Show the type of the return value in "hash"

[Locked-chess-official]

Feature or enhancement

Proposal:

Sometimes showing the type of the result can be helpful to debug:

class A:
    def hash(self):
        return exit
a = A()
m= {}
m[a] = 1

The new message is : "cannot use 'A' as a dict key (__hash__ method should return an integer, returned 'Quitter')"

Sometimes there may be one or more paths that __hash__ doesn't return an integer, show what was returned can be helpful to find the mistake.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

[skirpichev]

The new message is : "cannot use 'A' as a dict key (hash method should return an integer, returned 'Quitter')"

I don't see anything in your above example: A.__hash__ == object.__hash__ in your code.

I would suspect, reproducer will be like that:

>>> class B:
...     def __hash__(self):
...         return "exit"
...         
>>> b = B()
>>> m[b]
Traceback (most recent call last):
  File "<python-input-10>", line 1, in <module>
    m[b]
    ~^^^
TypeError: cannot use 'B' as a dict key (__hash__ method should return an integer)

The message looks for me simple and clear. Why do you think, that actual returned type will be helpful here? No matter which one it will be, if not an integer.

[Locked-chess-official]

The new message is : "cannot use 'A' as a dict key (hash method should return an integer, returned 'Quitter')"

I don't see anything in your above example: A.__hash__ == object.__hash__ in your code.

I would suspect, reproducer will be like that:

>>> class B:
...     def __hash__(self):
...         return "exit"
...         
>>> b = B()
>>> m[b]
Traceback (most recent call last):
  File "<python-input-10>", line 1, in <module>
    m[b]
    ~^^^
TypeError: cannot use 'B' as a dict key (__hash__ method should return an integer)

The message looks for me simple and clear. Why do you think, that actual returned type will be helpful here? No matter which one it will be, if not an integer.

In some code, the magic method's codes are very complete. In that case, if simple message will make me confused: Why it returnd another? What is returned? Show the type can make it clear:

class A:
    def __hash__(self):
        if condition1:
            #lots of codes
            return value1
        elif conditiob2:
            #lots of codes
            return value2
        # lots of 'elif' blocks
        else:
            return final value

[skirpichev]

In some code, the magic method's codes are very complete.

What you meant? Long? Complex?

In that case, if simple message will make me confused:

How? Any return statement in your hash dunder must return an integer.

I think that returned type will be mostly useless on debug. You want exact line, where problem return statement is, right? Then why not add few asserts in your code?

[Locked-chess-official]

In some code, the magic method's codes are very complete.

What you meant? Long? Complex?

In that case, if simple message will make me confused:

How? Any return statement in your hash dunder must return an integer.

I think that returned type will be mostly useless on debug. You want exact line, where problem return statement is, right? Then why not add few asserts in your code?

However, in the project, maybe you don't know who wrote this shit code and you are afraid that the code what you add lead to a crash and you will be dismissed by your boss. Sometimes even delete or add a note can make it.

[skirpichev]

Why do you think, that actual returned type will be helpful here?

Show and example. Talk is cheap.

[Locked-chess-official]

For example, If the messages said that returned 'str', I can locate the variable that type-hint is 'str' and found that when the value wrongly change to 'str' in the code. Or I will need to check "all of the code in hash" and make more mistakes.

The combine with the result and the hint is helpful in the large project.

class ComplexClass:
    def __init__(self, *args, **kwargs): #In real code it is too many var
        self.attr1: int=args[0]
        self.attr2: str=args[1]
        ...

Now the hash function use two custom prime number and use the attr to multiply. However, the attr wrote wrong: it type-hint is 'str' so it changes to repeat the string many times. The tips "returned 'str'" is helpful to locate which line the calculation went with wrong way.

[skirpichev]

Sorry, I'm not impressed by this example. If your IDE could infer variable types - you can just check return statements to see which one has a wrong type signature.

Or I will need to check "all of the code in hash"

That's a good idea. If you fixed one place where return type is wrong - maybe somewhere in the other branch it's wrong as well.