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Using pprint() to print dictionaries while preserving their key order
Anubhav Gain
2024-08-11
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Using pprint() to print dictionaries while preserving their key order#

While parsing a CSV file using csv.DictReader today I noticed the following surprising result:

import csv
from io import StringIO
from pprint import pprint


csv_content = """id,name,latitude,longitude,country
nyc,New York City,40.7128,-74.006,US
lon,London,51.5074,-0.1278,GB
tok,Tokyo,35.6895,139.6917,JP"""


rows = list(csv.DictReader(StringIO(csv_content)))
pprint(rows)

This output the following:

[{'country': 'US',
  'id': 'nyc',
  'latitude': '40.7128',
  'longitude': '-74.006',
  'name': 'New York City'},
 {'country': 'GB',
  'id': 'lon',
  'latitude': '51.5074',
  'longitude': '-0.1278',
  'name': 'London'},
 {'country': 'JP',
  'id': 'tok',
  'latitude': '35.6895',
  'longitude': '139.6917',
  'name': 'Tokyo'}]

I had expected DictReader() to preserve the order of keys from the CSV file, since Python has defaulted to ordered dictionaries for a while now.

It turns out DictReader() does do that… but pprint() does not! From the pprint documentation:

pprint.pprint(object, stream=None, indent=1, width=80, depth=None, *, compact=False, sort_dicts=True, underscore_numbers=False)

Note that sort_dicts defaults to True, presumably for historical reasons.

So the following does what I had expected in the first place:

pprint(rows, sort_dicts=False)

Output:

[{'id': 'nyc',
  'name': 'New York City',
  'latitude': '40.7128',
  'longitude': '-74.006',
  'country': 'US'},
 {'id': 'lon',
  'name': 'London',
  'latitude': '51.5074',
  'longitude': '-0.1278',
  'country': 'GB'},
 {'id': 'tok',
  'name': 'Tokyo',
  'latitude': '35.6895',
  'longitude': '139.6917',
  'country': 'JP'}]
Using pprint() to print dictionaries while preserving their key order
https://mranv.pages.dev/posts/using-pprint-to-print-dictionaries-while-preserving-their-key-order/
Author
Anubhav Gain
Published at
2024-08-11
License
CC BY-NC-SA 4.0
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