General support for properties

[JukkaL]

Add support for type checking properties.

We should initially support at least the decorator syntax for read-only properties and the assignment syntax for both read-only and read-write properties, as these seem to be the most frequently used variants:

class A:
    @property
    def foo(self) -> int: return 2

    def get_bar(self) -> int: return 3
    def set_bar(self, value: int) -> None: ...
    bar = property(get_bar, set_bar)

Also support defining a deleter and a docstring, at least when using the call syntax.

[JukkaL]

Generic descriptor support would also let us support properties, but the distinction between read-only and settable properties could be lost. A property would have type property[ObjT, ValT], where ObjT is the class that defines the property and ValT is the type of the property value.

In order to properly model settable properties, we should have something like property[ObjT, GetT, SetT], and allow SetT to be undefined. However, there is currently no way of letting the value of a type argument be undefined. We could also generally support default values for type variables, and undefined would a valid default value.

[JukkaL]

Read-only properties using @property work, but not the other forms.

[thomascellerier]

Any progress on this? I am using mypy to check some new production (yes i know :)) python daemon and it worked fine until I started using properties. Do you know of any workaround? Even using the old properties notation i get issues with setters.
A property cannot be overloaded
'overload' decorator expected

Thanks for this great tool.

[gvanrossum]

I'm guessing you're talking about an example like this:

class C:
    def __init__(self):
        self._x = None

    @property
    def x(self):
        """I'm the 'x' property."""
        return self._x

    @x.setter
    def x(self, value):
        self._x = value

    @x.deleter
    def x(self):
        del self._x

Indeed mypy goes crazy over this:

a.py, line 5: A property cannot be overloaded
a.py, line 5: 'overload' decorator expected
a.py, line 10: Name 'x' is not defined
a.py, line 10: 'overload' decorator expected
a.py, line 14: Name 'x' is not defined
a.py, line 14: 'overload' decorator expected

[JukkaL]

Yeah, this hasn't been implemented yet. It's been on my to-do list for a long time, but since the code I've worked with haven't used setters much I've never gotten around to actually implementing this. The implementation should not be very difficult, though.

I'll try to get this working reasonably soon (though probably only after PyCon, i.e. after April 15th).

[JukkaL]

I just pushed experimental setter support. The following code now works as expected:

class C:
    def __init__(self) -> None:
        self._x = 0

    @property
    def x(self) -> int:
        return self._x

    @x.setter
    def x(self, value: int) -> None:
        self._x = value

    @x.deleter
    def x(self) -> None:
        del self._x

c = C()
c.x = 2 * c.x
c.x = ''   # Error!

The implementation still needs refinement. For example, x = property(...) is not supported.

Let me know if/when you encounter additional issues.

[thomascellerier]

Thanks, the file containing the properties now typechecks.
However I have another file in the same module assigning the property to a variable which fails to typecheck with this error (given a property called x with a getter and setter):
a.py

class C:
    @property
    def x(self) -> Dict[str, Any]:
        return self._x

    @x.setter
    def x(self, value: Dict[str, Any]) -> None:
        self._x = value

b.py

def foo(c: a.C):
    x = c.x

Cannot determine type of 'x'.

[JukkaL]

Do you have cyclic imports? Mypy has trouble with them (#481) in general. However, it should be pretty easy to fix mypy to support your example -- assuming that this actually is due to cyclic imports.

[JukkaL]

This still doesn't work (reported by Tim Abbott):

class C:
    def __init__(self) -> None:
        self._x = 0

    @property
    def x(self) -> int:
        return self._x

    @x.setter
    def set_x(self, value: int) -> None:   # Note the name of the method! "def x" works.
        self._x = value

[JukkaL]

The last example seems to be only valid in Python 2, not Python 3.

[Deimos]

Here's some more examples of strange property-related behavior:

This code produces a mypy error as it should (the passwordconstructor argument was "accidentally" annotated as an int, but the property setter expects a str value):

class User:
    @property
    def password(self) -> str:
        raise AttributeError('Password is write-only')

    @property.setter
    def password(self, value: str) -> None:
        # TODO: add hashing
        self.password_hash = value

    def __init__(self, username: str, password: int) -> None:
        self.username = username
        self.password = password

Running mypy gives mypy_test.py:13: error: Incompatible types in assignment (expression has type "int", variable has type "str") as expected.

However, if I just rearrange the order of the methods so that the constructor is first and the property methods come after it, there's no error produced any more:

class User:
    def __init__(self, username: str, password: int) -> None:
        self.username = username
        self.password = password

    @property
    def password(self) -> str:
        raise AttributeError('Password is write-only')

    @property.setter
    def password(self, value: str) -> None:
        # TODO: add hashing
        self.password_hash = value

Running mypy on that code has no output.

In addition, even the "more correct" behavior from the original code was only because the property was annotated incorrectly. Because attempting to read .password always raises an error, the correct way to write this code should have been (annotation on the password constructor argument "fixed" to str as well):

from mypy_extensions import NoReturn

class User:
    @property
    def password(self) -> NoReturn:
        raise AttributeError('Password is write-only')

    @property.setter
    def password(self, value: str) -> None:
        # TODO: add hashing
        self.password_hash = value

    def __init__(self, username: str, password: str) -> None:
        self.username = username
        self.password = password

However, even though this code should now be correct, mypy produces mypy_test.py:15: error: Incompatible types in assignment (expression has type "str", variable has type NoReturn).

So it seems like mypy is using the return value of the "getter" to determine the value the "setter" can accept, but that won't necessarily always match up.

[gvanrossum]

Hm, I think we ran into a similar issue (about potential discrepancies between getter and setter) for descriptors (#2266).

[funkyfuture]

edit: previously contained #3011

[gvanrossum]

it'd be nice to support abc.abstractproperty as well.

