output is 3 then 3.5.

Check your understanding

What does this program print, line by line?

• A. 3 then 3.5 (correct)
• B. 3.5 then 3.5
• C. 3 then 3
• D. 4 then 3.5

Answer: A

Why: // is floor division: 7 // 2 throws away the remainder and gives the int 3. / is true division: 7 / 2 gives the float 3.5. Verified by execution: the output is 3 then 3.5.

Why B tempts people

Used / for both lines. // is floor division and drops the remainder, giving 3 - not 3.5.

Why C tempts people

Assumed // and / behave the same. Only // floors; / always produces a float (3.5 here).

Why D tempts people

Rounded 7 // 2 up to 4. Floor division truncates toward the lower whole number, giving 3.

Comparison matrix

From Check yourself: // vs /: refill the value column from what you know. The rest of the table is as it appeared.

Check your understanding

What does this print?

• A. True (correct)
• B. False
• C. 0
• D. TypeError

Answer: A

Why: "0" is a string with one character, so it is non-empty and therefore truthy: bool("0") is True. Only the empty string "" is falsy. The number 0 is falsy, but the text "0" is not the number 0. Verified by execution.

Why B tempts people

Confused the text "0" with the number 0. The number 0 is falsy, but any non-empty string - including "0" - is truthy.

Why C tempts people

bool() returns True or False, never the number 0. bool(0) would be False, but here the argument is the string "0".

Why D tempts people

bool() accepts any value without error - strings included. There is no type mismatch here.

Draw it

One page, no notation unless you need it: draw how these connect — The conversion recipe · Why types matter at camp · The three core types · A fourth type you'll meet: bool · Every value wears a label. Put an arrow wherever one of them is what makes another possible, and label the arrow with why.