58

There is a ->, or dash-greater-than symbol at the end of a python method, and I'm not sure what it means. One might call it an arrow as well.

Here is the example:

@property
def get_foo(self) -> Foo:
    return self._foo

where self._foo is an instance of Foo.

My guess is that it is some kind of static type declaration, to tell the interpreter that self._foo is of type Foo. But when I tested this, if self._foo is not an instance of Foo, nothing unusual happens. Also, if self._foo is of a type other than Foo, let's say it was an int, then type(SomeClass.get_foo()) returns int. So, what's the point of -> Foo?

This concept is hard to lookup because it is a symbol without a common name, and the term "arrow" is misleading.

styvane
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modulitos
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1 Answers1

50

This is function annotations. It can be use to attach additional information to the arguments or a return values of functions. It is a useful way to say how a function must be used. Functions annotations are stored in a function's __annotations__ attribute.

Use Cases (From documentation)

  • Providing typing information

    • Type checking
    • Let IDEs show what types a function expects and returns
    • Function overloading / generic functions
    • Foreign-language bridges
    • Adaptation
    • Predicate logic functions
    • Database query mapping
    • RPC parameter marshaling

  • Other information

    • Documentation for parameters and return values

From python-3.5 it can be used for Type Hints

Captain Trojan
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styvane
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