AttributeError: app.config.from_file missing or misused

AttributeError: app.config.from_file

$ flask --app app.py shell -c "from app import app; app.config.from_file('config.json', load=json.load)"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AttributeError: 'Config' object has no attribute 'from_file'

Why this happens

Older Flask versions (and some forks) may not offer Config.from_file. Or you tried to call it without providing a loader function (load=) for non-Python formats. In other cases, you replaced app.config with a plain dict, losing methods.

Fix

• Use a supported Flask version that includes Config.from_file, or use from_envvar/direct Python loading.
• Do not overwrite app.config with a dict.
• Provide a loader callable for the file format.

Wrong code

from flask import Flask
import json
app = Flask(__name__)

# Overwriting app.config removes methods
app.config = {}
app.config.from_file('config.json', load=json.load)

Fixed code

from flask import Flask
import json
app = Flask(__name__)

# Keep Config object and provide a loader
app.config.from_file('config.json', load=json.load)
# Or fallback:
with open('config.json') as f:
    app.config.update(json.load(f))

Tip

Prefer environment variables plus Config.from_mapping to avoid file IO during startup.