4

First of all, I'm still a newb to python so please take it easy on me.

I've done my research and I have a basic understanding of how to write a recursive function, but I'm totally confused at the task ahead of me.

I would really like some tips and pointers so I can do this myself to really understand.

Here's the task:

  1. Iterate through a dictionary
  2. check if there is a 'children' key
  3. if true, do something
  4. repeat

Here's the issue,

The 'option' key is nested in another key called 'children'. There for I would like to iterate through each 'children' key until they exist no more.

Here's what the code looks like without recursion. Of course it fails because its too explicate, too rigid.

#level(one/two/three) refer to the level of nesting, easier for me to know where I am     in the loop.
#qdf_v2 is a particular type of file which is essentially a dictionary 

for levelone in qdf_v2['children']:
    if 'children' in levelone:
        store = options
        for leveltwo in levelone['children']:
            if 'children' in leveltwo:
                store = options
                for levelthree in leveltwo['children']:
                    if 'children' in levellevel:
                        repeat....

I would like to loop through the dictionary and it's 'children' key until they don't exist anymore.

Here's some JSON you can copy and past this in http://www.jsoneditoronline.org/ to see what I'm working with.

{
    "base": "http://panoptic-fearless.ldc.yougov.net/questionnaires/Test_maj_demo_newG4_2/versions/12/",
    "children": [
        {
            "name": "one",
            "children": [
                {
                    "name": "measures",
                    "children": [
                        {
                            "type": "single",
                            "options": {
                                "sort_order": "asending",
                                "chart_layout": "5",
                                "chart_type": "bar",
                                "chart_color": "green"
                            },
                            "text": "How much would you say you like or dislike P.E. at school? ",
                            "name": "KYS_Q1",
                            "responses": {
                                "items": [
                                    {
                                        "code": 1,
                                        "text": "I love it",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 2,
                                        "text": "I like it",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 3,
                                        "text": "I don’t like it",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 4,
                                        "text": "I hate it",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 5,
                                        "text": "Don’t know",
                                        "element": "qdf:response"
                                    }
                                ],
                                "element": "qdf:response_group"
                            },
                            "element": "qdf:question",
                            "export": [
                                {
                                    "name": "KYS_Q1"
                                }
                            ]
                        },
                        {
                            "children": [
                                {
                                    "type": "single",
                                    "options": {
                                        "sort_order": "asending",
                                        "chart_layout": "4",
                                        "chart_type": "pie",
                                        "chart_color": "pink"
                                    },
                                    "text": "{multiple order=\"randomize\" max=3} You said that you $event P.E. at school...<br/> Which THREE of the following are your MAIN reasons for not liking P.E. at school? (Please tick up to three options)",
                                    "name": "KYS_Q2",
                                    "responses": {
                                        "items": [
                                            {
                                                "code": 1,
                                                "text": "I don't like going outside in bad weather (e.g. in the cold, rain etc.)",
                                                "element": "qdf:response"
                                            },
                                            {
                                                "code": 2,
                                                "text": "I’m always picked last to join a team",
                                                "element": "qdf:response"
                                            },
                                            {
                                                "code": 3,
                                                "text": "I don't like my P.E. teacher/ don’t think they are very good",
                                                "element": "qdf:response"
                                            },
                                            {
                                                "code": 4,
                                                "text": "I’m not very good at it",
                                                "element": "qdf:response"
                                            },
                                            {
                                                "code": 5,
                                                "text": "I find the games/ sports we play boring",
                                                "element": "qdf:response"
                                            }
                                        ],
                                        "element": "qdf:response_group"
                                    },
                                    "element": "qdf:question",
                                    "export": [
                                        {
                                            "name": "KYS_Q2"
                                        }
                                    ]
                                },
                                {
                                    "type": "single",
                                    "options": {
                                        "sort_order": "asending",
                                        "chart_layout": "5",
                                        "chart_type": "line",
                                        "chart_color": "green"
                                    },
                                    "text": "{multiple order=\"randomize\"} Below are some feelings that people can have about doing P.E. at school...<br/> Which of the following words describe how you feel about doing P.E. at school? (Please tick all that apply) ",
                                    "name": "KYS_Q3A",
                                    "responses": {
                                        "items": [
                                            {
                                                "code": 1,
                                                "text": "Stressed",
                                                "element": "qdf:response"
                                            },
                                            {
                                                "code": 2,
