Class name forward references

Python does not allow references to a class object before the class isdefined. Thus this code does not work as expected:

def f(x: A) -> None:  # Error: Name A not defined
    ...
 
class A:
    ...

In cases like these you can enter the type as a string literal — thisis a forward reference:

def f(x: 'A') -> None:  # OK
    ...
 
class A:
    ...

Of course, instead of using a string literal type, you could move thefunction definition after the class definition. This is not alwaysdesirable or even possible, though.

Any type can be entered as a string literal, and you can combinestring-literal types with non-string-literal types freely:

def f(a: List['A']) -> None: ...  # OK
def g(n: 'int') -> None: ...      # OK, though not useful
 
class A: pass

String literal types are never needed in # type: comments.

String literal types must be defined (or imported) later in the samemodule. They cannot be used to leave cross-module referencesunresolved. (For dealing with import cycles, seeImport cycles.)