flattening nested Json in pandas data frame

Question

I am trying to load the json file to pandas data frame. I found that there were some nested json. Below is the sample json:

{'events': [{'id': 142896214,
   'playerId': 37831,
   'teamId': 3157,
   'matchId': 2214569,
   'matchPeriod': '1H',
   'eventSec': 0.8935539999999946,
   'eventId': 8,
   'eventName': 'Pass',
   'subEventId': 85,
   'subEventName': 'Simple pass',
   'positions': [{'x': 51, 'y': 49}, {'x': 40, 'y': 53}],
   'tags': [{'id': 1801, 'tag': {'label': 'accurate'}}]}

I used the following code to load json into dataframe:

with open('EVENTS.json') as f:
    jsonstr = json.load(f)

df = pd.io.json.json_normalize(jsonstr['events'])

Below is the output of df.head()

But I found two nested columns such as positions and tags.

I tried using the following code to flatten it:

Position_data = json_normalize(data =jsonstr['events'], record_path='positions', meta = ['x','y','x','y'] )

It showed me an error as follow:

KeyError: "Try running with errors='ignore' as key 'x' is not always present"

Can you advise me how to flatten positions and tags ( those having nested data).

Thanks, Zep

Answer 1

If you are looking for a more general way to unfold multiple hierarchies from a json you can use recursion and list comprehension to reshape your data. One alternative is presented below:

def flatten_json(nested_json, exclude=['']):
    """Flatten json object with nested keys into a single level.
        Args:
            nested_json: A nested json object.
            exclude: Keys to exclude from output.
        Returns:
            The flattened json object if successful, None otherwise.
    """
    out = {}

    def flatten(x, name='', exclude=exclude):
        if type(x) is dict:
            for a in x:
                if a not in exclude: flatten(x[a], name + a + '_')
        elif type(x) is list:
            i = 0
            for a in x:
                flatten(a, name + str(i) + '_')
                i += 1
        else:
            out[name[:-1]] = x

    flatten(nested_json)
    return out

Then you can apply to your data, independent of nested levels:

New sample data

this_dict = {'events': [
  {'id': 142896214,
   'playerId': 37831,
   'teamId': 3157,
   'matchId': 2214569,
   'matchPeriod': '1H',
   'eventSec': 0.8935539999999946,
   'eventId': 8,
   'eventName': 'Pass',
   'subEventId': 85,
   'subEventName': 'Simple pass',
   'positions': [{'x': 51, 'y': 49}, {'x': 40, 'y': 53}],
   'tags': [{'id': 1801, 'tag': {'label': 'accurate'}}]},
 {'id': 142896214,
   'playerId': 37831,
   'teamId': 3157,
   'matchId': 2214569,
   'matchPeriod': '1H',
   'eventSec': 0.8935539999999946,
   'eventId': 8,
   'eventName': 'Pass',
   'subEventId': 85,
   'subEventName': 'Simple pass',
   'positions': [{'x': 51, 'y': 49}, {'x': 40, 'y': 53},{'x': 51, 'y': 49}],
   'tags': [{'id': 1801, 'tag': {'label': 'accurate'}}]}
]}

Usage

pd.DataFrame([flatten_json(x) for x in this_dict['events']])

Out[1]:
          id  playerId  teamId  matchId matchPeriod  eventSec  eventId  \
0  142896214     37831    3157  2214569          1H  0.893554        8   
1  142896214     37831    3157  2214569          1H  0.893554        8   

  eventName  subEventId subEventName  positions_0_x  positions_0_y  \
0      Pass          85  Simple pass             51             49   
1      Pass          85  Simple pass             51             49   

   positions_1_x  positions_1_y  tags_0_id tags_0_tag_label  positions_2_x  \
0             40             53       1801         accurate            NaN   
1             40             53       1801         accurate           51.0   

   positions_2_y  
0            NaN  
1           49.0  

Note that this flatten_json code is not mine, I have seen it here and here without much certainty of the original source.

Answer 2

• As noted in the accepted answer, flatten_json can be a great option, depending on the structure of the JSON, and how the structure should be flattened.
  • In this case the OP wants all the values for 1 event, to be on a single row, so flatten_json works
  • If the desired result is for each position in positions to have a separate row, then pandas.json_normalize is the better option.
• An issue with flatten_json is, if there are many positions, then the number of columns for each event in events can be very large.
• See How to flatten a nested JSON recursively, with flatten_json? for a more thorough explanation if using flatten_json.

Create 1 row for each dict in events

data = {'events': [{'id': 142896214,
                    'playerId': 37831,
                    'teamId': 3157,
                    'matchId': 2214569,
                    'matchPeriod': '1H',
                    'eventSec': 0.8935539999999946,
                    'eventId': 8,
                    'eventName': 'Pass',
                    'subEventId': 85,
                    'subEventName': 'Simple pass',
                    'positions': [{'x': 51, 'y': 49}, {'x': 40, 'y': 53}],
                    'tags': [{'id': 1801, 'tag': {'label': 'accurate'}}]}]}

Create the DataFrame

df = pd.DataFrame.from_dict(data)
df = df['events'].apply(pd.Series)

Flatten positions with pd.Series

df_p = df['positions'].apply(pd.Series)

df_p_0 = df_p[0].apply(pd.Series)
df_p_1 = df_p[1].apply(pd.Series)

Rename positions[0] & positions[1]:

df_p_0.columns = ['pos_0_x', 'pos_0_y']
df_p_1.columns = ['pos_1_x', 'pos_1_y']

Flatten tags with pd.Series:

df_t = df.tags.apply(pd.Series)
df_t = df_t[0].apply(pd.Series)
df_t_t = df_t.tag.apply(pd.Series)

Rename id & label:

df_t =  df_t.rename(columns={'id': 'tags_id'})
df_t_t.columns = ['tags_tag_label']

Combine them all with pd.concat:

df_new = pd.concat([df, df_p_0, df_p_1, df_t.tags_id, df_t_t], axis=1)

Drop the old columns:

df_new = df_new.drop(['positions', 'tags'], axis=1)

Create a separate row for each position in positions

# normalize events
df = pd.json_normalize(data, 'events')

# explode all columns with lists of dicts
df = df.apply(lambda x: x.explode()).reset_index(drop=True)

# list of columns with dicts
cols_to_normalize = ['positions', 'tags']

# if there are keys, which will become column names, overlap with excising column names
# add the current column name as a prefix
normalized = list()
for col in cols_to_normalize:
    
    d = pd.json_normalize(df[col], sep='_')
    d.columns = [f'{col}_{v}' for v in d.columns]
    normalized.append(d.copy())

# combine df with the normalized columns
df = pd.concat([df] + normalized, axis=1).drop(columns=cols_to_normalize)

# display(df)
          id  playerId  teamId  matchId matchPeriod  eventSec  eventId eventName  subEventId subEventName  positions_x  positions_y  tags_id tags_tag_label
0  142896214     37831    3157  2214569          1H  0.893554        8      Pass          85  Simple pass           51           49     1801       accurate
1  142896214     37831    3157  2214569          1H  0.893554        8      Pass          85  Simple pass           40           53     1801       accurate