                                                "text": "Scared",
                                                "element": "qdf:response"
                                            },
                                            {
                                                "code": 3,
                                                "text": "Worried",
                                                "element": "qdf:response"
                                            },
                                            {
                                                "code": 4,
                                                "text": "Sick",
                                                "element": "qdf:response"
                                            },
                                            {
                                                "code": 5,
                                                "text": "Tiring ",
                                                "element": "qdf:response"
                                            }
                                        ],
                                        "element": "qdf:response_group"
                                    },
                                    "element": "qdf:question",
                                    "export": [
                                        {
                                            "name": "KYS_Q3A"
                                        }
                                    ]
                                }
                            ],
                            "element": "qdf:section"
                        },
                        {
                            "type": "single",
                            "options": {
                                "sort_order": "desending",
                                "chart_layout": "5",
                                "chart_type": "bar",
                                "chart_color": "green"
                            },
                            "text": "{single order=\"randomize\"} You said that you $event P.E...<br/> Which ONE of the following is the main reason you like it? ",
                            "name": "KYS_Q4",
                            "responses": {
                                "items": [
                                    {
                                        "code": 1,
                                        "text": "I like getting outside/ not being in the classroom",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 2,
                                        "text": "I like my P.E. teacher",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 3,
                                        "text": "I’m good at it",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 4,
                                        "text": "I find the games/ sports fun",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 5,
                                        "text": "I find the games/ sports easy",
                                        "element": "qdf:response"
                                    }
                                ],
                                "element": "qdf:response_group"
                            },
                            "element": "qdf:question",
                            "export": [
                                {
                                    "name": "KYS_Q4"
                                }
                            ]
                        }
                    ],
                    "element": "qdf:page"
                }
            ],
            "element": "qdf:module"
        },
        {
            "name": "two",
            "children": [
                {
                    "name": "test",
                    "children": [
                        {
                            "type": "single",
                            "options": {
                                "sort_order": "asending",
                                "chart_layout": "5",
                                "chart_type": "bar",
                                "chart_color": "green"
                            },
                            "text": "{multiple order=\"randomize\"}Below are some feelings that people can have about doing P.E at school...<br/> Which of the following words describe how you feel about doing P.E. at school? (Please tick all that apply)   ",
                            "name": "KYS_Q5A",
                            "responses": {
                                "items": [
                                    {
                                        "code": 1,
                                        "text": "Energised/ Awake",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 2,
                                        "text": "Excited",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 3,
                                        "text": "Relaxed",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 4,
                                        "text": "Confident",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 5,
                                        "text": "Happy",
                                        "element": "qdf:response"
                                    }
                                ],
                                "element": "qdf:response_group"
                            },
                            "element": "qdf:question",
                            "export": [
                                {
                                    "name": "KYS_Q5A"
                                }
                            ]
                        },
                        {
                            "type": "single",
                            "options": {
                                "sort_order": "asending",
                                "chart_layout": "25",
                                "chart_type": "column",
                                "chart_color": "orange"
                            },
                            "text": "{multiple order=\"randomize\"} For the following question please do not be afraid to be as honest as possible, no one else will be told your personal answers. <br/> Which of the following have you EVER done in order to avoid taking part in P.E. at school? (Please tick all that apply) ",
                            "name": "KYS_Q6",
                            "responses": {
                                "items": [
                                    {
                                        "code": 1,
                                        "text": "Lied about forgetting my kit",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 2,
                                        "text": "Lied about an injury",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 3,
                                        "text": "Lied about feeling unwell",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 4,
                                        "text": "Faked a note from my parents/ guardian",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 5,
                                        "text": "Faked a note from my doctor/ health professional",
                                        "element": "qdf:response"
                                    }
                                ],
                                "element": "qdf:response_group"
                            },
                            "element": "qdf:question",
                            "export": [
                                {
                                    "name": "KYS_Q6"
                                }
