# Required kwarg in \_\_prepare\_\_

**URL:** <https://discuss.python.org/t/required-kwarg-in-prepare/17311>\
**Category:** Python Help\
**Created:** [July 12, 2022, 6:02pm UTC](https://discuss.python.org/t/required-kwarg-in-prepare/17311 "2022-07-12T18:02:42Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Melendowski](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/melendowski/32/2960_2.png) [@Melendowski](https://discuss.python.org/u/Melendowski)\
**Post date:** [July 12, 2022, 6:02pm UTC](https://discuss.python.org/t/required-kwarg-in-prepare/17311/1 "2022-07-12T18:02:42Z")

</div>

Hi I am attempt to learn the class creation process better. I am interested in the aspect of class parameterization. Why do I get an error if I make a required key word argument in ` __prepare__ `? If I just use the default value, the class is defined properly but if i explicitly pass it, I get an error in ` __init_subclass__ `?

```python
In [13]: class Meta(type):
    ...: @classmethod
    ...: def __prepare__ (mcs, *args, required_kwarg="foo", **kwargs):
    ...: print(required_kwarg)
    ...: return {}

In [14]: class Foo(metaclass=Meta):
    ...: ...
foo

In [15]: class Foo2(metaclass=Meta, required_kwarg="foo2"):
    ...: ...
foo2
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Input In [15], in <module>
----> 1 class Foo2(metaclass=Meta, required_kwarg="foo2"):
      2 ...

TypeError: Foo2. __init_subclass__ () takes no keyword arguments

```

If I remove the default value to `required_kwarg` then ` __prepare__ ` raises an error as I would expect but I am still getting the ` __init_subclass__ ` error when trying to create `Foo2`

```python
In [19]: class Meta(type):
    ...: @classmethod
    ...: def __prepare__ (mcs, *args, required_kwarg, **kwargs):
    ...: print(required_kwarg)
    ...: return {}

In [20]: class Foo(metaclass=Meta):
    ...: ...
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Input In [20], in <module>
----> 1 class Foo(metaclass=Meta):
      2 ...

TypeError: Meta. __prepare__ () missing 1 required keyword-only argument: 'required_kwarg'

In [21]: class Foo2(metaclass=Meta, required_kwarg="foo2"):
    ...: ...
foo2
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Input In [21], in <module>
----> 1 class Foo2(metaclass=Meta, required_kwarg="foo2"):
      2 ...

TypeError: Foo2. __init_subclass__ () takes no keyword arguments

```

It seems like passing any kind of keyword argument through the class definition throws an error with ` __init_subclass__ `.

```python
In [1]: class Meta(type):
   ...: @classmethod
   ...: def __prepare__ (mcs, *args, **kwargs):
   ...: if "required_kwarg" not in kwargs:
   ...: raise TypeError('missing "required_kwarg" keyword argument')
   ...: else:
   ...: required_kwarg = kwargs["required_kwarg"]
   ...: print(required_kwarg)
   ...: return {}

In [2]: class Foo2(metaclass=Meta, required_kwarg="foo2"):
   ...: ...
foo2
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Input In [2], in <module>
----> 1 class Foo2(metaclass=Meta, required_kwarg="foo2"):
      2 ...

TypeError: Foo2. __init_subclass__ () takes no keyword arguments

```

---

<div class="post-metadata">

**Author:** ![Melendowski](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/melendowski/32/2960_2.png) [@Melendowski](https://discuss.python.org/u/Melendowski)\
**Post date:** [July 14, 2022, 12:09am UTC](https://discuss.python.org/t/required-kwarg-in-prepare/17311/2 "2022-07-14T00:09:58Z")

</div>

I’ve been researching this more off and on and this is like the closest explanation I come of it. I don’t think its a direct cause, as I am using python 3.10, but could it be a similar problem?

[https://bugs.python.org/issue29581](https://bugs.python.org/issue29581)

> <https://stackoverflow.com/questions/42281697/typeerror-when-combining-abcmeta-with-init-subclass-in-python-3-6>

---

<div class="post-metadata">

**Author:** ![MegaIng](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/megaing/32/16162_2.png) [@MegaIng](https://discuss.python.org/u/MegaIng)\
**Post date:** [March 14, 2024, 11:20pm UTC](https://discuss.python.org/t/required-kwarg-in-prepare/17311/3 "2024-03-14T23:20:13Z")

</div>

You can avoid the issue by also defining a ` __new__ ` method that removes the arguments before passing them to `type. __new__ `:

```python
class Meta(type):
    def __prepare__ (mcs, *args, required_kwarg, **kwargs):
        print(required_kwarg)
        return {}

    def __new__ (mcs, *args, required_kwarg, **kwargs):
        return super(). __new__ (mcs, *args, **kwargs)

```

---

<div class="post-metadata">

**Author:** ![Melendowski](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/melendowski/32/2960_2.png) [@Melendowski](https://discuss.python.org/u/Melendowski)\
**Post date:** [March 14, 2024, 11:26pm UTC](https://discuss.python.org/t/required-kwarg-in-prepare/17311/4 "2024-03-14T23:26:35Z")

</div>

I see.

Interestingly I ran this in python 3.5, which is right before PEP487 with ` __init_subclass__ ` and I get a different error

```python
>>> class Meta(type):
... @classmethod
... def __prepare__ (mcs, *args, required_kwarg="foo", **kwargs):
... print(required_kwarg)
... return {}
...
>>> class Foo2(metaclass=Meta, required_kwarg="foo2"):
... ...
...
foo2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: type() takes 1 or 3 arguments

```

But I see this section PEP487 hinting at this sort of behavior. Glad I’ve finally figured this one out

> **[PEP 487 – Simpler customisation of class creation | peps.python.org](https://peps.python.org/pep-0487/#backward-compatibility-issues)**
>
> Python Enhancement Proposals (PEPs)