Does anyone know of a way to set (mock) the User-Agent of the request object provided by FLask (Werkzeug) during unit testing?

As it currently stands, when I attempt to obtain details such as the request.headers[‘User-Agent’] a KeyError is raised as the Flask test_client() doesn’t set these up. (See partial stack trace below)

When attempting to get the User-Agent from the request object in a Flask project during unit testing, a KeyError is raised.

File "/Users/me/app/rest/", line 515, in login
    if request.headers['User-Agent']:
File "/Users/me/.virtualenvs/app/lib/python2.7/site-packages/werkzeug/", line 1229, in __getitem__
    return self.environ['HTTP_' + key]
    KeyError: 'HTTP_USER_AGENT'


Along with the (accepted) solution below, the environ_base hint lead me to this other SO solution. The premise of this solution is to create a wrapper class for the Flask app and override the call method to automatically set the environment variables. This way, the variables are set for all calls. So, the solution I ended up implementing is creating this proxy class:

class FlaskTestClientProxy(object):
    def __init__(self, app): = app

    def __call__(self, environ, start_response):
        environ['REMOTE_ADDR'] = environ.get('REMOTE_ADDR', '')
        environ['HTTP_USER_AGENT'] = environ.get('HTTP_USER_AGENT', 'Chrome')
        return, start_response)

And then wrapping the WSGI container with that proxy:

app.wsgi_app = FlaskTestClientProxy(app.wsgi_app)
test_client = app.test_client()

You need to pass in environ_base when you call get() or post(). E.g.,

client = app.test_client()
response = client.get('/your/url/', 
                      environ_base={'HTTP_USER_AGENT': 'Chrome, etc'})

Then your request.user_agent should be whatever you pass in, and you can access it via request.headers['User-Agent'].

See for more info.

Even though what @chris-mckinnel wrote works, I wouldn’t opt to use environ_base in this case.

You can easily do something as shown below:

with app.test_request_context('url', headers={'User-Agent': 'Your value'}):

This should do the job and at the same time it keeps your code explicit – explicit is better than implicit.

If you want to know what you can pass to the test_request_context reference the EnvironBuilder definition; which can be found here.