Type Categories Revisited

Now that we’ve seen all of Python’s core built-in types in action, let’s wrap up our object types tour by reviewing some of the properties they share. Table 9-3 classifies all the major types we’ve seen so far according to the type categories introduced earlier. Here are some points to remember:

 

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  • Objects share operations according to their category; for instance, strings, lists, and tuples all share sequence operations such as concatenation, length, and indexing.
  • Only mutable objects (lists, dictionaries, and sets) may be changed in-place; you cannot change numbers, strings, or tuples in-place.
  • Files export only methods, so mutability doesn’t really apply to them—their state may be changed when they are processed, but this isn’t quite the same as Python core type mutability constraints.
  • “Numbers” in Table 9-3 includes all number types: integer (and the distinct long integer in 2.6), floating-point, complex, decimal, and fraction.
  • “Strings” in Table 9-3 includes str, as well as bytes in 3.0 and unicode in 2.6; the bytearray string type in 3.0 is mutable.
  • Sets are something like the keys of a valueless dictionary, but they don’t map to values and are not ordered, so sets are neither a mapping nor a sequence type; frozenset is an immutable variant of set.
  • In addition to type category operations, as of Python 2.6 and 3.0 all the types in Table 9-3 have callable methods, which are generally specific to their type.

Table 9-3. Object classifications

Object type

Category

Mutable?

Numbers (all)

Numeric

No

Strings

Sequence

No

Lists

Sequence

Yes

Dictionaries

Mapping

Yes

Tuples

Sequence

No

Files

Extension

N/A

Sets

Set

Yes

frozenset

Set

No

bytearray (3.0)

Sequence

Yes

Why You Will Care: Operator Overloading

In Part VI of this book, we’ll see that objects we implement with classes can pick and choose from these categories arbitrarily. For instance, if we want to provide a new kind of specialized sequence object that is consistent with built-in sequences, we can 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. We 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). Although it’s beyond this book’s scope, it’s also possible to implement new objects in an external language like C as C extension types. For these, we fill in C function pointer slots to choose between number, sequence, and mapping operation sets.