7.3 Type Categories Revisited

Now that we've seen all of Python's core built-in types, let's take a look at some of the properties they share.

Table 7-3 classifies all the types we've seen, according to the type categories we introduced earlier. Objects share operations according to their category—for instance, strings, lists, and tuples all share sequence operations. Only mutable objects may be changed in-place. You can change lists and dictionaries in-place, but not numbers, strings, or tuples. Files only export methods, so mutability doesn't really apply (they may be changed when written, but this isn't the same as Python type constraints).

Table 7-3. Object classifications

Object type	Category	Mutable?
Numbers	Numeric	No
Strings	Sequence	No
Lists	Sequence	Yes
Dictionaries	Mapping	Yes
Tuples	Sequence	No
Files	Extension	n/a

Why You Will Care: Operator Overloading Later, we'll see that objects we implement with classes can pick and choose from these categories arbitrarily. For instance, if you want to provide a new kind of specialized sequence object that is consistent with built-in sequences, code a class that overloads things like indexing and concatenation: class MySequence: def __getitem__(self, index): # Called on self[index], others def __add__(self, other): # Called on self + other and so on. You can also make the new object mutable or not, by selectively implementing methods called for in-place change operations (e.g., __setitem__ is called on self[index]=value assignments). It's also possible to implement new objects in C, as C extension types. For these, you fill in C function pointer slots to choose between number, sequence, and mapping operation sets.