python - not printing additional rows from json

Question

I have the below code but for some reason it's only writing the first row for car and ignoring every car after.

edit - I tried putting a

print field

in the row after

for field in required_fields:

and for some reason it only shows the field when parsing the first row. Every additional row it returns

[]

for each field.

What I see in the csv is the header and one row and then

,,,,

import csv

import json



car_list = [

  {

    "Year": 1997,

    "Make": "Ford",

    "Model": "E350",

    "Description": "ac, abs, moon",

    "Price": 3000

  },

  {

    "Year": 1999,

    "Make": "Chevy",

    "Model": "Venture \"Extended Edition\"",

    "Price": 4900

  },

  {

    "Year": 1999,

    "Model": "Venture \"Extended Edition, Very Large\"",

    "Description": "",

    "Price": 5000

  },

  {

    "Year": 1996,

    "Make": "Jeep",

    "Model": "Grand Cherokee",

    "Description": "MUST SELL!air, moon roof, loaded",

    "Price": 4799

  }

]





def _get_attr(dict_, attrs):

    try:

        src = attrs[:]

        root = attrs.pop(0)

        node = dict_[root]

        null = object()

        for i, attr in enumerate(attrs[:]):

            try:

                node = node.get(attr, null)

            except AttributeError:

                node = null

            if node is null:

                # i+2 pop and last element

                raise ValueError("%s not present (level %s)" % (attr, '->'.join(src[: i+2])))

        return node

    except KeyError:

        raise ValueError("%s not present" % root)



required_fields = [

    ['Year'],

    ['Make'],

    ['Model'],

    ['Description'],

    ['Price']

]



f = csv.writer(open('test.csv', 'wb+'))



f.writerow(['Year',

           'Make',

           'Model',

           'Description',

           'Price'])



for car in car_list:

    row_values = []

    for field in required_fields:

        try:

            row_values.append(_get_attr(car, field))

        except:

           row_values.append('')

    f.writerow(row_values)

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Answer 1

The reason this occurs is because of a combination of two things:

Lists (and more generally mutable objects) are passed by reference in Python. Mutating a list passed as an argument to a function mutates that list globally.
You're mutating attrs passed in to _get_attr by calling attrs.pop(0). This removes the field everywhere, including in required_fields.

A much better way to do what you want while using native Python:

required_fields = ['Year', 'Make', 'Model','Description','Price']

for car in car_list:
    row_values = [car.get(field, '') for field in required_fields]
    f.writerow(row_values)

This immediately takes care of possibly missing values by using the power of get() on dictionaries to supply a default value - '' in this case - if a key is missing, and requires no messy _get_attr function either.

python - not printing additional rows from json

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