Session 22 - Standard Library Tour

Discussion prompt

Before we open Session 22 - Standard Library Tour: without looking back, what was the main idea of Session 21 - Modules, Packages & pip, 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

The standard library is a large set of ready-made modules installed alongside Python itself. No download, no pip - it is already on your machine.

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 import brings a module in 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.

Comparison matrix

From Two ways to import: refill the then you call column from what you know. The rest of the table is as it appeared.

Comparison matrix

From Import and use a module: every row here is a choice with a cost. Fill the returns 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:

You call a function without importing its module first.

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

Correct: sqrt lives inside math, not in the plain language.

Import the module, then reach the tool through it.

Why: sqrt lives inside math, not in the plain language. With no import, the name is undefined and Python raises NameError.

Annotate

Walk the callouts on Trap: using a tool before importing it. Each one is a place this is easy to get subtly wrong.

• sqrt lives inside math, not in the plain language. With no import, the name is undefined and Python raises NameError.
• Verified by execution: prints 4.0. The name lives on the module, so you say math.sqrt, not sqrt.

Discussion prompt

Explain math handles the tricky arithmetic 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: math.sqrt(2) | 1.4142135623730951 · math.floor(3.7) | 3

In sqrt, floor, and ceil, given the rows so far: what is the next one — the row where call is math.ceil(3.2)?

Correct: math.ceil(3.2) | 4

Why: The relationship between the columns, not the individual numbers, is what generates the next row. Verified by execution: math.sqrt(2) is 1.4142135623730951 - a full-precision float.

Draw it

Draw what sqrt, floor, and ceil 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.

Sort into buckets

Every line below is part of the definition of standard library or of randint(a, b) — one or the other, never both. Put each where it belongs.

standard library

The collection of modules bundled with Python.; 'Batteries included' - common jobs like math, dates, files, and randomness already have tested tools.

randint(a, b)

A random integer from a to b, including both ends.; randint(1, 6) is a fair six-sided die.

b1

The collection of modules bundled with Python. 'Batteries included' - common jobs like math, dates, files, and randomness already have tested tools.

b2

A random integer from a to b, including both ends. randint(1, 6) is a fair six-sided die.

Sort into buckets

These are the pieces of Session 22 - Standard Library Tour, out of order. Put each one back under the part of the lesson it belongs to.

Batteries Included

A library comes with Python; import brings a module in; Two ways to import

math: Numbers Done Right

math handles the tricky arithmetic; sqrt, floor, and ceil; floor and ceil with negatives

random: Controlled Chance

random makes unpredictable values; seed makes it reproducible; Seeded dice rolls

s1

Batteries Included is where Session 22 - Standard Library Tour puts A library comes with Python, import brings a module in, Two ways to import. Knowing which part of the lesson a problem belongs to is most of knowing which method to reach for.

s2

math: Numbers Done Right is where Session 22 - Standard Library Tour puts math handles the tricky arithmetic, sqrt, floor, and ceil, floor and ceil with negatives. Knowing which part of the lesson a problem belongs to is most of knowing which method to reach for.

s3

random: Controlled Chance is where Session 22 - Standard Library Tour puts random makes unpredictable values, seed makes it reproducible, Seeded dice rolls. Knowing which part of the lesson a problem belongs to is most of knowing which method to reach for.

Fill in the blanks

From choice and shuffle — one line has had its right-hand side removed. Put it back.

import random

random.seed(7)
deck = [1, 2, 3, 4, 5]
random.shuffle(deck)
print(deck)

Why: deck is what everything below it consumes, so the wrong expression here fails later and somewhere else. It changes deck itself and returns None - so you print deck, not the call.

Predict first

A student writes this, and it looks reasonable:

No seed - so the same code gives different numbers each run, and a bug you saw once may never come back.

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

Correct: With no fixed starting point, you cannot reproduce a result.

Seed first - same seed, same sequence, every time.

Why: With no fixed starting point, you cannot reproduce a result. There is no single 'correct' output to trace - that is the whole problem.

Discussion prompt

The rule this trap just fixed:

Verified by execution: 18, then 18. Re-seeding rewinds the sequence to the same spot.

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.

Step through it

Step through Build a date and read its parts one row at a time. What is driving the change, and what would the row after the last one be?

1. Step 1: expression is d
2. Step 2: expression is d.year
3. Step 3: expression is d.month
4. Step 4: expression is d.day

Fill in the blanks

Add days with timedelta, with the scaffolding fading: two lines are gone now — fill both.

from datetime import date, timedelta

d = date(2026, 7, 15)
later = d + timedelta(days=10)
print(later)

Why: Reproducing these unaided, rather than reading them, is what tells you the method has transferred. timedelta(days=10) is the length; adding it rolls the calendar forward, handling month ends for you.

Comparison matrix

From Add days with timedelta: refill the value column from what you know. The rest of the table is as it appeared.

