How to Parse Query String to Dict with Duplicate Keys in Python

Convert a URL query string into a Python dictionary, merging duplicate keys into lists while keeping single values as scalars.

Easy Python 3.9+ Aug 9, 2026 Dictionaries & sets 13 views 0 copies

query-string dict url-parsing urllib

Python code

12 lines
Python 3.9+
Run it 
from urllib.parse import parse_qs


def parse_query_to_dict(query_string):
    parsed = parse_qs(query_string, keep_blank_values=True)
    return {key: values if len(values) > 1 else values[0] for key, values in parsed.items()}


if __name__ == "__main__":
    query = "name=John&name=Jane&age=30&city=&city=Paris&empty_prop="
    result = parse_query_to_dict(query)
    print(result)

Output

stdout 
{'name': ['John', 'Jane'], 'age': '30', 'city': ['', 'Paris'], 'empty_prop': ''}

How it works

The parse_qs function from urllib.parse splits a query string into key-value pairs, automatically handling URL decoding and duplicate keys by grouping values into lists. The dictionary comprehension then converts single-element lists back to plain values while keeping multi-element lists intact. keep_blank_values=True ensures empty values like city= and empty_prop= are preserved in the output. This approach keeps the result compact for common single-value parameters while explicitly showing duplicates.

Common mistakes

  • Forgetting `keep_blank_values=True` drops empty parameters like `city=`
  • Assuming duplicate keys are lost; always remember parse_qs groups them into lists
  • Overlooking that all values are lists by default, requiring conversion logic
  • Not URL-decoding values, though parse_qs handles percent-encoding automatically

Variations

  1. Use `urllib.parse.parse_qsl` and manually group with a defaultdict for custom merge rules
  2. Return flat lists for all keys: `{k: v for k, v in parse_qs(q).items()}` if uniform handling is preferred

Real-world use cases

  • Parsing multi-value form submissions like checkboxes or multi-select filters in a web framework.
  • Extracting repeated query parameters in API request handling where users may pass the same filter twice.
  • Normalizing incoming webhook URLs for analytic tracking where campaign tags can appear more than once.

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