INTERNAL ERROR reported if a Final isn't given specific float type

[cflynn3]

Crash Report

Mypy reports an INTERNAL ERROR without information around the issue in the code that is incorrect.

Traceback

 File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "C:\EvQrZ-py3.13\Lib\site-packages\mypy\__main__.py", line 16, in console_entry
    main()
    ~~~~^^
  File "mypy\main.py", line 154, in main
  File "mypy\main.py", line 244, in run_build
  File "mypy\build.py", line 422, in build
  File "mypy\build.py", line 537, in build_inner
  File "mypy\build.py", line 4150, in dispatch
  File "mypy\build.py", line 4618, in process_graph
  File "mypy\build.py", line 1492, in wait_for_done
  File "mypy\build.py", line 4824, in process_stale_scc
  File "mypy\build.py", line 3608, in write_cache
  File "mypy\build.py", line 2327, in write_cache
  File "mypy\nodes.py", line 597, in write
  File "mypy\nodes.py", line 5118, in write
  File "mypy\nodes.py", line 5023, in write
  File "mypy\nodes.py", line 4315, in write
  File "mypy\cache.py", line 558, in write_json
  File "mypy\cache.py", line 540, in write_json_value
  File "mypy\cache.py", line 529, in write_json_value
  File "mypy\cache.py", line 540, in write_json_value
  File "mypy\cache.py", line 540, in write_json_value
  File "mypy\cache.py", line 540, in write_json_value
TypeError: union[None, int, str, bool, list, dict, tuple] object expected; got float

note: use --pdb to drop into pdb

To Reproduce

Run mypy on the code file.

from dataclasses import dataclass
from typing import Final


@dataclass(kw_only=True, repr=False)
class MyDataClass:
    MY_CONSTANT: Final = 1.234

Your Environment

• Mypy version used: 2.1.0
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.13
• Operating system and version: Windows + Linux

[beanscg]

I checked the traceback path against current source. The crash lands in cache serialization, not in normal type checking:

• mypy/cache.py:520-542 serializes None, int, str, list, tuple, and dict, but not float.
• write_json walks dict values at 553-558.
• The issue traceback comes through mypy/nodes.py symbol/type-info serialization (TypeInfo.write around 4318-4324, symbol-table serialization around 5119-5124).

That fits the minimal repro: MY_CONSTANT: Final = 1.234 leaves a float literal value in serialized metadata/cache data. A regression should run mypy twice on the dataclass repro so cache write/read is exercised, then assert no internal error. The acceptable user-facing result can be a normal type result or diagnostic; the key failure is letting a float literal reach write_json_value unsupported.

[ilevkivskyi]

Yeah, this is my fault (I forgot floats are valid in JSON, LOL). Fortunately, fix is simple, I will make a PR later today.