Unit 3 · Heavy
The four built-in collections and the habit of choosing between them. Most of the awkwardness in beginner Python comes from solving a dictionary problem with parallel lists.
What a strong answer looks like
A strong Unit 3 answer names the collection that matches the problem and says what that choice makes cheap.
Know
Lists are ordered and mutable. A slice takes start, stop, and step, and the stop index is excluded.
Apply
Mark both slice boundaries before evaluating, and use a negative step to reverse.
Watch out
Treating the stop index as included, which yields a slice one item short of what you expected.
Study move
Predict five slices of one list, including a negative index and a negative step.
Know
A dictionary maps keys to values with constant average lookup. Keys must be hashable, which excludes lists.
Apply
Reach for a dictionary whenever you would otherwise keep two parallel lists in step.
Watch out
Indexing a missing key raises rather than returning nothing; use a default lookup when absence is expected.
Study move
Convert a pair of parallel lists into one dictionary and rewrite a lookup against it.
Know
A set stores unique unordered items with fast membership testing. A tuple is an immutable sequence, which makes it hashable and usable as a dictionary key.
Apply
Use a set to deduplicate or test membership, and a tuple for a fixed record.
Watch out
Expecting a set to preserve order. It does not, and relying on the order you observe is fragile.
Study move
Deduplicate a list with a set and explain what is lost.
Know
A comprehension builds a collection from an iterable in one expression, and is clearer than an accumulating loop when the logic is simple.
Apply
Use a comprehension for map-and-filter work; keep a loop when the body needs several statements.
Watch out
Compressing complex logic into one comprehension, which trades readability for brevity.
Study move
Rewrite an accumulating loop as a comprehension, then find one that should stay a loop.
Connections and techniques that receive extra attention in this unit.
Related topics some schools attach to this unit and others leave out. Covered on request rather than assumed.
Modeling drill
Given a list of student names with repeated entries, produce a count per name and the list of unique names, choosing structures that make each cheap.
