Use enum trick for dataclasses.MISSING #7127
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hauntsaninja
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sobolevn
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The goal of this change is to fix the behavior of
`dataclasses.Field`. Several attributes of a `Field` may have a value
of `MISSING` (a sentinel value). As a result, attributes of Field may
be checked against `MISSING` as in
[f for f in fields(obj) if f.default is MISSING]
`default`, `default_factory`, and `kw_only` are the attributes
which may have a value of `MISSING`.
This workaround of defining `_MISSING_TYPE` as an enum, and `MISSING`
as its only member, allows `... | Literal[_MISSING_TYPE.MISSING]` to
be used in type annotations for these attributes. This is very
slightly inaccurate -- primarily in that `_MISSING_TYPE` isn't really
an enum -- but this allows for use in `Literal`.
After PEP 661 (Sentinel Values), there may be a more accurate way of
writing this, but for now this works.
This adjustment is only applied to the attributes of Field, not the
arguments to functions and methods.
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According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉 |
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The goal of this change is to fix the behavior of
dataclasses.Field. Several attributes of aFieldmay have a value ofMISSING(a sentinel value). As a result, attributes of Field may be checked againstMISSINGas indefault,default_factory, andkw_onlyare the attributes which may have a value ofMISSING.This workaround of defining
_MISSING_TYPEas an enum, andMISSINGas its only member, allows... | Literal[MISSING]to be used in type annotations for these attributes. This is very slightly inaccurate -- primarily in that_MISSING_TYPEisn't really an enum -- but this allows for use inLiteral. After PEP 661 (Sentinel Values), there may be a more accurate way of writing this, but for now this works.This adjustment is only applied to the attributes of Field, not the arguments to functions and methods.
There have been two previous attempts to handle this, #5900 and #7126.
Per those PRs, PEP 661 (Sentinel Values) will provide the best solution for this in the future. The enum+Literal workaround used here was suggested as a way to handle
dataclasses.MISSINGwhile we wait for that PEP.