gh-91673: Missing import in dataclasses documentation #91672
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In this example: ``` @DataClass class D: x: List = [] def add(self, element): self.x += element ``` the import ```from typing import List``` is missing
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xandrade
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Is there a corresponding issue? |
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Ah, it's #91673. |
xandrade
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@ericvsmith also noted there is a trailing whitespace and the Azure Pipelines PR is failing (library/dataclasses.rst:704). |
Doc/library/dataclasses.rst
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| @@ -700,6 +700,8 @@ variable ``x``, as expected. | |||
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| Using dataclasses, *if* this code was valid:: | |||
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| from typing import List | |||
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| @dataclass | |||
| class D: | |||
| x: List = [] | |||
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As @ericvsmith said on the issue, it's easier to just put x: list = [] here.
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@JelleZijlstra I think they really mean List, from typing. But I can be wrong. With x: list = [], you get the following error:
ValueError: mutable default <class 'list'> for field x is not allowed: use default_factory
In that case, it should be (I think):
x: list = field(default_factory=list)
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Using list is fine in recent versions; see PEP-585.
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Noted, but x: list = [] doesn't work either (tried in 3.7, 3.8 and 3.9). Maybe in 3.10?
from dataclasses import dataclass
@dataclass
class D:
x: list = []
throws the following exception:
ValueError Traceback (most recent call last)
Input In [2], in <cell line: 4>()
1 from dataclasses import dataclass, field
2 from typing import List
4 @dataclass
----> 5 class D:
6 x: list = []
File /opt/conda/lib/python3.9/dataclasses.py:1021, in dataclass(cls, init, repr, eq, order, unsafe_hash, frozen)
1018 return wrap
1020 # We're called as @dataclass without parens.
-> 1021 return wrap(cls)
File /opt/conda/lib/python3.9/dataclasses.py:1013, in dataclass.<locals>.wrap(cls)
1012 def wrap(cls):
-> 1013 return _process_class(cls, init, repr, eq, order, unsafe_hash, frozen)
File /opt/conda/lib/python3.9/dataclasses.py:863, in _process_class(cls, init, repr, eq, order, unsafe_hash, frozen)
858 cls_annotations = cls.__dict__.get('__annotations__', {})
860 # Now find fields in our class. While doing so, validate some
861 # things, and set the default values (as class attributes) where
862 # we can.
--> 863 cls_fields = [_get_field(cls, name, type)
864 for name, type in cls_annotations.items()]
865 for f in cls_fields:
866 fields[f.name] = f
File /opt/conda/lib/python3.9/dataclasses.py:863, in <listcomp>(.0)
858 cls_annotations = cls.__dict__.get('__annotations__', {})
860 # Now find fields in our class. While doing so, validate some
861 # things, and set the default values (as class attributes) where
862 # we can.
--> 863 cls_fields = [_get_field(cls, name, type)
864 for name, type in cls_annotations.items()]
865 for f in cls_fields:
866 fields[f.name] = f
File /opt/conda/lib/python3.9/dataclasses.py:747, in _get_field(cls, a_name, a_type)
739 # Should I check for other field settings? default_factory
740 # seems the most serious to check for. Maybe add others. For
741 # example, how about init=False (or really,
(...)
744
745 # For real fields, disallow mutable defaults for known types.
746 if f._field_type is _FIELD and isinstance(f.default, (list, dict, set)):
--> 747 raise ValueError(f'mutable default {type(f.default)} for field '
748 f'{f.name} is not allowed: use default_factory')
750 return f
ValueError: mutable default <class 'list'> for field x is not allowed: use default_factory
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The point of the example is that it doesn't work.
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I agree with @JelleZijlstra. Since the example isn't supposed to work anyway, I think changing List to list is good enough. No other changes are needed.
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Thanks guys for your time
Updated ```x: list = []``` by ```x: list = field(default_factory=list)``` on the example to avoid error ```ValueError: mutable default <class 'list'> for field x is not allowed: use default_factory```
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As reported in #91673, the import is missed for the typing
Listat dataclasses.rst file.In this example:
the import
from typing import Listis missing