python: type hinting

Programming_Languages/Python/Concepts/Type_hinting.md

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+---
+categories:
+  - Programming Languages
+tags: [python, types]
+---
+
+# Type hinting in Python
+
+With type hinting we can add type information to variables, functions, and classes. This is not enforced by the Python interpreter but can be used by external tools like `mypy` to check the code.
+
+## Basic syntax
+
+### Variables
+
+```py
+name: str = "John"
+age: int = 30
+```
+
+### Functions
+
+```py
+def greet(name: str) -> str:
+    return "Hello, " + name
+```
+
+### Complex data types
+
+// TODO: Give examples of lists, tuples, dictionaries, sets
+
+### Classes
+
+// TODO: Add examples
+
+### Optional types
+
+`Optional` can be used to indicate that a variable can be `None` or the specified type.
+
+```py
+from typing import Optional
+
+def find_index(numbers: List[int], target: int) -> Optional[int]:
+    try:
+        return numbers.index(target)
+    except ValueError:
+        return None
+```
+
+The function above returns an `int` or `None`.
+
+### Union types
+
+`Union` can be used to indicate that a variable can be one of several types.
+
+```py
+from typing import Union
+
+def add(a: Union[int, float], b: Union[int, float]) -> Union[int, float]:
+    return a + b
+```
+
+The function above returns an `int` or `float`.
+
+> Note that in the case of `Optional` and `Union` the type is specified using square brackets. This is a syntactic covnention and does not denote that the return type is a list. Below is an example of how we would use type hinting to indicate that the return type actually is a list:
+
+```py
+from typing import List, Union
+def my_function() -> List[int]:
+    return [1, 2, 3]
+
+# And to show a return type of a list of integers or floats:
+
+def my_function() -> List[Union[int, float]]:
+    return [1, 2, 3]
+```