Types (CMUCL User’s Manual)

Python’s support for types is unusual in three major ways:

Precise type checking encourages the specific use of type declarations as a form of run-time consistency checking. This speeds development by localizing type errors and giving more meaningful error messages. See precise-type-checks. Python produces completely safe code; optimized type checking maintains reasonable efficiency on conventional hardware (see type-check-optimization.)
Comprehensive support for the Common Lisp type system makes complex type specifiers useful. Using type specifiers such as or and member has both efficiency and robustness advantages. See advanced-type-stuff.
Type inference eliminates the need for some declarations, and also aids compile-time detection of type errors. Given detailed type declarations, type inference can often eliminate type checks and enable more efficient object representations and code sequences. Checking all types results in fewer type checks. See sections type-inference and non-descriptor.