第169页 | Learning Python | 阅读 ‧ 电子书库

Types Live with Objects, Not Variables

To see how object types come into play, watch what happens if we assign a variable multiple times:

>>> a = 3             # It's an integer
>>> a = 'spam'        # Now it's a string
>>> a = 1.23          # Now it's a floating point

This isn’t typical Python code, but it does work—a starts out as an integer, then becomes a string, and finally becomes a floating-point number. This example tends to look especially odd to ex-C programmers, as it appears as though the type of a changes from integer to string when we say a = 'spam'.

However, that’s not really what’s happening. In Python, things work more simply. Names have no types; as stated earlier, types live with objects, not names. In the preceding listing, we’ve simply changed a to reference different objects. Because variables have no type, we haven’t actually changed the type of the variable a; we’ve simply made the variable reference a different type of object. In fact, again, all we can ever say about a variable in Python is that it references a particular object at a particular point in time.

Objects, on the other hand, know what type they are—each object contains a header field that tags the object with its type. The integer object 3, for example, will contain the value 3, plus a designator that tells Python that the object is an integer (strictly speaking, a pointer to an object called int, the name of the integer type). The type designator of the 'spam' string object points to the string type (called str) instead. Because objects know their types, variables don’t have to.

To recap, types are associated with objects in Python, not with variables. In typical code, a given variable usually will reference just one kind of object. Because this isn’t a requirement, though, you’ll find that Python code tends to be much more flexible than you may be accustomed to—if you use Python well, your code might work on many types automatically.

I mentioned that objects have two header fields, a type designator and a reference counter. To understand the latter of these, we need to move on and take a brief look at what happens at the end of an object’s life.