                            ]
                        },
                        {
                            "type": "single",
                            "options": {
                                "sort_order": "asending",
                                "chart_layout": "5",
                                "chart_type": "bar",
                                "chart_color": "green"
                            },
                            "text": "{multiple order=\"randomize\"} For the following question, please think about the teacher who has taught you P.E. MOST recently and if you have more than one teacher teaching you P.E. please think about the one who has taught you most often. <br/>  Which of the following statements do you agree with? (Please tick all that apply)",
                            "name": "KYS_Q7",
                            "responses": {
                                "items": [
                                    {
                                        "code": 1,
                                        "text": "My P.E. teacher inspires me to do more exercise",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 2,
                                        "text": "My P.E. teacher motivates me to take part in P.E. lessons",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 3,
                                        "text": "My P.E. teacher scares me",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 4,
                                        "text": "My P.E. teacher makes P.E. boring",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 5,
                                        "text": "My P.E. teacher makes P.E. fun",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 6,
                                        "text": "My P.E. teacher makes P.E. difficult/ hard",
                                        "element": "qdf:response"
                                    }
                                ],
                                "element": "qdf:response_group"
                            },
                            "element": "qdf:question",
                            "export": [
                                {
                                    "name": "KYS_Q7"
                                }
                            ]
                        },
                        {
                            "type": "single",
                            "options": {
                                "sort_order": "asending",
                                "chart_layout": "5",
                                "chart_type": "bar",
                                "chart_color": "green"
                            },
                            "text": "{multiple order=\"randomize\"} What would make you like P.E. more? (Please tick all that apply)",
                            "name": "KYS_Q8",
                            "responses": {
                                "items": [
                                    {
                                        "code": 1,
                                        "text": "If the P.E. teacher was nicer to me",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 2,
                                        "text": "If more lessons took place indoors (e.g. in the sports hall or a gym)",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 3,
                                        "text": "If more lessons took place outdoors",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 4,
                                        "text": "If we did more interesting sports/ activities",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 5,
                                        "text": "If we had more choice of the types of sport we had to do",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "code": 6,
                                        "text": "If we could wear what we wanted for the lesson",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "options": {
                                            "fixed": true
                                        },
                                        "children": [
                                            {
                                                "export": [
                                                    {
                                                        "name": "KYS_Q8other"
                                                    }
                                                ],
                                                "name": "KYS_Q8other",
                                                "type": "open",
                                                "element": "qdf:question"
                                            }
                                        ],
                                        "code": 7,
                                        "text": "Other ",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "options": {
                                            "xor": true,
                                            "fixed": true
                                        },
                                        "code": 8,
                                        "text": "Don’t know",
                                        "element": "qdf:response"
                                    },
                                    {
                                        "options": {
                                            "xor": true,
                                            "fixed": true
                                        },
                                        "code": 9,
                                        "text": "Not applicable – nothing would make me like P.E. more than I already do",
                                        "element": "qdf:response"
                                    }
                                ],
                                "element": "qdf:response_group"
                            },
                            "element": "qdf:question",
                            "export": [
                                {
                                    "name": "KYS_Q8"
                                }
                            ]
                        }
                    ],
                    "element": "qdf:page"
                }
            ],
            "element": "qdf:module"
        },
        {
            "children": [
                {
                    "children": [
                        {
                            "text": "​\n",
                            "element": "qdf:text"
                        }
                    ],
                    "element": "qdf:page"
                }
            ],
            "element": "qdf:module"
        }
    ],
    "panelid": 13,
    "created": "#2014-01-10T15:57:18.594000+00:00#",
    "typespec": {
        "src": "/questionnaires/typespecs/initial.qdf",
        "element": "qdf:typespec"
    },
    "qdf-ext:ext": [
        "meaningful-page-names"
    ],
    "element": "qdf:questionnaire",
    "builtin": {
        "src": "/questionnaires/builtins/initial.qdf",
        "element": "qdf:typedef"
    },
    "committer": "majeed.sahebzadha",
    "name": "Test_maj_demo_newG4_2",
    "commit message": "added a new module within a module"
}
3
  • Could you share an example dictionary? Commented Jan 10, 2014 at 16:43
  • And an expected output? Commented Jan 10, 2014 at 16:44
  • the output is slightly difficult to display. But basically I store the values of a 'option' key in the 'children' key and send it to another file via a function. But i just needed to understand how to repeat the process. Commented Jan 10, 2014 at 17:01

