Session 3 - Input & Output

Discussion prompt

Before we open Session 3 - Input & Output: without looking back, what was the main idea of Session 2 - Variables & Types, and what could you do by the end of it that you could not do before?

Hint: One sentence for the idea, one for the skill. If the second one is blank, that is the part to revisit.

Discussion prompt

input("...") shows your message, then pauses and waits for the user to type and press Enter.

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 Store the answer in a variable by analogy to something with no Python Fundamentals 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 A simple greeter 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

Explain The prompt is just a string 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.

Predict first

The table runs: 12 | "12" | str · hello | "hello" | str

In Proof: it is a str, given the rows so far: what is the next one — the row where user types is 3.5?

Correct: 3.5 | "3.5" | str

Why: The relationship between the columns, not the individual numbers, is what generates the next row. Digits, words, decimals - all arrive as a str.

Comparison matrix

From Proof: it is a str: refill the type(age) column from what you know. The rest of the table is as it appeared.

Predict first

A student writes this, and it looks reasonable:

Adding to input() directly. The user types 5.

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

Correct: input() returned a string. Adding the number 1 to text stops the program.

Wrap the input in int() to get a real number.

Why: input() returned a string. Adding the number 1 to text stops the program.

Put in order

Put the moves of Next year's age into the order they have to happen.

1. Line 1: int() wraps the input, so age is a number
2. Line 2: now the math works
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. input() returns "12"; int("12") makes it 12.

Comparison matrix

From Next year's age: every row here is a choice with a cost. Fill the type column, then say which row you would actually pick and what you give up for it.

Predict first

A student writes this, and it looks reasonable:

Using int() on text that has a decimal point. The user types 3.5.

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

Correct: "3.5" has a decimal point, which int() refuses.

Use float() for anything that might have a decimal.

Why: "3.5" has a decimal point, which int() refuses. The program stops with a ValueError.

Discussion prompt

The rule this trap just fixed:

Use float() for anything that might have a decimal.

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.

Discussion prompt

In Two inputs: rectangle area this move is made:

Then compute and show

Why is that legal? Name the rule or definition it rests on before you read on.

Hint: If you can only say "because that is what you do", the rule is the thing to go and find.

Annotate

Walk the callouts on Two inputs: rectangle area. Each one is a place this is easy to get subtly wrong.

• length becomes 12, width becomes 8 - both ints.
• Verified by execution: 12 * 8 is 96, prints Area: 96.

Predict first

A student writes this, and it looks reasonable:

Joining a number onto text with + but no str().

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

Correct: "Score: " is a str and score is an int.

Convert with str() - or just use a comma, or an f-string.

Why: "Score: " is a str and score is an int. Python stops rather than guess.

Sort into buckets

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

Holds up

input("...") shows your message, then pauses and waits for the user to type and press Enter.; The answer is only useful if you keep it: name = input("Your name? ").; The text inside input("...") is an ordinary string - end it with a space so the user's typing does not touch your question.

Breaks

Adding to input() directly. The user types 5.; Using int() on text that has a decimal point. The user types 3.5.

sound

These are stated as this lesson states them — each one survives the edge cases Session 3 - Input & Output 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.

Sort into buckets

These are the pieces of Session 3 - Input & Output, out of order. Put each one back under the part of the lesson it belongs to.

Asking with input()

input() asks a question; Store the answer in a variable; A simple greeter

Converting the Answer

int(input(...)) for whole numbers; Next year's age; float(input(...)) for decimals

Building Output

str(): number to text; Joining with str(); f-strings: fill in the blanks

s1

Asking with input() is where Session 3 - Input & Output puts input() asks a question, Store the answer in a variable, A simple greeter. Knowing which part of the lesson a problem belongs to is most of knowing which method to reach for.

s2

Converting the Answer is where Session 3 - Input & Output puts int(input(...)) for whole numbers, Next year's age, float(input(...)) for decimals. Knowing which part of the lesson a problem belongs to is most of knowing which method to reach for.

s3

Building Output is where Session 3 - Input & Output puts str(): number to text, Joining with str(), f-strings: fill in the blanks. Knowing which part of the lesson a problem belongs to is most of knowing which method to reach for.

Fill in the blanks

From An f-string in action — one line has had its right-hand side removed. Put it back.

name = "Sam"
badges = 3
print(f"___ has ___ badges")

Why: name is what everything below it consumes, so the wrong expression here fails later and somewhere else. {name} becomes Sam, {badges} becomes 3 - converted for you.

Comparison matrix

From Two decimal places: refill the shows column from what you know. The rest of the table is as it appeared.

Discussion prompt

In Echoing a text answer this move is made:

Drop it into a message

Why is that legal? Name the rule or definition it rests on before you read on.

Hint: If you can only say "because that is what you do", the rule is the thing to go and find.

Discussion prompt

Read, then convert — 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: raw is the text "25"

Discussion prompt

Explain Overwriting the same name is fine 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 Convert onto the same name — one line has had its right-hand side removed. Put it back.

age = input("Age? ")
age = int(age)
print(type(age))

Why: age is what everything below it consumes, so the wrong expression here fails later and somewhere else. Verified by execution: type is now <class 'int'>.

Draw it

Draw what Convert onto the same name 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

Explain Mixing a text and a number answer by analogy to something with no Python Fundamentals 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.

Fill in the blanks

From Name plus score — one line has had its right-hand side removed. Put it back.

name = input("Name: ")
score = int(input("Score: "))
print(f"Hi ___, you scored ___ - ___ to go!")

Why: name is what everything below it consumes, so the wrong expression here fails later and somewhere else. name is "Alex"; score is 90 so we can subtract with it.

