Added python notes to the knowledge base

knowledge base/python.md

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+# Python
+
+## String formatting
+
+```python
+# f-strings
+f"Hello, {name}. You are {age}."
+F"{name.lower()} is funny."
+```
+
+## Web servers
+
+### Flask
+
+- `request.args` gets query arguments
+- `request.form` gets POST arguments
+
+```python
+from flask import request, jsonify
+
+@app.route('/get/questions/', methods=['GET', 'POST','DELETE', 'PATCH'])
+def question():
+    if request.method == 'GET':
+        start = request.args.get('start', default=0, type=int)
+        limit_url = request.args.get('limit', default=20, type=int)
+        data = [doc for doc in questions]
+        return jsonify(
+            isError = False,
+            message = "Success",
+            statusCode = 200,
+            data= data
+        ), 200
+    if request.method == 'POST':
+        question = request.form.get('question')
+        topics = request.form.get('topics')
+        return jsonify(
+            isError = True,
+            message = "Conflict",
+            statusCode = 409,
+            data = data
+        ), 409
+```
+
+### WSGI server
+
+You can use `waitress`:
+
+```python
+from flask import Flask
+
+app = Flask(__name__)
+
+@app.route("/")
+def index():
+    return "<h1>Hello!</h1>"
+
+if __name__ == "__main__":
+    from waitress import serve
+    serve(app, host="0.0.0.0", port=8080)
+```
+
+```shell
+pip install flask waitress
+python hello.py
+```
+
+## Maintenance
+
+```shell
+# generate a list of all outdated packages
+pip list --outdated
+
+# upgrade all packages (oneliner)
+pip install --requirement <(pip freeze | sed 's/==/>=/') --upgrade
+
+# remove orphaned dependencies
+# after installation of pip-autoremove
+pip-autoremove
+
+# upgrade onboard pip on mac os x
+pip3 install --user --upgrade pip
+echo 'export PATH="${HOME}/Library/Python/3.8/bin:${PATH}"' >> ${HOME}/.zprofile
+```
+
+## Further readings
+
+- [flask at first run: do not use the development server in a production environment]
+- [f-strings]
+- [data types]
+- [flask example with POST]
+- [multi-value query parameters with flask]
+- [How To Update All Python Packages]
+- [invl/pip-autoremove]
+
+[data types]: https://www.w3schools.com/python/python_datatypes.asp
+[f-strings]: https://realpython.com/python-f-strings/
+[flask at first run: do not use the development server in a production environment]: https://stackoverflow.com/questions/51025893/flask-at-first-run-do-not-use-the-development-server-in-a-production-environmen#54381386
+[flask example with POST]: https://stackoverflow.com/questions/22947905/flask-example-with-post#53725861
+[how to update all python packages]: https://www.activestate.com/resources/quick-reads/how-to-update-all-python-packages/
+[multi-value query parameters with flask]: https://dev.to/svencowart/multi-value-query-parameters-with-flask-3a92
+[invl/pip-autoremove]: https://github.com/invl/pip-autoremove