Python - Function Annotations

 

def func(s:str, i:int, j:int) -> str:

return s[i:j]

The parameter s is supposed to be a string, so we place a colon after the parameter name and then write str. Parameters i and j are supposed to be integers so we write int for them.

Return type is specified in between the closing parentheses and the colon and it is preceded by an arrow. This function returns a value of type str, so we have written ->str between the closing parentheses and the colon.

We can write descriptive annotations, too. For example, if we want the user to send only values from 0 to 5 for the parameter i, then we can write the annotation in string form.

def func(s:str, i:'int 0 to 5', j:int) -> str:

return s[i:j]

Similarly, the annotation for the return type can also be descriptive. So, annotations not only document the expected types but also allow us to specify any type of metadata about the parameters and return type.

If there is default value for any parameter, then it is written after the annotation. In our function func, if we want default value for the last two parameters, we can write it like:

def func(s:str, i:'int 0 to 5'=0, j:int=3) -> str:

return s[i:j]

The function annotations are optional, and they are used just for documentation purpose, they do not enforce any type checking by the interpreter.

 

>>> func.__annotations__