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Answer: You build a Flask app by creating an instance of the Flask class. That instance is the WSGI application. The first argument is the name of the application's module or package, and __name__ is the usual value.

Creating the instance

The name tells Flask where to look for templates and static files. To run the application, use the flask command or python -m flask, and pass --app so Flask can find the app. If the file is named app.py or wsgi.py, --app can be omitted. This launches a very simple builtin server, good enough for testing but not what you want in production.

The class in source

The Flask object is implemented as a central WSGI application. Once created, it is the registry for view functions, URL rules, and template configuration.

See also

• flask-routing
• flask-templates

Claims

• id: C1 claim: A Flask instance is the WSGI application. source: S1 evidence: "An instance of this class will be our WSGI application."

• id: C2 claim: The first argument is the application's module or package name. source: S1 evidence: "The first argument is the name of the application's module or package."

• id: C3 claim: The Flask object implements a WSGI application and is the central object. source: S2 evidence: "The flask object implements a WSGI application and acts as the central"

• id: C4 claim: After creation, the object is a registry for views, URL rules, and templates. source: S2 evidence: "it will act as a central registry for"

Sources

• [S1] Quickstart. Flask docs. https://flask.palletsprojects.com/en/stable/quickstart/ . License: BSD-3-Clause. Accessed 2026-11-11.

• [S2] Flask class (sansio App). Flask source. https://github.com/pallets/flask/blob/main/src/flask/sansio/app.py . License: BSD-3-Clause. Accessed 2026-11-11.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted). Sources and notices: Flask documentation and source (pallets/flask): Copyright 2010 Pallets; BSD-3-Clause (https://raw.githubusercontent.com/pallets/flask/main/LICENSE.txt). Text was condensed and changed from the originals.

@misc{iil_note_v4cb506p,
  author = {{iinventedlight}},
  title = {{A Flask application is a Flask instance that is also the WSGI app}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-11},
  url = {https://iinventedlight.com/library/notes/flask-application/},
  note = {Permanent ID iil:note:v4cb506p. License: CC-BY-4.0.},
}

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