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Three gentle ones to start with. Nothing is locked: the editor and the Check button work on every exercise, in any order, and the hints cost nothing.

01Getting the basics right10 exercises
Keep a number inside a range min and max, guard clauses ~10 min Initials from a name str.split, join ~10 min Read a yes or no from text truthiness versus meaning, sets for membership ~12 min What percentage is this of that true division, guarding the divisor ~10 min One file, two files f-strings, optional parameters ~10 min Convert without crashing try / except, naming the exception ~10 min File sizes people can read f-string format specs, loops with early return ~15 min 1st, 2nd, 3rd, 11th modulo, special case before general rule ~15 min Round to the nearest step banker’s rounding, floor division tricks ~15 min Capitalise a title properly why str.title() is wrong, word-level rules ~15 min
02Working with sequences12 exercises
The last few, and the zero that is not zero negative slicing, the -0 trap ~12 min Chunk a sequence slicing, range with a step ~15 min Remove duplicates, keep the order sets for membership, the seen pattern ~15 min Flatten one level of nesting nested comprehensions, reading order ~15 min Rotate a list without falling over modulo for wrapping, slice and rejoin ~15 min The total so far, at every step carrying state through a loop, accidentally quadratic ~15 min One cut, two pieces, nothing lost complementary slices, slices clamp ~15 min Taking turns until everyone is done *args, round-robin ~20 min The longest stretch of the same thing consecutive grouping, best-so-far ~20 min Page through a list 1-based vs 0-based, slicing ~20 min Rows become columns zip(*rows), unpacking into a call ~18 min Sliding windows over a sequence overlapping slices, the range bound ~20 min
03Text wrangling12 exercises
Show only the last four string repetition, negative slicing ~12 min camelCase to snake_case re.sub with backreferences, two passes ~20 min Titles into URL slugs regular expressions, re.sub ~15 min Back the other way, without flattening the capitals capitalize() lowercases the rest, leading underscores ~16 min Shorten text without cutting a word in half rsplit, slicing ~15 min The words people actually used tokenising, Counter ~18 min Finding the addresses in a wall of text findall, greedy boundaries ~22 min Marking the words somebody searched for re.escape, alternation order ~22 min Parse a key=value string str.split with maxsplit, skipping empties ~20 min Getting the text out of the markup greedy versus lazy matching, entity decoding order ~22 min Breaking a paragraph into lines greedy algorithms, accumulating a buffer ~22 min The comma that is not a separator character scanning, state machines ~28 min
04Shaping data12 exercises
Tally records by a field dict.get with a default, accumulating ~15 min Index records by a unique field dict as an index, detecting duplicates ~15 min Pull one field out of every record comprehension over records, a default that means absent ~12 min New names, same places rebuilding a dict, keeping order ~16 min The best few, without disturbing the rest sort stability, reverse= versus reversing ~18 min Reach into nested data safely walking nested dicts, defaults over exceptions ~20 min Group records by a field dict.setdefault, grouping ~20 min Turning a lookup around the inverse of a function, silent collisions ~20 min Sorting by several fields at once stable sort, successive passes ~24 min What changed between these two set algebra on keys, three-part results ~25 min Nested data, one level deep recursion over a tree, path building ~25 min Merge configuration, deeply recursion, deep vs shallow copy ~30 min
05Dates and money8 exercises
Days between two dates date.fromisoformat, timedelta ~15 min Cents into something readable divmod on negatives, format specs ~15 min Read 1h30m as seconds finditer, consuming the whole string ~25 min Three minutes ago ladder of thresholds, rounding a duration ~24 min Split money without losing a cent integer cents, divmod ~20 min Add months to a date calendar arithmetic, divmod for wrapping ~30 min Counting the days that count weekday(), inclusive ranges ~26 min Collapsing the bookings that touch sort then sweep, comparing against the last kept item ~26 min
06Functions that take functions10 exercises
Transform every value in a mapping functions as arguments, dict comprehensions ~15 min Sorting when some of the keys are missing decorate-sort-undecorate, None does not compare ~18 min Chain two functions into one returning a function, closures ~20 min A decorator that keeps a tally attributes on a function, functools.wraps ~22 min Find the first one that fits next with a default, generator expressions ~20 min Run it the first time and never again state that outlives a call, None is a real result ~20 min Both halves, one pass higher-order functions, calling a predicate once ~20 min Remember what you already worked out decorators, closures over a cache ~28 min Steps in the order you read them returning a function, left to right ~24 min Retry something that fails sometimes functions as arguments, try/except in a loop ~25 min
