Every sequence type this topic has covered so far —
str, tuple, range, list — is indexed by position: element 0, element 1, and so on. Many real problems, though, need lookup by label, not position: a ticker symbol mapping to a company name, a currency pair mapping to an exchange rate, a date mapping to a closing price. Dictionaries provide exactly this: a mutable collection of key/value pairs, with fast lookup by key. Sets, this topic's closing structure, are the special case of "just the keys, no values attached" — a collection whose only question is membership. Together, dictionaries and sets are the natural representation, named directly in docs/programme-profiles/library/introduction-to-python.md's Stage 4, for labelled financial series and parameter maps.Learning objectives. By the end of this note you should be able to:
- construct, index, and mutate a dictionary, and correctly iterate over its keys, values, or key/value pairs;
- explain the hashability requirement for dictionary keys, and state why tuples (but not lists) can serve as keys;
- construct a set, test membership, and compute unions, intersections, and differences using both operator and method syntax;
- choose correctly among
dict,set,list, andtuplefor a given labelled- or membership-based data problem.
In this lesson
- Dictionaries: key/value pairs
- Iterating over a dictionary
- Hashability: why tuples, not lists, can be dictionary keys
- Sets: membership without values
- Choosing among the four structures
Before you start
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