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4.5.2 Precise Type Checking

With the default compilation policy, all type assertions⁶ are precisely checked. Precise checking means that the check is done as though typep had been called with the exact type specifier that appeared in the declaration. Python uses policy to determine whether to trust type assertions (see compiler-policy). Type assertions from declarations are indistinguishable from the type assertions on arguments to built-in functions. In Python, adding type declarations makes code safer.

If a variable is declared to be (integer 3 17), then its value must always always be an integer between 3 and 17. If multiple type declarations apply to a single variable, then all the declarations must be correct; it is as though all the types were intersected producing a single and type specifier.

Argument type declarations are automatically enforced. If you declare the type of a function argument, a type check will be done when that function is called. In a function call, the called function does the argument type checking, which means that a more restrictive type assertion in the calling function (e.g., from the) may be lost.

The types of structure slots are also checked. The value of a structure slot must always be of the type indicated in any :type slot option.⁷ Because of precise type checking, the arguments to slot accessors are checked to be the correct type of structure.

In traditional Common Lisp compilers, not all type assertions are checked, and type checks are not precise. Traditional compilers blindly trust explicit type declarations, but may check the argument type assertions for built-in functions. Type checking is not precise, since the argument type checks will be for the most general type legal for that argument. In many systems, type declarations suppress what little type checking is being done, so adding type declarations makes code unsafe. This is a problem since it discourages writing type declarations during initial coding. In addition to being more error prone, adding type declarations during tuning also loses all the benefits of debugging with checked type assertions.

To gain maximum benefit from Python’s type checking, you should always declare the types of function arguments and structure slots as precisely as possible. This often involves the use of or, member and other list-style type specifiers. Paradoxically, even though adding type declarations introduces type checks, it usually reduces the overall amount of type checking. This is especially true for structure slot type declarations.

Python uses the safety optimization quality (rather than presence or absence of declarations) to choose one of three levels of run-time type error checking: see optimize-declaration. See advanced-type-stuff for more information about types in Python.