16.7. Glossary

base case

A branch of the conditional statement in a recursive function that does not give rise to further recursive calls.

data structure

An organization of data for the purpose of making it easier to use.

immutable data type

A data type which cannot be modified. Assignments to elements or slices of immutable types cause a runtime error.

infinite recursion

A function that calls itself recursively without ever reaching the base case. Eventually, an infinite recursion causes a runtime error.

mutable data type

A data type which can be modified. All mutable types are compound types. Lists and dictionaries (see next chapter) are mutable data types; strings and tuples are not.


The process of calling the function that is already executing.

recursive call

The statement that calls an already executing function. Recursion can even be indirect — function f can call g which calls h, and h could make a call back to f.

recursive definition

A definition which defines something in terms of itself. To be useful it must include base cases which are not recursive. In this way it differs from a circular definition. Recursive definitions often provide an elegant way to express complex data structures.


A data type that contains a sequence of elements of any type, like a list, but is immutable. Tuples can be used wherever an immutable type is required, such as a key in a dictionary (see next chapter).

tuple assignment

An assignment to all of the elements in a tuple using a single assignment statement. Tuple assignment occurs in parallel rather than in sequence, making it useful for swapping values.

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