Learning Python - Mark Lutz [173]
Besides being of mixed case, the first three entries in Table 11-3, True, False, and None, are somewhat unusual in meaning—they also appear in the built-in scope of Python described in Chapter 17, and they are technically names assigned to objects. They are truly reserved in all other senses, though, and cannot be used for any other purpose in your script other than that of the objects they represent. All the other reserved words are hardwired into Python’s syntax and can appear only in the specific contexts for which they are intended.
Furthermore, because module names in import statements become variables in your scripts, variable name constraints extend to your module filenames too. For instance, you can code files called and.py and my-code.py and run them as top-level scripts, but you cannot import them: their names without the “.py” extension become variables in your code and so must follow all the variable rules just outlined. Reserved words are off-limits, and dashes won’t work, though underscores will. We’ll revisit this idea in Part V of this book.
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Python’s Deprecation Protocol
It is interesting to note how reserved word changes are gradually phased into the language. When a new feature might break existing code, Python normally makes it an option and begins issuing “deprecation” warnings one or more releases before the feature is officially enabled. The idea is that you should have ample time to notice the warnings and update your code before migrating to the new release. This is not true for major new releases like 3.0 (which breaks existing code freely), but it is generally true in other cases.
For example, yield was an optional extension in Python 2.2, but is a standard keyword as of 2.3. It is used in conjunction with generator functions. This was one of a small handful of instances where Python broke with backward compatibility. Still, yield was phased in over time: it began generating deprecation warnings in 2.2 and was not enabled until 2.3.
Similarly, in Python 2.6, the words with and as become new reserved words for use in context managers (a newer form of exception handling). These two words are not reserved in 2.5, unless the context manager feature is turned on manually with a from__future__import (discussed later in this book). When used in 2.5, with and as generate warnings about the upcoming change—except in the version of IDLE in Python 2.5, which appears to have enabled this feature for you (that is, using these words as variable names does generate errors in 2.5, but only in its version of the IDLE GUI).
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Naming conventions
Besides these rules, there is also a set of naming conventions—rules that are not required but are followed in normal practice. For instance, because names with two leading and trailing underscores (e.g., __name__) generally have special meaning to the Python interpreter, you should avoid this pattern for your own names. Here is a list of the conventions Python follows:
Names that begin with a single underscore (_X) are not imported by a from module import * statement (described in Chapter 22).
Names that have two leading and trailing underscores (__X__) are system-defined names that have special meaning to the interpreter.
Names that begin with two underscores and do not end with two more (__X) are localized (“mangled”) to enclosing classes (see the discussion of pseudoprivate attributes in Chapter 30).
The name that is just a single underscore (_) retains the result of the last expression when working interactively.
In addition to these Python interpreter conventions, there are various other conventions that Python programmers usually follow. For instance, later in the book we’ll see that class names commonly start with an uppercase letter and module names with a lowercase letter, and that the name self, though not reserved, usually has a special role in classes. In Chapter 17 we’ll also