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5.2.1 More Types Meaningful

Common Lisp has a very powerful type system, but conventional Common Lisp implementations typically only recognize the small set of types special in that implementation. In these systems, there is an unfortunate paradox: a declaration for a relatively general type like fixnum will be recognized by the compiler, but a highly specific declaration such as (integer 3 17) is totally ignored.

This is obviously a problem, since the user has to know how to specify the type of an object in the way the compiler wants it. A very minimal (but rarely satisfied) criterion for type system support is that it be no worse to make a specific declaration than to make a general one. Python goes beyond this by exploiting a number of advantages obtained from detailed type information.

Using more restrictive types in declarations allows the compiler to do better type inference and more compile-time type checking. Also, when type declarations are considered to be consistency assertions that should be verified (conditional on policy), then complex types are useful for making more detailed assertions.

Python “understands” the list-style or, member, function, array and number type specifiers. Understanding means that:

• If the type contains more information than is used in a particular context, then the extra information is simply ignored, rather than derailing type inference.
• In many contexts, the extra information from these type specifier is used to good effect. In particular, type checking in Python is precise, so these complex types can be used in declarations to make interesting assertions about functions and data structures (see precise-type-checks.) More specific declarations also aid type inference and reduce the cost for type checking.

For related information, see numeric-types for numeric types, and section array-types for array types.