3 Answers 3

8

I would approach this as follows:

def recursive(input, output=None):
    if output is None:
        output = {} # container to store results
    if 'children' in input:
        # do whatever, add things to output
        recursive(input['children'], output)
    return output

This way, the output dictionary is accessible to all depths of iteration and gets returned with all contents at the end. This means that you don't have to explicitly deal with the return values of the recursive calls.

Depending on exactly what you have, it may look more like:

def recursive(input, output=None):
    if output is None:
        output = {} # container to store results
    if 'children' in input:
        for child in input['children']:
            # do whatever, add things to output
            recursive(child, output)
    return output    

And output may be a different container (e.g. list, set).

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4 Comments

Does this ever add anything to output? I'm confused.
@adsmith: since we have no idea what the OP wants to output, this answer leaves that to be filled in.
My example doesn't explicitly add anything to it, but then I don't know what the OP wants to extract! The point is that the same object is shared among all recursive calls, so you can collect all results together.
Since OP replied explaining he wanted the values from another key in each recursion, this is hands-down the preferred implementation.
0
def recurseDict(dictionary):
    if "children" in dictionary:
        return recurseDict(dictionary['children'])
    else:
        return "Whatever you want to return here -- you didn't specify"

This will take a dictionary, check if "children" is one of its keys, then recurse through the dictionary dictionary['children'] and etc etc until it gets to a point where there isn't a 'children' key (e.g. the dictionary has no further branches) where it goes into the else branch and you can return whatever the heck you want -- you didn't specify.

4 Comments

-1 -- if "children" in dictionary.keys() this is absolutely the wrong way of checking if a key is in a dictionary.
@MartijnPieters oops! I'm rusty with self-recursing functions. They never feel like the right answer to me. @Bakurio: changed to if 'children' in dictionary -- I never remember if I can do that or not so I tend to default to building a list of keys and checking for that instead.
@Bakuriu I suppose I could just do try: return recurseDict(dictionary['children']) except KeyError: return "whatever OP wants" if you think that would be more Pythonic.
No, you are missing the point. The right way to check if a key is in a dictionary is to write it in English: key in dictionary. Without calling .keys().
-1

Here is an example approach to recursively go through a dictionary, assuming that each child is also a dictionary:

def compute(my_dict):
    if 'children' in my_dict:
        print my_dict['children']
        for child in my_dict['children']:
            compute(child)

4 Comments

if 'children' in my_dict will fail if my_dict isn't an iterable and print my_dict['children'] will fail if my_dict isn't dict-like. You need to either check the type of object (using is_instance) or put these expressions in try ... except blocks. I prefer duck-typing, i.e. using try ... except.
@SteinarLima if this function isn't exposed to the user, there's no reason to expect anything but a dict to be passed to it. If you're incredibly concerned about it, you could write the first line of def compute to be assert typeof(my_dict) == dict which will instantly throw an AssertionError if you've given it anything it can't deal with.
What about my_dict = {'children': ['children', 'of', 'men']}. This is a recursive function, so it's not enough that the initial my_dictis a dict, all subitems need to be a dict as well.
Yes, which is how a data structure should be designed. When the children key is present it should contain child nodes, not values. Duck typing should only be used when needed. For example, for external data structures this can be necessary but if you design the data structure then it doesn't make sense to mix child nodes and values at different levels for the key children.

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