Learning Python - Mark Lutz [231]
Scope Example
Let’s look at a larger example that demonstrates scope ideas. Suppose we wrote the following code in a module file:
# Global scope
X = 99 # X and func assigned in module: global
def func(Y): # Y and Z assigned in function: locals
# Local scope
Z = X + Y # X is a global
return Z
func(1) # func in module: result=100
This module and the function it contains use a number of names to do their business. Using Python’s scope rules, we can classify the names as follows:
Global names: X, func
X is global because it’s assigned at the top level of the module file; it can be referenced inside the function without being declared global. func is global for the same reason; the def statement assigns a function object to the name func at the top level of the module.
Local names: Y, Z
Y and Z are local to the function (and exist only while the function runs) because they are both assigned values in the function definition: Z by virtue of the = statement, and Y because arguments are always passed by assignment.
The whole point behind this name-segregation scheme is that local variables serve as temporary names that you need only while a function is running. For instance, in the preceding example, the argument Y and the addition result Z exist only inside the function; these names don’t interfere with the enclosing module’s namespace (or any other function, for that matter).
The local/global distinction also makes functions easier to understand, as most of the names a function uses appear in the function itself, not at some arbitrary place in a module. Also, because you can be sure that local names will not be changed by some remote function in your program, they tend to make programs easier to debug and modify.
The Built-in Scope
We’ve been talking about the built-in scope in the abstract, but it’s a bit simpler than you may think. Really, the built-in scope is just a built-in module called builtins, but you have to import builtins to query built-ins because the name builtins is not itself built-in....
No, I’m serious! The built-in scope is implemented as a standard library module named builtins, but that name itself is not placed in the built-in scope, so you have to import it in order to inspect it. Once you do, you can run a dir call to see which names are predefined. In Python 3.0:
>>> import builtins
>>> dir(builtins)
['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException',
'BufferError', 'BytesWarning', 'DeprecationWarning', 'EOFError', 'Ellipsis',
...many more names omitted...
'print', 'property', 'quit', 'range', 'repr', 'reversed', 'round', 'set',
'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple',
'type', 'vars', 'zip']
The names in this list constitute the built-in scope in Python; roughly the first half are built-in exceptions, and the second half are built-in functions. Also in this list are the special names None, True, and False, though they are treated as reserved words. Because Python automatically searches this module last in its LEGB lookup, you get all the names in this list “for free;” that is, you can use them without importing any modules. Thus, there are really two ways to refer to a built-in function—by taking advantage of the LEGB rule, or by manually importing the builtins module:
>>> zip # The normal way
>>> import builtins # The hard way >>> builtins.zip The second of these approaches is sometimes useful in advanced work. The careful reader might also notice that because the LEGB lookup procedure takes the first occurrence of a name that it finds, names in the local scope may override variables of the same name in both the global and built-in scopes, and global names may override built-ins. A function can, for instance, create a local variable called open by