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devel / comp.lang.python / Re: Advantages of Default Factory in Dataclasses

SubjectAuthor
* Re: Advantages of Default Factory in DataclassesDavid Lowry-Duda
`* Re: Advantages of Default Factory in DataclassesAlan Bawden
 `- Re: Advantages of Default Factory in DataclassesDavid Lowry-Duda

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Re: Advantages of Default Factory in Dataclasses

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From: david@lowryduda.com (David Lowry-Duda)
Newsgroups: comp.lang.python
Subject: Re: Advantages of Default Factory in Dataclasses
Date: Tue, 16 Nov 2021 18:00:05 -0500
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 by: David Lowry-Duda - Tue, 16 Nov 2021 23:00 UTC

On Tue, Nov 16, 2021 at 05:04:05PM +0400, Abdur-Rahmaan Janhangeer wrote:
> A simple question: why do we need field(default_factory ) in
> dataclasses?

For the same reason that the following code doesn't do what some people
might expect it to:

```python
def add_to(elem, inlist=[]):
inlist.append(elem)
return inlist

list1 = add_to(1)
list2 = add_to(2)
print(list1) # prints [1]
print(list2) # prints [1, 2], potentially confusing
```

Mutable attributes are created once, upon definition. Reusing the same
function (or instantiating objects of the same class) and modifying this
attribute can lead to possibly unexpected side effects as above.

- DLD

Re: Advantages of Default Factory in Dataclasses

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From: alan@csail.mit.edu (Alan Bawden)
Newsgroups: comp.lang.python
Subject: Re: Advantages of Default Factory in Dataclasses
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 by: Alan Bawden - Tue, 16 Nov 2021 23:24 UTC

David Lowry-Duda <david@lowryduda.com> writes:

...

For the same reason that the following code doesn't do what some people
might expect it to:

```python
def add_to(elem, inlist=[]):
inlist.append(elem)
return inlist

list1 = add_to(1)
list2 = add_to(2)
print(list1) # prints [1]
print(list2) # prints [1, 2], potentially confusing
```

Not only does it not print what "most people" expect. It also doesn't
print what _you_ expect! (But you made your point.)

Re: Advantages of Default Factory in Dataclasses

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From: david@lowryduda.com (David Lowry-Duda)
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Subject: Re: Advantages of Default Factory in Dataclasses
Date: Thu, 18 Nov 2021 10:43:10 -0500
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 by: David Lowry-Duda - Thu, 18 Nov 2021 15:43 UTC

On Tue, Nov 16, 2021 at 06:24:43PM -0500, Alan Bawden wrote:
> ```python
> def add_to(elem, inlist=[]):
> inlist.append(elem)
> return inlist
>
> list1 = add_to(1)
> list2 = add_to(2)
> print(list1) # prints [1]
> print(list2) # prints [1, 2], potentially confusing
> ```
>
> Not only does it not print what "most people" expect. It also doesn't
> print what _you_ expect! (But you made your point.)

Haha, you're right. I would guess that I reordered the statements when I
quickly checked this in the interpreter. But indeed, both list1 and
list2 point to inlist, and thus are the same. Whoops!

- DLD

devel / comp.lang.python / Re: Advantages of Default Factory in Dataclasses

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