[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan  added the comment:

The revised behaviour now makes the error messages consistent with each other:

>>> class TooManyArgs():
... def __new__(cls):
... super().__new__(cls, 1)
... 
>>> TooManyArgs()
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 3, in __new__
TypeError: object.__new__() takes exactly one argument (the type to instantiate)
>>> class NotEnoughArgs():
... def __new__(cls):
... super().__new__()
... 
>>> NotEnoughArgs()
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 3, in __new__
TypeError: object.__new__(): not enough arguments
>>> class TooManyInitArgs():
... def __init__(self):
... super().__init__(1, 2, 3)
... 
>>> TooManyInitArgs()
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 3, in __init__
TypeError: object.__init__() takes exactly one argument (the instance to 
initialize)
>>> class NotEnoughInitArgs():
... def __init__(self):
... object.__init__()
... 
>>> NotEnoughInitArgs()
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 3, in __init__
TypeError: descriptor '__init__' of 'object' object needs an argument

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[issue31506] Improve the error message logic for object_new & object_init

[miss-islington]

miss-islington  added the comment:


New changeset 64ca72822338e0ba6e4f14d0a1cd3a9dcfa6c9ac by Miss Islington (bot) 
in branch '3.7':
bpo-31506: Clarify error messages for object.__new__ and object.__init__ 
(GH-11641)
https://github.com/python/cpython/commit/64ca72822338e0ba6e4f14d0a1cd3a9dcfa6c9ac


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[issue31506] Improve the error message logic for object_new & object_init

[miss-islington]

Change by miss-islington :


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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan  added the comment:


New changeset 5105483acb3aca318304bed056dcfd7e188fe4b5 by Nick Coghlan (Sanyam 
Khurana) in branch 'master':
bpo-31506: Clarify error messages for object.__new__ and object.__init__ 
(GH-11641)
https://github.com/python/cpython/commit/5105483acb3aca318304bed056dcfd7e188fe4b5


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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan  added the comment:

Paolo: it still won't be completely clear, since there's still the subtle issue 
that __new__ is a static method rather than a class method, so the correct 
calls up to the base class are respectively:

super(Singleton, cls).__new__(cls) # Static method, cls needs to be passed 
explicitly

super(Singleton, self).__init__() # Bound method, self filled in 
automatically

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[issue31506] Improve the error message logic for object_new & object_init

[Sanyam Khurana]

Change by Sanyam Khurana :


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stage: needs patch -> patch review

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[issue31506] Improve the error message logic for object_new & object_init

[Sanyam Khurana]

Change by Sanyam Khurana :


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[issue31506] Improve the error message logic for object_new & object_init

[Sanyam Khurana]

Change by Sanyam Khurana :


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[issue31506] Improve the error message logic for object_new & object_init

[Paolo Taddonio]

Paolo Taddonio  added the comment:

I am not sure if the following is resolved by your proposal, I post it just in 
case:
The following code works:
1. class Singleton(object):
2. def __new__(cls, *args, **kwargs):
3. if not hasattr(cls, 'instance'):
4. cls.instance = super(Singleton, cls).__new__(cls)
5. cls.instance._init_pointer = 
cls.instance._init_properties
6. else:
7. cls.instance._init_pointer = lambda *args, **kwargs: 
None # do nothing
8. return cls.instance
9. def __init__(self, *args, **kwargs):
10. super(Singleton, self).__init__()
11. self._init_pointer(*args, **kwargs)
12. def _init_properties(self, tag):
13. self.info = tag
14. #
15. if __name__ == '__main__':
16. S1 = Singleton('I am S1')
17. print('S1 info is:' + S1.info)
18. S2 = Singleton('Am I S2?')
19. print('S2 info is:' + S2.info)
However if I change line 4 into this code (which works in Python 2 by the way):
cls.instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
I get:
TypeError: object.__new__() takes no arguments
But if I change line 4 into this (no arguments as suggested):
cls.instance = super(Singleton, cls).__new__()
I get:
TypeError: object.__new__(): not enough arguments
Line 10 has the same issue when changed to:
super(Singleton, self).__init__(*args, **kwargs)

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan  added the comment:

We added the method names to help provide a nudge that the issue is likely to 
be a missing method implementation in the subclassing case, so I'd like to keep 
them if we can find a way to make the messages accurate again.

