Description
When using use_type_alias=True together with reuse_model=True on large OpenAPI specs, the TypeStatement.jinja2 template crashes with list object has no element 0. This happens because reuse_model can create TypeStatement models with an empty fields list, but the template unconditionally accesses fields[0] on line 11.
Error
jinja2.exceptions.UndefinedError: list object has no element 0
Traceback points to TypeStatement.jinja2 line 11:
type {{ class_name }} = {{ get_type_annotation(fields[0]) }}Reproduction
The bug triggers on large, real-world OpenAPI specs where reuse_model deduplicates enough schemas that some get converted to TypeStatement with empty fields. I was not able to produce a minimal synthetic repro — the interaction between reuse_model and schema deduplication requires a critical mass of schemas.
Using the public Amazon Advertising Sponsored Products spec (669 schemas):
from pathlib import Path
from datamodel_code_generator import DataModelType, LiteralType, PythonVersion, generate
# Download the spec first:
# curl -o sp_v3.json 'https://d1y2lf8k3vrkfu.cloudfront.net/openapi/en-us/dest/SponsoredProducts_prod_3p.json'
generate(
input_=Path("sp_v3.json"),
output=Path("output.py"),
output_model_type=DataModelType.PydanticV2BaseModel,
target_python_version=PythonVersion.PY_314,
use_type_alias=True,
reuse_model=True,
enum_field_as_literal=LiteralType.All,
use_annotated=True,
field_constraints=True,
)Other Amazon Advertising specs that also crash: Sponsored Brands, Sponsored Display, DSP Campaigns, DSP Creatives.
Suggested fix
Guard the fields[0] access in TypeStatement.jinja2:
{%- if fields %}
type {{ class_name }} = {{ get_type_annotation(fields[0]) }}{% if comment is defined %} # {{ comment }}{% endif %}
{%- if description %}
...
{%- endif %}
{%- else %}
type {{ class_name }} = {{ base_class }}{% if comment is defined %} # {{ comment }}{% endif %}
{%- if description %}
...
{%- endif %}
{%- endif %}Environment
- datamodel-code-generator version: 0.55.0
- Python: 3.14
Description
When using
use_type_alias=Truetogether withreuse_model=Trueon large OpenAPI specs, theTypeStatement.jinja2template crashes withlist object has no element 0. This happens becausereuse_modelcan createTypeStatementmodels with an emptyfieldslist, but the template unconditionally accessesfields[0]on line 11.Error
Traceback points to
TypeStatement.jinja2line 11:type {{ class_name }} = {{ get_type_annotation(fields[0]) }}Reproduction
The bug triggers on large, real-world OpenAPI specs where
reuse_modeldeduplicates enough schemas that some get converted toTypeStatementwith empty fields. I was not able to produce a minimal synthetic repro — the interaction betweenreuse_modeland schema deduplication requires a critical mass of schemas.Using the public Amazon Advertising Sponsored Products spec (669 schemas):
Other Amazon Advertising specs that also crash: Sponsored Brands, Sponsored Display, DSP Campaigns, DSP Creatives.
Suggested fix
Guard the
fields[0]access inTypeStatement.jinja2:Environment