Quote DataType::Struct field names in Display formatting
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Display/Debug formatting
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This is part of an attempt to improve the error reporting of `arrow-rs`,
`datafusion`, and any other 3rd party crates.
I believe that error messages should be as readable as possible. Aim for
`rustc` more than `gcc`.
Here's an example of how this PR improves some existing error messages:
Before:
> Casting from Map(Field { name: "entries", data_type: Struct([Field {
name: "key", data_type: Utf8, nullable: false, dict_id: 0,
dict_is_ordered: false, metadata: {} }, Field { name: "value",
data_type: Interval(DayTime), nullable: true, dict_id: 0,
dict_is_ordered: false, metadata: {} }]), nullable: false, dict_id: 0,
dict_is_ordered: false, metadata: {} }, false) to Map(Field { name:
"entries", data_type: Struct([Field { name: "key", data_type: Utf8,
nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} },
Field { name: "value", data_type: Duration(Second), nullable: false,
dict_id: 0, dict_is_ordered: false, metadata: {} }]), nullable: false,
dict_id: 0, dict_is_ordered: false, metadata: {} }, true) not supported
After:
> Casting from Map(Field { "entries": Struct(key Utf8, value nullable
Interval(DayTime)) }, false) to Map(Field { "entries": Struct(key Utf8,
value Duration(Second)) }, true) not supported
# Which issue does this PR close?
- Closes #7048
- Continues and closes #7051
- Continues #7469
- More improvements coming in
#8291
- Sibling PR: apache/datafusion#17565
- Part of #8351
# Rationale for this change
DataType:s are often shown in error messages. Making these error
messages readable is _very important_.
# What changes are included in this PR?
## ~Unify `Debug` and `Display`~
~The `Display` and `Debug` of `DataType` are now the SAME.~
~Why? Both are frequently used in error messages (both in arrow, and
`datafusion`), and both benefit from being readable yet reversible.~
Reverted based on PR feedback. I will try to improve the `Debug`
formatting in a future PR, with clever use of
https://doc.rust-lang.org/std/fmt/struct.Formatter.html#method.debug_struct
## Improve `Display` of lists
Improve the `Display` formatting of
* `DataType::List`
* `DataType::LargeList`
* `DataType::FixedSizeList`
**Before**: `List(Field { name: \"item\", data_type: Int32, nullable:
true, dict_id: 0, dict_is_ordered: false, metadata: {} })`
**After**: `List(nullable Int32)`
**Before**: `FixedSizeList(Field { name: \"item\", data_type: Int32,
nullable: false, dict_id: 0, dict_is_ordered: false, metadata: {} }, 5)`
**After**: `FixedSizeList(5 x Int32)`
### Better formatting of `DataType::Struct`
The formatting of `Struct` is now reversible, including nullability and
metadata.
- Continues #7469
### ~Improve `Debug` format of `Field`~
~Best understood with this diff for an existing test:~
<img width="1140" height="499" alt="Screenshot 2025-09-07 at 18 30 44"
src="https://github.com/user-attachments/assets/794b4de9-8459-4ee7-82d2-8f5ae248614c"
/>
**EDIT**: reverted
# Are these changes tested?
Yes - new tests cover them
# Are there any user-facing changes?
`Display/to_string` has changed, and so this is a **BREAKING CHANGE**.
Care has been taken that the formatting contains all necessary
information (i.e. is reversible), though the actual `FromStr`
implementation is still not written (it is missing on `main`, and
missing in this PR - so no change).
----
Let me know if I went to far… or not far enough 😆
---------
Co-authored-by: irenjj <renj.jiang@gmail.com>
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Display/Debug formattingDataType::Struct field names in Display formatting
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Sep 24, 2025
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Which issue does this PR close?
DisplayforDataTypeandField#8290 (merge that first, and the diff of this PR will drop)DisplayforDataType#8351Rationale for this change
We would previously format structs like so:
Struct(name1 type1, name2 nullable type2)This will break badly whenever the field name is anything but a simple identifier. In other words: it allows string injection if the field name contains an end-paranthesis.
Except for that, it is also difficult to debug mistakingly bad field names like " " or "\n".
What changes are included in this PR?
We change the
DisplayandDebugformatting ofStructBefore:
Struct(name1 type1, name2 nullable type2)After:
Struct("name1": type1, "name2": nullable type2)Are these changes tested?
Yes - I've updated the existing tests.
Are there any user-facing changes?
Yes, changing the
Displayformatting is a breaking change