What if we updated the offending format strings in typeobject.c to state the 
exact nature of the expected argument that is missing?

PyErr_SetString(PyExc_TypeError, "object.__init__() takes exactly one 
argument (the instance to initialize)");

PyErr_Format(PyExc_TypeError, "%.200s.__init__() takes exactly one argument 
(the instance to initialize)", type->tp_name);

PyErr_SetString(PyExc_TypeError, "object.__new__() takes exactly one 
argument (the type to instantiate)")

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[issue31506] Improve the error message logic for object_new & object_init

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I think this change should be reverted.

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[issue31506] Improve the error message logic for object_new & object_init

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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[issue31506] Improve the error message logic for object_new & object_init

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Error messages "object.__init__() takes no arguments" and "object.__new__() 
takes no arguments" are wrong. They contradicts the following error messages:

>>> object.__init__()
Traceback (most recent call last):
  File "", line 1, in 
TypeError: descriptor '__init__' of 'object' object needs an argument
>>> object.__new__()
Traceback (most recent call last):
  File "", line 1, in 
TypeError: object.__new__(): not enough arguments

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[issue31506] Improve the error message logic for object_new & object_init

[Sanyam Khurana]

Sanyam Khurana  added the comment:

Serhiy, can you please elaborate on that a bit? I'll try to fix this.

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[issue31506] Improve the error message logic for object_new & object_init

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

780acc89bccf332d334a27887684cc942eb6 reintroduced the part of the original 
bug fixed in a6c0c0695614177c8b6e1840465375eefcfee586. object.__new__() and 
object.__init__() require an argument (cls and self correspondingly).

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan  added the comment:

Thanks for the feedback and updates folks! If we decide to make any further 
changes, I think they will be best handled as a new issue :)

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan  added the comment:


New changeset 780acc89bccf332d334a27887684cc942eb6 by Nick Coghlan (Sanyam 
Khurana) in branch 'master':
bpo-31506: Improve the error message logic for class instantiation (GH-4740)
https://github.com/python/cpython/commit/780acc89bccf332d334a27887684cc942eb6


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[issue31506] Improve the error message logic for object_new & object_init

[Sanyam Khurana]

Change by Sanyam Khurana :


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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan  added the comment:

Aye, I think Sanyam's proposed messages look good, and the "C().__init__() 
takes no arguments" wording is easier to follow than my suggested "C.__init__() 
takes exactly one argument" wording (as interpreting the latter currently 
requires noticing that it's referring to the *unbound* method taking one 
argument: the instance).

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[issue31506] Improve the error message logic for object_new & object_init

[Sanyam Khurana]

Sanyam Khurana  added the comment:

Nick,

I think the error messages are incorrect. We expect error message to be `takes 
no argument` rather than `takes exactly one argument`. Can you please confirm 
that?

I think for the class without any method overrides, the functionality should be 
something like this:

 >>> class C:
... pass
...
>>> C(42)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: C() takes no arguments
>>> C.__new__(C, 42)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: C() takes no arguments
>>> C().__init__(42)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: C().__init__() takes no arguments
>>> object.__new__(C, 42)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: C() takes no arguments
>>> object.__init__(C(), 42)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: C().__init__() takes no arguments



Is that correct?

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[issue31506] Improve the error message logic for object_new & object_init

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I think the main problem is not with coding, but with design. And from this 
point of view this may be not so easy issue. Let wait until Nick has a time to 
work on it.

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[issue31506] Improve the error message logic for object_new & object_init

[Sanyam Khurana]

Sanyam Khurana  added the comment:

I'll work on a fix for this and issue a PR.

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan added the comment:

Aye, the "C.__new__" example omitting the first arg was just an error in that 
example.

And that's a good point about the current "object.__init__()" error message 
actually being incorrect, since the *methods* each take exactly one argument - 
it's only the "object(*args, **kwds)" form that genuinely expects zero 
arguments.

