Type Hints
See: PEP 484 - Python Type Hints
- Snippet from Wikipedia: Type system
A programming language consists of a system of allowed sequences of symbols (constructs) together with rules that define how each construct is interpreted. For example, a language might allow expressions representing various types of data, expressions that provide structuring rules for data, expressions representing various operations on data, and constructs that provide sequencing rules for the order in which to perform operations.
A simple type system for a programming language is a set of rules that associates a data type (for example, integer, floating point, string) with each term (data-valued expression) in a computer program. In more ambitious type systems, a variety of constructs, such as variables, expressions, functions, and modules, may be assigned types.
Type systems formalize and enforce the otherwise implicit categories the programmer uses for algebraic data types, data structures, or other data types, such as "string", "array of float", "function returning boolean".
The main purpose of a type system in a programming language is to reduce possibilities for bugs in computer programs due to mismatches in how values are interpreted in different parts of a program. The aim is to prevent operations expecting a certain kind of value from being applied to values for which that operation does not make sense (validity errors). A type system can detect and prevent some of these mismatches. When a type mismatch is detected, it is called a type error.
The type of a term constrains the contexts in which it may be used. For a variable, the type system determines the allowed values of that variable. For that variable be presented as a parameter to an operation, the operation must be able to accept in that parameter any value that the type of the variable allows.
Type systems are typically specified as part of programming language design. They are built into interpreters and compilers for the language. In some languages, the type system can be extended by optional tools that perform added checks using the language's original type syntax and grammar.
Type systems allow defining interfaces between different parts of a computer program, and then checking that the parts have been connected in a consistent way. This checking can happen statically (at compile time), dynamically (at run time), or as a combination of both.
Type systems have other purposes as well, such as expressing business rules, enabling certain compiler optimizations, allowing for multiple dispatch, and providing a form of documentation.