Why does the Flask bool query parameter always evaluate to true?

Question

I have an odd behavior for one of my endpoints in my Flask application which accepts boolean query parameters. No matter what I pass to it, such as asfsdfd or true or false, it is considered true. Only by leaving it empty does it become false.

full_info = request.args.get("fullInfo", default=False, type=bool)
if full_info:
    # do stuff

It seems to be that either any input is considered to be true. Is there any way to make this work with the Flask intended way of defining the type, or do I need to accept a string and compare it?

Answer 1

The type parameter of request.args.get is a bit misleading, as it's not for specifying the value's type, but for specifying a callable:

• type – A callable that is used to cast the value in the MultiDict. If a ValueError is raised by this callable the default value is returned.

It accepts a callable (ex. a function), applies that callable to the query parameter value, and returns the result. So the code

request.args.get("fullInfo", default=False, type=bool)

calls bool(value) where value is the query parameter value. In Flask, the query parameter values are always stored as strings. And unfortunately, calling bool() on a non-empty string will always be True:

In [10]: bool('true')
Out[10]: True

In [11]: bool('false')
Out[11]: True

In [12]: bool('any non-empty will be true')
Out[12]: True

In [13]: bool('')
Out[13]: False

Instead of bool, you can pass a function that explicitly checks if the string is equal to the literal string true (or whichever value your API rules consider as true-ish):

full_info = request.args.get('fullInfo', default=False, type=lambda v: v.lower() == 'true')

return jsonify({'full_info': full_info})

$ curl -XGET http://localhost:5000/test?fullInfo=false
{"full_info":false}

$ curl -XGET http://localhost:5000/test?fullInfo=adasdasd
{"full_info":false}

$ curl -XGET http://localhost:5000/test?fullInfo=11431423
{"full_info":false}

$ curl -XGET http://localhost:5000/test?fullInfo=
{"full_info":false}

$ curl -XGET http://localhost:5000/test?fullInfo=true
{"full_info":true}

$ curl -XGET http://localhost:5000/test?fullInfo=TRUE
{"full_info":true}

$ curl -XGET http://localhost:5000/test
{"full_info":false}

Answer 2

A "trick" is to use json.loads as type. It will act as a "factory" that will build a bool True/False from the strings 'true'/'false'

Answer 3

This is expected as query string is an actual string, hence when you get a string no matter what it is, if it's not empty, it will be true. As in:

>>>bool('False')
True

You will have to do string comparison if you want to get a boolean.