Learning Python

General Object Properties

Now that we’ve seen all of Python’s built-in types, let’s take a quick look at some of the properties they share. Some of this section is a review of ideas we’ve already seen at work.

Type Categories Revisited

Table 2.11 classifies all the types we’ve seen, according to the type categories we introduced earlier. As we’ve seen, objects share operations according to their category—for instance, strings, lists, and tuples all share sequence operations. As we’ve also seen, only mutable objects may be changed in place. You can change lists and dictionaries in place, but not numbers, strings, or tuples.^[17] 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 2-11. 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 ourselves 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, slicing, and concatenation:

class MySequence: def __getitem__(self, index): # called on self[index], for x in self, x in self def __getslice__(self, low, high): # called on self[low:high] ...