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Shared References
So far, we’ve seen what happens as a single variable is assigned references to objects. Now let’s introduce another variable into our interaction and watch what happens to its names and objects:
>>> a = 3
>>> b = a
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Typing these two statements generates the scene captured in Figure 6-2. The second line causes Python to create the variable b; the variable a is being used and not assigned here, so it is replaced with the object it references (3), and b is made to reference that object. The net effect is that the variables a and b wind up referencing the same object (that is, pointing to the same chunk of memory). This scenario, with multiple names referencing the same object, is called a shared reference in Python.
Figure 6-2. Names and objects after next running the assignment b = a. Variable b 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.
Next, suppose we extend the session with one more statement:
>>> a = 3
>>> b = a
>>> a = 'spam'
As with all Python assignments, this statement simply makes a new object to represent the string value 'spam' and sets a to reference this new object. It does not, however, change the value of b; b still references the original object, the integer 3. The resulting reference structure is shown in Figure 6-3.
Figure 6-3. Names and objects after finally running the assignment a = ‘spam’. Variable a references the new object (i.e., piece of memory) created by running the literal expression ‘spam’, but variable b still refers to the original object 3. Because this assignment is not an in-place change to the object 3, it changes only variable a, not b.
The same sort of thing would happen if we changed b to 'spam' instead—the assignment would change only b, not a. This behavior also occurs if there are no type differences at all. For example, consider these three statements:
>>> a = 3
>>> b = a
>>> a = a + 2
In this sequence, the same events transpire. Python makes the variable a reference the object 3 and makes b reference the same object as a, as in Figure 6-2; as before, the last assignment then sets a to a completely different object (in this case, the integer 5, which is the result of the + expression). It does not change b as a side effect. In fact, there is no way to ever overwrite the value of the object 3—as introduced in Chapter 4, integers are immutable and thus can never be changed in-place.
One way to think of this is that, unlike in some languages, in Python variables are always pointers to objects, not labels of changeable memory areas: setting a variable to a new value does not alter the original object, but rather causes the variable to reference an entirely different object. The net effect is that assignment to a variable can impact only the single variable being assigned. When mutable objects and in-place changes enter the equation, though, the picture changes somewhat; to see how, let’s move on.