If we were to correct that error as well, we'd end up with the following:

# Without any method overrides
class C:
pass

C(42) -> "TypeError: C() takes no arguments"
C.__new__(C, 42) -> "TypeError: C() takes no arguments"
C().__init__(42) -> "TypeError: C.__init__() takes exactly one argument"
# These next two quirks are the price we pay for the nicer errors above
object.__new__(C, 42) -> "TypeError: C() takes no arguments"
object.__init__(C(), 42) -> "TypeError: C.__init__() takes exactly one 
argument"

# With method overrides
class D:
def __new__(cls, *args, **kwds):
super().__new__(cls, *args, **kwds)
def __init__(self, *args, **kwds):
super().__init__(*args, **kwds)

D(42) -> "TypeError: object.__new__() takes exactly one argument"
D.__new__(D, 42) -> "TypeError: object.__new__() takes exactly one argument"
D().__init__(42) -> "TypeError: object.__init__() takes exactly one 
argument"
object.__new__(C, 42) -> "TypeError: object.__new__() takes exactly one 
argument"
object.__init__(C(), 42) -> "TypeError: object.__init__() takes exactly one 
argument"

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[issue31506] Improve the error message logic for object_new & object_init

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

C.__new__(42) emits different error, "TypeError: object.__new__(X): X is not a 
type object (int)". Perhaps you meant C.__new__(C, 42) which now emits 
"TypeError: C() takes no arguments".

Messages "object.__new__() takes no arguments" and "object.__init__() takes no 
arguments" are not correct since both object.__new__() and object.__init__() 
take one argument -- a class and an instance correspondingly.

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan added the comment:

I filed issue 31527 as a follow-up issue to see whether or not it might be 
possible to amend the way these custom errors are generated to benefit from the 
work that has gone into improving the error responses from 
PyArg_ParseTupleAndKeywords.

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan added the comment:

Reopening, as I was a little hasty with the merge button: the merged PR *also* 
changed the `__init__` error message to drop the method name, but I don't think 
that's what we want.

I'm also wondering if we should change the up-call case to *always* report the 
method name.

That is, we'd implement the following revised behaviour:

# Without any method overrides
class C:
pass

C(42) -> "TypeError: C() takes no arguments"
C.__new__(42) -> "TypeError: C() takes no arguments"
C().__init__(42) -> "TypeError: C.__init__() takes no arguments"
# These next two quirks are the price we pay for the nicer errors above
object.__new__(C, 42) -> "TypeError: C() takes no arguments"
object.__init__(C(), 42) -> "TypeError: C.__init__() takes no arguments"

# With method overrides
class D:
def __new__(cls, *args, **kwds):
super().__new__(cls, *args, **kwds)
def __init__(self, *args, **kwds):
super().__init__(*args, **kwds)

D(42) -> "TypeError: object.__new__() takes no arguments"
D.__new__(42) -> "TypeError: object.__new__() takes no arguments"
D().__init__(42) -> "TypeError: object.__init__() takes no arguments"
object.__new__(C, 42) -> "TypeError: object.__new__() takes no arguments"
object.__init__(C(), 42) -> "TypeError: object.__init__() takes no 
arguments"

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resolution: fixed -> 
stage: resolved -> needs patch
status: closed -> open

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Changes by Nick Coghlan :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan added the comment:


New changeset a6c0c0695614177c8b6e1840465375eefcfee586 by Nick Coghlan (Serhiy 
Storchaka) in branch 'master':
bpo-31506: Improve the error message logic for object.__new__ and 
object.__init__. (GH-3650)
https://github.com/python/cpython/commit/a6c0c0695614177c8b6e1840465375eefcfee586


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[issue31506] Improve the error message logic for object_new & object_init

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


--
keywords: +patch
pull_requests: +3643
stage: needs patch -> patch review

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan added the comment:

Those would both report "C() takes no parameters" without further enhancements 
(which would be out of scope for this issue).

