GraphQLFloat should accept Python Decimal for database ORM compatibility #280

graphql-core's GraphQLFloat scalar rejects Python Decimal objects, which are the standard return type from database ORMs (SQLAlchemy, Django ORM, etc.) for:

• Numeric/Decimal column types
• Aggregate functions (SUM, AVG) even on Integer/Float columns
• Any high-precision arithmetic

Current Behavior:

from decimal import Decimal
from graphql import GraphQLFloat

GraphQLFloat.serialize(Decimal("123.45"))
# Raises: GraphQLError: Float cannot represent non numeric value: <Decimal instance>

Expected Behavior:
Decimal should be coerced to float, just as int is already accepted.

Why This Matters:

1. Python's Decimal is a numeric type - it's part of the standard library for precise decimal arithmetic
2. Database ORMs universally return Decimal for precision:
   • SQLAlchemy: Numeric, DECIMAL columns → Decimal
   • Django ORM: DecimalField → Decimal
   • PostgreSQL aggregates often return NUMERIC → Decimal
3. Precedent exists: GraphQLInt already accepts bool (coerces True/False → 1/0)
4. Backwards compatibility: Earlier versions were more lenient

Workarounds:

• Manually float() every Decimal in resolvers (verbose, error-prone)
• Monkey-patch GraphQLFloat (fragile)
• Custom scalar type (requires client-side changes)

Impact:
Every Python GraphQL server using database ORMs must implement workarounds. This makes graphql-core less pythonic and forces type coercion at the application layer.

Environment:

• graphql-core: 3.x
• Python: 3.12