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[JSap0914]

Bug

Config.from_prefixed_env silently crashes with an opaque TypeError when a flat env var and a nested env var share the same config key prefix.

Root cause: env vars are loaded in sorted() order, so FLASK_FOO=bar is always processed before FLASK_FOO__X=1. After the flat key sets self["FOO"] = "bar", the nested traversal does:

current = self["FOO"]   # "bar" (str)
current["X"] = value    # TypeError: 'str' object does not support item assignment

The internal TypeError gives no hint which env var caused the conflict.

Fix

Add an isinstance check before descending into the intermediate dict. When the existing value is not a dict, raise a ValueError that names the conflicting environment variable.

Verification

$ pytest tests/test_config.py -v -k prefixed
tests/test_config.py::test_from_prefixed_env PASSED
tests/test_config.py::test_from_prefixed_env_custom_prefix PASSED
tests/test_config.py::test_from_prefixed_env_nested PASSED
tests/test_config.py::test_from_prefixed_env_non_dict_intermediate PASSED
4 passed in 0.07s

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