The proposed improvement here isn't "Let's make the error message exactly 
correct in all cases" (that's probably impossible, since we've lost relevant 
information by the time the argument processing happens).

Instead, it's "let's make the error message more helpful in the most common 
case for beginners, and let the folks performing the more advanced operation of 
calling __new__ directly do the translation if they need to"

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[issue31506] Improve the error message logic for object_new & object_init

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

What do you expect for:

class C: pass

object.__new__(C, 1)
C.__new__(C, 1)

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan added the comment:

For this issue, I'm not proposing to make any change other than to solve the 
specific problem reported in the blog post: when the method itself isn't 
overridden, then the error message should report the name of the most derived 
class, not "object", to help users more readily find the likely source of their 
problem (a missing "__init__" method definition).

Making these custom errors consistent with Python 3's otherwise improved 
argument unpacking errors would be a separate issue (and I agree *that* change 
wouldn't qualify as being easy).

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[issue31506] Improve the error message logic for object_new & object_init

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

It is not so easy to make an error message conforming with error messages for 
similar types. This may require changing error messages in other code.

First, "takes no arguments" instead of "takes no parameters".

For normal __new__ and __init__ you never got "takes no arguments". They  take 
at least one argument -- a class or an instance.

>>> tuple.__new__(tuple, 1, 2, 3, 4)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: tuple expected at most 1 arguments, got 4
>>> list.__init__([], 1, 2, 3, 4)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: list expected at most 1 arguments, got 4
>>> class C:
... def __new__(cls): return object.__new__(cls)
... def __init__(self): pass
... 
>>> C.__new__(C, 1, 2, 3, 4)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: __new__() takes 1 positional argument but 5 were given
>>> C.__init__(C(), 1, 2, 3, 4)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: __init__() takes 1 positional argument but 5 were given

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Nick Coghlan added the comment:

Fortunately, the logic is already well encapsulated: there's a "if (excess_args 
&& (case A || case B)) {... report error ...}" check at the start of each of 
object_new and object_init, where "case A" = "the other function in the 
object_new/object_init pair has *not* been overriden" and "case B" is "this 
function *has* been overridden".

That means the only change needed is to include the type name in an updated 
error message in case A, while retaining the current error messages for case B.

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[issue31506] Improve the error message logic for object_new & object_init

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Not sure this is easy issue. It requires taking to account many different cases 
and analyzing many arguments checking code scattered around many files.

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Changes by Nick Coghlan :


--
keywords: +easy (C)

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

Changes by Nick Coghlan :


--
components: +Interpreter Core
stage:  -> needs patch
type:  -> enhancement
versions: +Python 3.7

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[issue31506] Improve the error message logic for object_new & object_init

[Nick Coghlan]

New submission from Nick Coghlan:

As described in 
https://blog.lerner.co.il/favorite-terrible-python-error-message/, object_new 
and object_init currently have "object" hardcoded in the error messages they 
raise for excess parameters:


>>> class C: pass
... 
>>> C(10)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: object() takes no parameters
>>> c = C()
>>> c.__init__(10)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: object.__init__() takes no parameters

This hardcoding makes sense for the case where that particular method has been 
overridden, and the interpreter is reporting an error in the subclass's call up 
to the base class, rather than in the call to create an instance of the 
subclass:

>>> class D:
... def __init__(self, *args):
... return super().__init__(*args)
... 
>>> D(10)
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 3, in __init__
TypeError: object.__init__() takes no parameters


However, it's misleading in the case where object_new is reporting an error 
because it knows object_init hasn't been overridden (or vice-versa), and hence 
won't correctly accept any additional arguments: in those cases, it would be 
far more useful to report "type->tp_name" in the error message, rather than 
hardcoding "object".

If we split the error message logic that way, then the first two examples above 
would become:

>>> class C: pass
... 
>>> C(10)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: C() takes no parameters
>>> c = C()
>>> c.__init__(10)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: C.__init__() takes no parameters

while the subclassing cases would be left unchanged.

--
messages: 302439
nosy: ncoghlan
priority: normal
severity: normal
status: open
title: Improve the error message logic for object_new & object_init

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