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I have a simple code to fetch users from db using sqlalchemy and return them as json. My problem is how to format the output to get something like this:

{"results": [{"id":1, "username":"john"},{"id":2,"username":"doe"}]}

my code outputs an error which I cant seem to fix being a newbie in python:

d = [] for user in Users.query.all(): v = {} for columnName in Users.__table__.columns.keys(): v[columnName] = getattr( user, columnName ) d.append( v ) return jsonify( d )

The code says:

ValueError: dictionary update sequence element #0 has length 11; 2 is required

Thanks.

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  • 1
    In which line do you get this error? Commented May 30, 2011 at 19:20
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    Could you paste the whole stacktrace? Commented May 30, 2011 at 19:20
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    Not the reason for your error, but if you want to have that data structure, you need to change the return line to: return jsonify( {"results": d} ) Commented May 30, 2011 at 19:27
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    Full stack trace at: paste.pocoo.org/show/397870 Full code at: paste.pocoo.org/show/397871 Commented May 30, 2011 at 19:32
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    Also, you're using a property that's supposed to be private (__table__), there are other ways to get that metadata, using the API plus some helpers. Commented May 30, 2011 at 19:43

4 Answers 4

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10

I solved this error by simply saying

return jsonify( results = d )

instead of 

return jsonify( d )
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Nice. The reason behind this is that the jsonify in flask will not encode a raw array (in square brackets) at the top level. I don't know exactly why, but that is considered unsafe.
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Ah, now that your code has been pasted, I can see that the fundamental problem is indeed coming from jsonify. The below workaround should be satisfactory. 

>>> import json >>> json.dumps({"results": [{"id":1, "username":"john"},{"id":2,"username":"doe"}]}) '{"results": [{"username": "john", "id": 1}, {"username": "doe", "id": 2}]}'

Replace jsonify with json.dumps, and let me know if that doesn't fix the problem.

But if you'd prefer to use flask.jsonify, then you should take a look at the flask documentation. The argument to jsonify should be the same as the argument to any dict constructor -- i.e. a dict or an iterable of tuples. So that's the problem.

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BTW, simplejson lets you register additional encoders with it so you can extend it with more complex objects.
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You're calling dict() with *args, which is going to expand args to positional parameters. That is a typical cause of that error you are seeing. Leave that out.

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He is somewhere, or is being called with some parameters he provided.
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The error is what you'd expect when creating a dict incorrectly - see Python dictionary creation error

Edited: See answer from senderle that identifies the root cause...

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