07Objects and classes10 exercises
The class you barely have to write @dataclass, generated __eq__ and __repr__ ~18 min A Stack that knows how long it is __len__ and free truthiness, private attributes ~20 min Settings nobody can change behind your back frozen dataclass, hashable values ~22 min Keys that do not care about capitals __getitem__ and friends, normalising at the boundary ~24 min Stock that cannot go negative methods that guard state, zero is not a stock level ~24 min A Money value that behaves like one __eq__ and NotImplemented, __add__ ~25 min A Temperature that cannot be impossible @property, computed attributes ~25 min An object you can loop over twice __iter__, iterable versus iterator ~24 min A set that keeps the order you added things operator dunders, returning new objects ~28 min Versions that sort like versions tuple comparison, functools.total_ordering ~30 min
08Files and formats10 exercises
Swap a file’s extension pathlib without touching disk, with_suffix ~15 min Three fields and everything else maxsplit, fixed fields plus a free-text tail ~18 min Directory, name, stem, suffix rpartition, the last dot, not the first ~18 min The last few lines splitlines, negative slicing ~15 min Turn CSV rows into records zip, header rows ~20 min The metadata block at the top of the file a fence is only a fence in the right place, returning a pair ~22 min Make two JSON payloads comparable json.loads and dumps, objects unordered, arrays ordered ~20 min Sections, settings and the lines that are neither line-oriented parsing, state across lines ~24 min Back out to rows and columns a header is a decision, missing versus empty ~20 min A file too big to hold iterables you can only read once, single pass ~26 min
09Making code trustworthy8 exercises
Comparing floats without lying float equality, relative versus absolute tolerance ~15 min An error message somebody can act on what a message must contain, !r for ambiguous values ~18 min Report every missing field at once collecting errors, present but falsy ~15 min Everything arrives as a string converting at the boundary, bool('False') is True ~24 min One shape in, one shape out accepting more than one shape, one cleaning path ~20 min Rules as functions, errors as data rules you can pass around, collecting every failure ~24 min Code that asks for the time instead of taking it injecting the clock, reading a value once ~25 min A block of code that has to leave something alone context managers, post-conditions ~26 min
10Data without the library14 exercises
The last batch is smaller generators, the ragged final batch ~18 min Four numbers that describe a classifier true and false, positive and negative, counting in one pass ~18 min The average, and the other average mean versus median, the even-length middle ~18 min Squeezing a column into nought and one rescaling, the zero-range column ~18 min Ninety-nine per cent accurate and completely uselessdebug accuracy on rare events, the majority-class baseline ~20 min Perfect in validation, useless in productiondebug data leakage, fit on train, apply to test ~24 min Categories a model can actually add up categories are not numbers, a fixed vocabulary ~24 min Two numbers that disagree, and the one that combines them precision versus recall, undefined metrics ~24 min Holding some data back shuffling with a seed, reproducibility ~24 min Dividing by n, or by n minus one Bessel’s correction, sample versus population ~22 min How unusual is this number standardising, units of standard deviation ~20 min The same patient on both sides of the splitdebug group leakage, the unit of independence ~26 min Every row gets a turn at being the test set cross-validation, distributing a remainder ~26 min Turning scores into probabilities without overflowing numerical stability, exp overflow ~26 min
11Working with arrays12 exercises
Selecting with a condition instead of a loop boolean arrays, & rather than and ~20 min Running totals and the gaps between readings cumsum, diff shortens the array ~20 min An if statement with no if in it np.where, both branches are evaluated ~20 min Where the largest value is, not what it is argmax returns a position, unravel_index ~24 min A slice of an array is not a copy of it views versus copies, assigning into a slice ~22 min Shapes that stretch to fit broadcasting rules, trailing dimensions ~24 min Brightening a pixel until it goes dark fixed-width integers, silent wraparound ~24 min Picking rows in an order you choose indexing with an array, fancy indexing copies ~22 min The value that is not equal to itself nan propagation, isnan rather than == ~24 min The same numbers, a different shape reshape, the -1 dimension ~20 min Three lists into three columns stack adds a dimension, concatenate joins one ~22 min The at sign is not the star @ versus *, inner dimensions must match ~26 min