Step through it

Step through Name plus score one row at a time. What is driving the change, and what would the row after the last one be?

1. Step 1: blank is {name}
2. Step 2: blank is {score}
3. Step 3: blank is {100 - score}

Discussion prompt

A tidy program has three phases: gather the inputs, compute the results, then show the output.

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.

Put in order

Put the moves of Build it: a trainer profile into the order they have to happen.

1. Gather: one text input, one number input
2. Compute: badges remaining
3. Show: one f-string ties it together

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. Verified by execution: Sam has 3 badges, 5 to go.

Step through it

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

1. Step 1: variable is name
2. Step 2: variable is badges
3. Step 3: variable is left

Put in order

Put the moves of Build it: a tip calculator into the order they have to happen.

1. Read the bill as a float
2. Compute tip and total
3. Show with money formatting

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. Money can have decimals, so float() is the right choice.

Annotate

Walk the callouts on Build it: a tip calculator. Each one is a place this is easy to get subtly wrong.

• Money can have decimals, so float() is the right choice. bill is 40.0.
• Verified by execution: Tip: $8.00 Total: $48.00.

Predict first

The table runs: {a} | 20 · {b} | 22

In Build it: a two-number adder, given the rows so far: what is the next one — the row where blank is {a + b}?

Correct: {a + b} | 42

Why: The relationship between the columns, not the individual numbers, is what generates the next row. a is 20, b is 22. Without int(), the next line would join text instead of add.

Comparison matrix

From Build it: a two-number adder: every row here is a choice with a cost. Fill the fills with column, then say which row you would actually pick and what you give up for it.

Discussion prompt

Outside this lesson: where does Session 3 - Input & Output 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 Choosing the right conversion 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 type(score) print?

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

Answer: B

Why: input() always returns a str, even for digits. score is the text "20", so type(score) is <class 'str'>. Verified by execution.

Why A tempts people

input() never returns an int on its own; only int(input(...)) does.

Why C tempts people

Typing 20 gives the text "20", not 20.0.

Why D tempts people

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

Check your understanding

What happens?

• A. It prints 6
• B. It prints 42
• C. TypeError: can only concatenate str (not "int") to str (correct)
• D. It prints 4

Answer: C

Why: n is the text "4" because input() returns a str. Adding the number 2 mixes a str and an int, so Python stops. int(input(...)) would make it print 6. Verified by execution.

Why A tempts people

6 needs conversion: int(n) + 2. As written, n is text, so the addition never runs.

Why B tempts people

Joining would give "42" only if both sides were strings, like n + "2". Here 2 is a number.

Why D tempts people

It does not silently print n - it errors before printing.

Check your understanding

Now what prints?

• A. 6 (correct)
• B. 42
• C. TypeError
• D. 4

Answer: A

Why: int(input(...)) converts "4" to the number 4, so 4 + 2 is 6. Wrapping input in int() is exactly what enables the math. Verified by execution.

Why B tempts people

"42" is string joining. Here n is an int, so + adds instead of joins.

Why C tempts people

No type mismatch now - both sides are numbers, so no error.

Why D tempts people

The + 2 runs, so it prints 6, not the original 4.

Check your understanding

What happens?

• A. x becomes 2
• B. x becomes 3
• C. ValueError: invalid literal for int() with base 10: '2.5' (correct)
• D. x becomes 2.5

Answer: C

Why: int() only parses whole-number text. "2.5" has a decimal point, so int() refuses and raises a ValueError. Use float() for decimals. Verified by execution.

Why A tempts people

int() does not truncate text - it rejects it. int(2.5) on a float would give 2, but this is text.

Why B tempts people

int() neither rounds nor accepts "2.5" as text - it errors.

Why D tempts people

int() cannot return a decimal. To keep 2.5 you would use float().

Check your understanding

What does this print?

• A. 6 and 4 make 10 (correct)
• B. {a} and {b} make {a + b}
• C. 6 and 4 make 64
• D. a and b make a + b

Answer: A

Why: The f before the quote makes each {...} a blank filled with that value's text: 6, 4, and the computed 6 + 4 = 10. Verified by execution.

Why B tempts people

That is what prints WITHOUT the f. The f is what turns the braces into blanks.

Why C tempts people

{a + b} computes the sum (10), it does not glue 6 and 4 into 64.

Why D tempts people

The braces are replaced by the variables' values, not their names.

Comparison matrix

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

Check your understanding

What does this print?

• A. $5.00 (correct)
• B. $5
• C. $5.0
• D. $5.2f

Answer: A

Why: :.2f formats the number with exactly two decimal places, so 5 shows as 5.00. The value is unchanged - only its display. Verified by execution.

Why B tempts people

Without :.2f you would get $5, but the format spec forces two decimals.

Why C tempts people

:.2f means two decimal places, not one, so it is 5.00 not 5.0.

Why D tempts people

The :.2f is a format instruction, not literal text - it never appears in the output.

Check your understanding

What should fill the blank?

• A. float (correct)
• B. int
• C. str
• D. nothing - leave it as input()

Answer: A

Why: A bill like 19.99 has a decimal point, so float() is the right converter. int() would raise a ValueError on "19.99". Verified by reasoning about int()'s decimal restriction.

Why B tempts people

int() rejects decimal text like "19.99" with a ValueError, so it cannot read a price with cents.

Why C tempts people

str() keeps it as text, so you could not do math (like adding tax) on it.

Why D tempts people

Leaving it as input() gives text; any arithmetic on it would be a TypeError.

Draw it

One page, no notation unless you need it: draw how these connect — Asking with input() · Converting the Answer · Building Output · Text Answers & Converting Later · Putting It Together · Check Yourself. Put an arrow wherever one of them is what makes another possible, and label the arrow with why.