Cast Flask form value to int

Question

I have a form that posts a personId int to Flask. However, request.form['personId'] returns a string. Why isn’t Flask giving me an int?

I tried casting it to an int, but the route below either returned a 400 or 500 error. How can I get I get the personId as an int in Flask?

@app.route('/getpersonbyid', methods = ['POST'])
def getPersonById():
    personId = (int)(request.form['personId'])
    print personId

Asked By: Amritha Dilip

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Answer 1

HTTP form data is a string, Flask doesn’t receive any information about what type the client intended each value to be. So it parses all values as strings.

You can call int(request.form['personId']) to get the id as an int. If the value isn’t an int though, you’ll get a ValueError in the log and Flask will return a 500 response. And if the form didn’t have a personId key, Flask will return a 400 error.

personId = int(request.form['personId'])

Instead you can use the MultiDict.get() method and pass type=int to get the value if it exists and is an int:

personId = request.form.get('personId', type=int)

Now personId will be set to an integer, or None if the field is not present in the form or cannot be converted to an integer.

There are also some issues with your example route.

A route should return something, otherwise it will raise a 500 error. print outputs to the console, it doesn’t return a response. For example, you could return the id again:

@app.route('/getpersonbyid', methods = ['POST'])
def getPersonById():
    personId = int(request.form['personId'])
    return str(personId)  # back to a string to produce a proper response

The parenthesis around int are not needed in Python and in this case are ignored by the parser. I’m assuming you are doing something meaningful with the personId value in the view; otherwise using int() on the value is a little pointless.

Answered By: Martijn Pieters

Cast Flask form value to int

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