12Tables with pandas12 exercises
How the answers split, as a percentage value_counts, normalize for proportions ~20 min The update that quietly did nothingdebug chained assignment, copy versus view ~24 min Half the league scored nothingdebug Series arithmetic aligns on the index, NaN from a missing label ~24 min The region that vanished from the reportdebug groupby drops missing keys, dropna=False ~24 min The total came out as a sentencedebug object dtype is not a number, to_numeric with errors='coerce' ~26 min The latest status, out of an event log drop_duplicates with subset and keep, sort before you dedupe ~26 min One row per channel, and a cost per click groupby with named aggregations, reset_index turns keys back into a column ~26 min A row-by-row loop over a table whole-column arithmetic, np.where and Series.clip ~26 min The month we sold everything twicedebug many-to-many merges, validate= ~26 min The grid averaged what it should have addeddebug pivot_table defaults to mean, pivot raises where pivot_table averages ~30 min Every month, including the quiet ones to_datetime and the .dt accessor, grouping by a period ~30 min The best three in every region groupby().head() takes the first rows, not the best, sort by several keys with one call ~28 min
13Reading broken code8 exercises
The basket that adds up to almost rightdebug float equality, tolerance ~20 min The leaderboard where 9 beats 100debug sort keys, strings that look like numbers ~20 min 'int' object is not callabledebug shadowing a built-in, name lookup order ~18 min The basket that remembersdebug default arguments, when defaults are evaluated ~20 min The snippet that is short at one end and wrong at the otherdebug exclusive slice ends, negative indices wrap ~22 min The cleanup that crashes halfway throughdebug mutating while iterating, build rather than delete ~20 min The filter that misses every other onedebug iterating while modifying, how for walks a list ~20 min Three discount buttons, one discountdebug closures, late binding ~25 min
14Reading tracebacks8 exercises
The stock count that tried to count the word quantitydebug reading a ValueError, the quoted value ~15 min The chart that crashes on the last daydebug reading an IndexError, the last iteration ~15 min The total that crashes on a price without a pound signdebug reading a traceback, a function that falls off the end ~15 min The podium that sorted into nothingdebug reading a NoneType error, methods that return None ~15 min The setting that is there and cannot be founddebug reading a KeyError, repr of a key ~15 min The digit sum that never reaches zerodebug reading a RecursionError, a base case that is never reached ~20 min The class list that will not sortdebug errors raised inside a built-in, None in data ~20 min The balance that exists and cannot be readdebug reading an UnboundLocalError, scope ~20 min
15Making it better8 exercises
The flag that should be any() any and all, short-circuiting ~15 min The string built one piece at a time str.join, immutable strings ~15 min The if-chain that should be data dict as a lookup table, data versus control flow ~15 min The counter that enumerate already keeps enumerate with start, manual counters ~15 min Unwinding the arrow guard clauses, early return ~20 min Numbers that do not say what they are named constants, one definition per rule ~18 min The three lines that are one line setdefault, defaultdict ~18 min The lookup that scans everything dict as an index, nested loops ~20 min
16Interview classics8 exercises
Brackets that close in the right order a list as a stack, matching pairs ~20 min The best day to buy and the best day to sell running minimum, one pass ~20 min The first letter that appears only once count first, then look, Counter ~15 min The one ticket number nobody handed in the sum you expect, arithmetic instead of searching ~15 min Two numbers that add up to a target dict as a lookup, one pass ~20 min The best run of consecutive days Kadane’s algorithm, extend or start again ~25 min The longest stretch with no letter repeated sliding window, last seen position ~30 min Merge the meetings that overlap sort first, compare with the last kept ~25 min
17Recursion9 exercises
Flatten a list of lists of lists recursion, base case ~20 min Add up numbers hidden at any depth recursion, base case ~15 min How deep does it go? recursion, max of the answers ~15 min Every way to choose from a list recursion that branches, building new lists ~25 min Every order the letters can go in permutations, recursion that branches ~25 min Find it in a sorted list by halving binary search, recursion on a range ~25 min How much space is this folder using? recursion over a tree, two kinds of node ~20 min Move the tower, one disc at a time recursion with two calls, trusting the smaller call ~25 min Sort a list by splitting it in half divide and conquer, merging two sorted lists ~30 min