Correct Type annotation for __init__
Question:
What is the correct type annotation for a __init__ function in python?
class MyClass:
...
Which of the following would make more sense?
def __init__(self):
# type: (None) -> None
def __init__(self):
# type: (MyClass) -> MyClass
def __init__(self):
# type: (None) -> MyClass
Since we would normally instantiate as myclass = MyClass(), but the __init__ function itself has no return value.
Answers:
self should be omitted from the annotation when it is given as a comment, and __init__() should be marked as -> None. This is all specified explicitly in PEP-0484.
If you’re using Python 3 (I hope you do), no need to annotate __init__ method at all since mypy 0.641 release if there’s at least one annotated argument, it must always be None, because it returns nothing. In other cases, mypy will raise an error. This behavior annoyed people for years but eventually it was fixed.
This is what Guido says:
New Feature: Allow Omitting Return Type for __init__ It is now
possible to omit the return type of an annotated __init__ method
without getting an error message. For example:
class Circuit:
def __init__(self, voltage: float):
self.voltage = voltage
In previous mypy versions this would elicit an error message:
error: The return type of "__init__" must be None
This error was just annoying as the only legal return declaration for
__init__ is -> None, so we’ve removed it. Note that this only works if there’s at least one annotated argument! For __init__ methods without
arguments you must still add -> None, otherwise the method will be
treated as untyped, and its body will not be type checked at all.
Examples:
class UntypedExample:
# This method is not type-checked at all!
def __init__(self):
self.voltage = 0.0
class TypedExample:
# This is how to ensure that a 0-argument __init__ is type-checked:
def __init__(self) -> None:
self.voltage = 0.0
Related discussions:
What is the correct type annotation for a __init__ function in python?
class MyClass:
...
Which of the following would make more sense?
def __init__(self):
# type: (None) -> None
def __init__(self):
# type: (MyClass) -> MyClass
def __init__(self):
# type: (None) -> MyClass
Since we would normally instantiate as myclass = MyClass(), but the __init__ function itself has no return value.
self should be omitted from the annotation when it is given as a comment, and __init__() should be marked as -> None. This is all specified explicitly in PEP-0484.
If you’re using Python 3 (I hope you do), no need to annotate __init__ method at all since mypy 0.641 release if there’s at least one annotated argument, it must always be None, because it returns nothing. In other cases, mypy will raise an error. This behavior annoyed people for years but eventually it was fixed.
This is what Guido says:
New Feature: Allow Omitting Return Type for
__init__It is now
possible to omit the return type of an annotated__init__method
without getting an error message. For example:class Circuit: def __init__(self, voltage: float): self.voltage = voltageIn previous mypy versions this would elicit an error message:
error: The return type of "__init__" must be NoneThis error was just annoying as the only legal return declaration for
__init__is -> None, so we’ve removed it. Note that this only works if there’s at least one annotated argument! For__init__methods without
arguments you must still add -> None, otherwise the method will be
treated as untyped, and its body will not be type checked at all.
Examples:class UntypedExample: # This method is not type-checked at all! def __init__(self): self.voltage = 0.0 class TypedExample: # This is how to ensure that a 0-argument __init__ is type-checked: def __init__(self) -> None: self.voltage = 0.0
Related discussions: