gh-106625 : Add missing code to tutorial 4.6 example

[RustyNail]

This is a pull request for the sample code in section 4.6.

I ran the following based on the sample code and got an error.

The sample code in the tutorial may be an excerpt of some code.
However, given that we will try "Point(1, var)" immediately following, I believe the user will easily understand that there is a necessary dataclass import statement.

Executed code :

class Point:
    x: int
    y: int

def where_is(point):
    match point:
        case Point(x=0, y=0):
            print("Origin")
        case Point(x=0, y=y):
            print(f"Y={y}")
        case Point(x=x, y=0):
            print(f"X={x}")
        case Point():
            print("Somewhere else")
        case _:
            print("Not a point")

## Added code
p = Point(1, 1)

Errors that occurred :

    p = Point(1, 1)
        ^^^^^^^^^^^
TypeError: Point() takes no arguments

---

📚 Documentation preview 📚: https://cpython-previews--106623.org.readthedocs.build/

• Issue: Error when executing the sample in chapter 4.6 #106625

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[bedevere-bot]

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I made the requested change

[methane]

However, given that we will try "Point(1, var)" immediately following, I believe the user will easily understand that there is a necessary dataclass import statement.

That example is after the statement "You can use positional parameters with some builtin classes that provide an ordering for their attributes (e.g. dataclasses)."

Using dataclass before the statemnt is bit misleading.

In your case, following code would work.

p = Point()
p.x = 0
p.y = 42
where_is(p)

Of course, it is ugly. But the first example should demonstrate that named argument pattern like Point(x=y, y=y) will work for regular classes. So we shouldn't use dataclass here.

Instead, I think this makes the first example better.

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

With this Point class, user will understand that even typehint for attributes are not necessary for Point(x=x, y=y) pattern.

[terryjreedy]

Reading @methane 's comment after fixing the example as a dataclass, I agree. The example follows "f you are using classes to structure your data you can use the class name followed by an argument list resembling a constructor, but with the ability to capture attributes into variables:". To me also, this implies a regular class with a contructor (__init__ method). The mention of dataclasses follows the example and I read that as applying to further examples.

[RustyNail]

Thanks for the review.
I will change this Pull Request change to initialization code according to the constructor using __init__.

[terryjreedy]

@RustyNail I already made the change. Note that dunder names with double underscores need to be surrounded by backticks to display properly in GitHub comments. Otherwise the underscores are seen as markup. Compare 'init' with bolder 'init' with __init__.

[RustyNail]

@terryjreedy
Thanks for the update.
I too understood your guys intent to use constructor initialization better.

[ericvsmith]

Reading @methane 's comment after fixing the example as a dataclass, I agree.

I agree, too.

[miss-islington]

Thanks @RustyNail for the PR, and @terryjreedy for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[bedevere-bot]

GH-106636 is a backport of this pull request to the 3.12 branch.

[bedevere-bot]

GH-106637 is a backport of this pull request to the 3.11 branch.