Fill in the blanks

Days between two dates, with the scaffolding fading: two lines are gone now — fill both.

from datetime import date

start = date(2026, 1, 1)
end = date(2026, 7, 15)
gap = end - start
print(gap.days)

Why: Reproducing these unaided, rather than reading them, is what tells you the method has transferred. end - start is a span; printing it whole shows '195 days, 0:00:00'.

Step through it

Step through Format a date with strftime one row at a time. What is driving the change, and what would the row after the last one be?

1. Step 1: format string is %Y-%m-%d
2. Step 2: format string is %B %d, %Y
3. Step 3: format string is %A

Step through it

Step through name, suffix, and stem one row at a time. What is driving the change, and what would the row after the last one be?

1. Step 1: attribute is p.name
2. Step 2: attribute is p.suffix
3. Step 3: attribute is p.stem

Fill in the blanks

From Check whether a path exists — one line has had its right-hand side removed. Put it back.

from pathlib import Path

p = Path("notes.txt")
print(p.exists())
missing = Path("ghost.txt")
print(missing.exists())

Why: missing is what everything below it consumes, so the wrong expression here fails later and somewhere else. It returns True if a real file or folder is there, False otherwise - a safe check before you read.

Discussion prompt

Explain Counter tallies for you 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

Sort these by count, from memory, without looking back at most_common finds the top item. Telling them apart on the spot is the skill; the table is only where the answer happens to be written down.

3

the

1

cat; dog; bird

g1

count is "3" for the — that is what the table on "most_common finds the top item" records, and it is the single property separating this group from the rest.

g2

count is "1" for cat, dog, bird — that is what the table on "most_common finds the top item" records, and it is the single property separating this group from the rest.

Match the pairs

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

• t1. standard library
• t2. randint(a, b)
• t3. Path
• t4. defaultdict(factory)

• d1. The collection of modules bundled with Python. 'Batteries included' - common jobs like math, dates, files, and randomness already have tested tools.
• d2. A random integer from a to b, including both ends. randint(1, 6) is a fair six-sided die.
• d3. A pathlib object standing for a filesystem path. It exposes parts like .name and .suffix and can check .exists() - no manual string slicing.
• d4. A dict that auto-creates a missing key using factory() - int gives 0, list gives []. The factory is the type you pass in.

Why: These are the working definitions of standard library, randint(a, b), Path, defaultdict(factory) as Session 22 - Standard Library Tour uses them. Pairing them correctly is the test of whether you could state each one with the slide switched off.

Draw it

Draw what Group items with defaultdict(list) 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.

Fill in the blanks

From Count letters with defaultdict(int) — one line has had its right-hand side removed. Put it back.

from collections import defaultdict

counts = defaultdict(int)
for ch in "hello":
counts[ch] += 1
print(dict(counts))

Why: counts is what everything below it consumes, so the wrong expression here fails later and somewhere else. counts[ch] += 1 works even the first time: the missing key defaults to 0 before adding.

Annotate

In Count letters with defaultdict(int), before reading the notes: mark where the time actually goes. Which line dominates?

• counts[ch] += 1 works even the first time: the missing key defaults to 0 before adding.
• Verified by execution: final dict is {'h': 1, 'e': 1, 'l': 2, 'o': 1}.

Predict first

A student writes this, and it looks reasonable:

You read a key that a normal dict has never seen.

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

Correct: Ordinary dicts have no default, so scores['ben'] raises KeyError: 'ben' and the program stops.

A defaultdict(int) fills in 0 instead of crashing.

Why: Ordinary dicts have no default, so scores['ben'] raises KeyError: 'ben' and the program stops.

Annotate

Walk the callouts on Trap: a plain dict raises KeyError. Each one is a place this is easy to get subtly wrong.

• Ordinary dicts have no default, so scores['ben'] raises KeyError: 'ben' and the program stops.
• Verified by execution: prints 0, and the key 'new' is now stored as 0. Pick the container that matches how you read it.

Discussion prompt

Explain os and sys reach outside your code 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.

Predict first

The table runs: sys.version_info.major | 3 · sys.version_info.minor | 12

In Peek at the environment, given the rows so far: what is the next one — the row where expression is sys.platform?

Correct: sys.platform | win32

Why: The relationship between the columns, not the individual numbers, is what generates the next row. Its .major and .minor fields are the version numbers - useful when code needs a certain Python.

Comparison matrix

From Peek at the environment: every row here is a choice with a cost. Fill the value (this run) column, then say which row you would actually pick and what you give up for it.

Discussion prompt

Before you hand-write square roots, date math, counting loops, or path parsing, check the library. The built-in version is tested, faster, and handles edge cases you would miss.

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

A student writes this, and it looks reasonable:

Guessing a square root by hand - slow, and only right for perfect squares you happen to know.

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

Correct: This prints 144 - it confirms 12 squared, but it never actually finds the root of a number you do not already know.

