Python in a Nutshell, 2nd Edition

Set Operations

Python provides a variety of operations applicable to sets. Since sets are containers, the built-in len function can take a set as its single argument and return the number of items in the set object. A set is iterable, so you can pass it to any function or method that takes an iterable argument. In this case, the items of the set are iterated upon, in some arbitrary order. For example, for any set S, min(S) returns the smallest item in S.

Set Membership

The k in S operator checks whether object k is one of the items of set S. It returns True if it is and False if it isn’t. Similarly, k not in S is just like not (k in S).

Set Methods

Set objects provide several methods, as shown in Table 4-4. Nonmutating methods return a result without altering the object to which they apply and can also be called on instances of type frozenset, while mutating methods may alter the object to which they apply and can be called only on instances of type set. In Table 4-4, S and S1 indicate any set object, and x any hashable object.

Table 4-4. Set object methods

Method	Description
Nonmutating methods
S.copy( )	Returns a shallow copy of the set (a copy whose items are the same objects as S’s, not copies thereof)
S.difference(S1)	Returns the set of all items of S that aren’t in S1
S.intersection(S1)	Returns the set of all items of S that are also in S1
S.issubset(S1)	Returns True if all items of S are also in S1; otherwise, returns False
S.issuperset(S1)	Returns True if all items of S1 are also in S; otherwise, ...