Should already be supported, see #263.

[gvanrossum]

@funkyfuture Please file a separate issue.

[douglas-treadwell]

@JukkaL

So I've seen this commit a103f25 that closed #263. It seems to check for exactly "property" or "abc.abstractproperty" for property support.

Is there a way to support non-stdlib property decorators such as SqlAlchemy's hybrid_property, which has getter, setter, deleter, as well as expression and comparator?

I'd rather not need to litter my code with # type: ignore.

[douglas-treadwell]

Also, I'd be happy to fix this myself if someone @JukkaL can give me enough context to do so in less time than it'd take just to fix it themselves.

[ilevkivskyi]

Another thing that doesn't work came in #6045: accessing attributes of a property object in the class body (for example fset, fget etc). This can be probably partially solved by some special casing in checkmember.py.

[Deimos]

In my comment above from March 9, 2017, I noted that putting __init__ before property getter/setters caused mypy to miss a (false-positive) error that it otherwise reported.

I don't know if it's useful information at all, but I just wanted to point out that the new semantic analyzer no longer has that particular issue, and it will report the same error (which isn't actually one) regardless of whether __init__ is at the top or bottom of the class.

Specifically, here's the code:

from mypy_extensions import NoReturn

class User:
    def __init__(self, username: str, password: str) -> None:
        self.username = username
        self.password = password

    @property
    def password(self) -> NoReturn:
        raise AttributeError('Password is write-only')

    @property.setter
    def password(self, value: str) -> None:
        # TODO: add hashing
        self.password_hash = value

And the output from both analyzers:

$ mypy test_mypy.py
$ mypy --new-semantic-analyzer test_mypy.py
test_mypy.py:6: error: Incompatible types in assignment (expression has type "str", variable has type "NoReturn")

[gonzaloamadio]

I have this file

import operator
from functools import total_ordering, wraps

def _time_required(f):
    @wraps(f)
    def wrapper(self, other):
        if not isinstance(other, Time):
            raise TypeError("Can only operate on Time objects.")
        return f(self, other)
    return wrapper
def _int_required(f):
    @wraps(f)
    def wrapper(self, value):
        if not isinstance(value, int):
            raise TypeError("An integer is required.")
        return f(self, value)
    return wrapper
def _balance(a, b):
    if b >= 0:
        while b >= 60:
            a += 1
            b -= 60
    elif b < 0:
        while b < 0:
            a -= 1
            b += 60
    return a, b

@total_ordering
class Time:

    def __init__(self, h: int = 0, m: int = 0, s: int = 0):
        self._h = h
        self._m = m
        self._s = s

    @property
    def h(self):
        return self._h

    @h.setter
    @_int_required
    def h(self, value):
        self._h = value

    @property
    def m(self):
        return self._m

    @m.setter
    @_int_required
    def m(self, value):
        self._m = value
        self._h, self._m = _balance(self._h, self._m)

    @property
    def s(self):
        return self._s

    @s.setter
    @_int_required
    def s(self, value):
        self._s = value
        self._m, self._s = _balance(self._m, self._s)

    def _operation(self, other, method):
        h = method(self.h, other.h)
        m = method(self.m, other.m)
        s = method(self.s, other.s)
        return Time(h, m, s)

    @_time_required
    def __add__(self, other):
        return self._operation(other, operator.add)

    @_time_required
    def __sub__(self, other):
        return self._operation(other, operator.sub)

    @_time_required
    def __eq__(self, other):
        return (self.h == other.h and
                self.m == other.m and
                self.s == other.s)

    @_time_required
    def __lt__(self, other):
        if self.h < other.h:
            return True
        if self.h > other.h:
            return False
        if self.m < other.m:
            return True
        if self.m > other.m:
            return False
        return self.s < other.s

    def __repr__(self):
        return f"<Time {self.h:02}:{self.m:02}:{self.s:02}>"

if __name__ == '__main__':
    a = Time(14, 23, 10)
    try:
        a.h = "Hello, world!"
    except Exception as err:
        print(err)

And when I run mypy, I have the following errors.

class.py:47: error: Decorated property not supported     **<-- Line @h.setter**
class.py:56: error: Decorated property not supported     **<-- Line @m.settet**
class.py:66: error: Decorated property not supported     **<-- Line @s.setter**
class.py:110: error: Property "h" defined in "Time" is read-only  **<-- Line:  a.h = "Hello, world!"**

If I comment out the decorators under the setters, mypy check runs OK. No errors

[gvanrossum]

@gonzaloamadio Thanks for the report. I can't help you with a quick fix. The problem is that property (both get and set functionality) is special-cased in mypy, and currently doesn't support additional decorators at all. Sorry!

[ZahlGraf]

@gvanrossum I have the same issue. In my project I use additional decorators for properties very often. Thus mypy sends me hundreds of error with "error: Decorated property not supported".

First of all, I don't understand the error message. Does it mean mypy does not support decorated properties, or is decorating a property related to any code quality issue in general? Because if just mypy is not supporting decorated properties, but the pattern in general is not considered as bad practice, I don't know, why mypy is giving me an error here. It should be at maximum a warning right?

Second, if it is just a mypy issue, I really want to ignore it. Is there any way to ignore exactly this error on a global level? At the moment, I'm only aware of ignoring type errors for a single line, but since this pattern is extremely common in my code, I don't want to add the "ignore line" comment on hundreds of lines in my code.

Thanks in advance for any suggestions and assistance.

[JukkaL]

Decorated property has a separate issue: #1362

[JukkaL]

I'm closing as this covers several related issues, some of which have already been fixed, and it's hard to keep track of what's still open. I created separate issues for the remaining problems. Feel free to create additional issues if I missed something.