Ask the library for the root directly.

Why: This prints 144 - it confirms 12 squared, but it never actually finds the root of a number you do not already know.

Sort into buckets

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

Holds up

The standard library is a large set of ready-made modules installed alongside Python itself. No download, no pip - it is already on your machine.; A module sits idle until you import it. After import math, you reach its tools through the module name: math.sqrt(...).; import math brings in the whole module; you then write math.sqrt. from datetime import date pulls one name in so you write date directly.

Breaks

You call a function without importing its module first.; No seed - so the same code gives different numbers each run, and a bug you saw once may never come back.

sound

These are stated as this lesson states them — each one survives the edge cases Session 22 - Standard Library Tour 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

Outside this lesson: where does Session 22 - Standard Library Tour 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 Reach for the library first 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 print?

• A. 4 (correct)
• B. 5
• C. 4.9
• D. 4.0

Answer: A

Why: floor always rounds down to the nearest whole number, so math.floor(4.9) is 4 (an int). Verified by execution.

Why B tempts people

That is rounding to nearest, or ceil. floor never rounds up, even from 4.9.

Why C tempts people

floor returns a whole number, not the original decimal.

Why D tempts people

floor returns an int (4), not a float (4.0).

Check your understanding

What is the second line of output?

• A. 18 (correct)
• B. A different random number
• C. 1
• D. None

Answer: A

Why: Re-seeding with the same value 1 rewinds the sequence, so the next randint is identical: 18 both times. Verified by execution.

Why B tempts people

It would differ only if the seed were not reset. The same seed guarantees the same number.

Why C tempts people

1 is the seed value, not the number drawn. randint(1, 100) returns 18 here.

Why D tempts people

randint returns an integer, not None.

Comparison matrix

From Check: seed reproducibility: refill the prints column from what you know. The rest of the table is as it appeared.

Check your understanding

What does this print?

• A. 4 (correct)
• B. 2
• C. 11
• D. KeyError

Answer: A

Why: Counter tallies each character; 'mississippi' has four s's, so c['s'] is 4. Verified by execution.

Why B tempts people

There are four s's in mississippi, not two - count them: mi-ss-i-ss-ippi.

Why C tempts people

11 is the length of the whole word, not the count of one letter.

Why D tempts people

Counter returns 0 for missing keys and a real count for present ones - never a KeyError.

Check your understanding

What does this print?

• A. 2026-02-28 (correct)
• B. 2026-03-00
• C. 2026-02-29
• D. 2026-02-31

Answer: A

Why: One day before March 1, 2026 is the last day of February. 2026 is not a leap year, so February ends on the 28th. Verified by execution.

Why B tempts people

There is no day 0 - datetime rolls back to the previous real calendar day.

Why C tempts people

Feb 29 exists only in leap years; 2026 is not a leap year.

Why D tempts people

February never has 31 days; datetime knows each month's real length.

Check your understanding

What does .suffix return here?

• A. .gz (correct)
• B. .tar.gz
• C. .tar
• D. gz

Answer: A

Why: .suffix is only the last extension, so it is '.gz'. To get both, .suffixes returns ['.tar', '.gz']. Verified by execution.

Why B tempts people

.suffix returns just the final piece; the full list of both is .suffixes, not .suffix.

Why C tempts people

.tar is the first extension, but .suffix reports the last one.

Why D tempts people

The suffix keeps its leading dot: '.gz', not 'gz'.

Check your understanding

What does this print?

• A. 0 (correct)
• B. KeyError
• C. None
• D. int

Answer: A

Why: defaultdict(int) creates a missing key using int(), which is 0, so d['new'] is 0 and no error is raised. Verified by execution.

Why B tempts people

A plain dict would raise KeyError, but defaultdict supplies a default instead.

Why C tempts people

The factory is int, which produces 0, not None. A defaultdict(lambda: None) would give None.

Why D tempts people

The factory int is called - int() - producing the value 0, not the type name.

Check your understanding

What does this produce?

• A. NameError: name 'sqrt' is not defined (correct)
• B. 4.0
• C. 4
• D. AttributeError

Answer: A

Why: sqrt lives in the math module. Without import math and the math. prefix, the bare name is undefined, raising NameError: name 'sqrt' is not defined. Verified by execution.

Why B tempts people

It would print 4.0 only after import math and calling math.sqrt(16).

Why C tempts people

sqrt returns a float (4.0) once imported, but here it never runs at all.

Why D tempts people

AttributeError is for a missing name on an object (like math.square); an undefined bare name is a NameError.

Draw it

One page, no notation unless you need it: draw how these connect — Batteries Included · math: Numbers Done Right · random: Controlled Chance · datetime: Dates & Days · pathlib: Paths as Objects · collections: Better Containers. Put an arrow wherever one of them is what makes another possible, and label the arrow with why.