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Variables, Objects, and References
As you’ve seen in many of the examples used so far in this book, when you run an assignment statement such as a = 3 in Python, it works even if you’ve never told Python to use the name a as a variable, or that a should stand for an integer-type object. In the Python language, this all pans out in a very natural way, as follows:
Variable creation
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A variable (i.e., name), like a, is created when your code first assigns it a value. Future assignments change the value of the already created name. Technically, Python detects some names before your code runs, but you can think of it as though initial assignments make variables.
Variable types
A variable never has any type information or constraints associated with it. The notion of type lives with objects, not names. Variables are generic in nature; they always simply refer to a particular object at a particular point in time.
Variable use
When a variable appears in an expression, it is immediately replaced with the object that it currently refers to, whatever that may be. Further, all variables must be explicitly assigned before they can be used; referencing unassigned variables results in errors.
In sum, variables are created when assigned, can reference any type of object, and must be assigned before they are referenced. This means that you never need to declare names used by your script, but you must initialize names before you can update them; counters, for example, must be initialized to zero before you can add to them.
This dynamic typing model is strikingly different from the typing model of traditional languages. When you are first starting out, the model is usually easier to understand if you keep clear the distinction between names and objects. For example, when we say this:
>>> a = 3
at least conceptually, Python will perform three distinct steps to carry out the request. These steps reflect the operation of all assignments in the Python language:
- Create an object to represent the value 3.
- Create the variable a, if it does not yet exist.
- Link the variable a to the new object 3.
The net result will be a structure inside Python that resembles Figure 6-1. As sketched, variables and objects are stored in different parts of memory and are associated by links (the link is shown as a pointer in the figure). Variables always link to objects and never to other variables, but larger objects may link to other objects (for instance, a list object has links to the objects it contains).
Figure 6-1. Names and objects after running the assignment a = 3. Variable a becomes a reference to the object 3. Internally, the variable is really a pointer to the object’s memory space created by running the literal expression 3.
These links from variables to objects are called references in Python—that is, a reference is a kind of association, implemented as a pointer in memory.[16] Whenever the variables are later used (i.e., referenced), Python automatically follows the variable-to-object links. This is all simpler than the terminology may imply. In concrete terms:
- Variables are entries in a system table, with spaces for links to objects.
- Objects are pieces of allocated memory, with enough space to represent the values for which they stand.
- References are automatically followed pointers from variables to objects.
At least conceptually, each time you generate a new value in your script by running an expression, Python creates a new object (i.e., a chunk of memory) to represent that value. Internally, as an optimization, Python caches and reuses certain kinds of unchangeable objects, such as small integers and strings (each 0 is not really a new piece of memory—more on this caching behavior later). But, from a logical perspective, it works as though each expression’s result value is a distinct object and each object is a distinct piece of memory.
Technically speaking, objects have more structure than just enough space to represent their values. Each object also has two standard header fields: a type designator used to mark the type of the object, and a reference counter used to determine when it’s OK to reclaim the object. To understand how these two header fields factor into the model, we need to move on.
