Conversion error with Python float value conversion to UNION containing float #115

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1. Hunknown

   added

   needs triage

   on Oct 9, 2025
2. paultiq commented on Oct 9, 2025

   

   paultiq

   on Oct 9, 2025

   Contributor

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   The cause seems to be that PythonValueConversion doesn't have the DefaultCastAs branch that the other datatypes have:
   Consider HandleDouble:
   

   duckdb-python/src/duckdb_py/native/python_conversion.cpp

   Lines 478 to 479 in 3e79791

   	default:
   	throw ConversionException("Could not convert 'float' to type %s", target_type.ToString());

   vs HandleBigint:
   

   duckdb-python/src/duckdb_py/native/python_conversion.cpp

   Lines 503 to 505 in 3e79791

   	default:
   	res = Value::BIGINT(value).DefaultCastAs(target_type);
   	break;

   Possible fix: this seems to fix it, provided the original return types are disambiguated with return_type=DuckDBPyType("MAP(VARCHAR, UNION(int_val BIGINT, float_val DOUBLE, str_val VARCHAR))[]"):

     	default:
        	    result = Value::DOUBLE(val).DefaultCastAs(target_type);
               break;

   Alternate Solution

   If the types are constant for each mapping, where a's are ints and b's are floats, could use a struct instead:

   import duckdb
   import numpy as np
   from duckdb.typing import (
   	DuckDBPyType
   )
   import duckdb.typing as dt
   
   arr = [{"a": 1, "b": 1.2}, {"a": 3, "b": 2.4}]
   
   def test():
       return arr
   
   with duckdb.connect() as conn:
       conn.create_function("test", test, return_type="STRUCT(a BIGINT, b DOUBLE)[]")
   
       print(conn.sql("select test()").df())
       print(conn.sql("describe select test()").df())

   👍React with 👍1Reacted by Hunknown
3. Tishj

   changed the title [-]Conversion error with float in Python UDF[/-] [+]Conversion error with Python `float` value conversion to `UNION` containing float[/+] on Oct 9, 2025
4. Hunknown commented on Oct 10, 2025

   

   Hunknown

   on Oct 10, 2025

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   Hi @paultiq

   Yes, I've just tested both solutions and I can confirm that the alternate solution does work.

   But the first one still raise an error, so as @Tishj mentioned by changing the title of the issue, it's only when the float variable is in an Union that it doesn't work.
5. paultiq commented on Oct 10, 2025

   

   paultiq

   on Oct 10, 2025

   Contributor

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   The first one requires a change/build of the source code to change the default case. If you only tried with the updated return type without rebuilding from source, it wouldn't work.

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6. evertlammerts commented on Feb 19, 2026

   

   evertlammerts

   on Feb 19, 2026

   Member

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   Fixed in #332. We now handle casts for unions as well. The hardest part is to disambiguate the union if it contains the same data type. I don't see a reliable way of doing so. We now just pick the first compatible type by first checking for an exact match, and then a type we can cast to.

   from typing import Union
   import duckdb
   import numpy as np
   from duckdb.sqltypes import DuckDBPyType
   
   arr = [{"a": 1, "b": 1.2}, {"a": 3, "b": 2.4}]
   
   def test():
       return arr
   
   return_type = DuckDBPyType(list[dict[str, Union[str, int, np.float64, np.float32, float]]])
   
   duckdb.create_function("test", test, return_type=return_type)
   
   print(duckdb.sql("select test()"))

   Outputs:

   ┌──────────────────────────────────────────────────────────────────────────────┐
   │                                    test()                                    │
   │ map(varchar, union(u1 varchar, u2 bigint, u3 double, u4 float, u5 double))[] │
   ├──────────────────────────────────────────────────────────────────────────────┤
   │ [{a=1, b=1.2}, {a=3, b=2.4}]                                                 │
   └──────────────────────────────────────────────────────────────────────────────┘
   

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7. Hunknown commented on Feb 19, 2026

   

   Hunknown

   on Feb 19, 2026

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   Awesome, thank you very much!
8. evertlammerts

   added a commit that references this issue on Feb 19, 2026

   Fix numeric conversion logic (#332)

   ...

   This issue will close once commit 58e68f6 is merged into the 'v2.0-cyanoptera' branch.

   58e68f6
9. evertlammerts

   closed this as completedon Feb 19, 2026