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Extracts a specific query parameter value from a URL string, with compatibility for both Python 2 and Python 3.

Purpose

This function parses a URL to extract the value of a specific query string parameter. It handles the differences between Python 2's urlparse module and Python 3's urllib.parse module by checking the Python version and using the appropriate import. The function returns the first value of the specified query parameter key.

Source Code

def parse_query_string(url, key):
    if is_py2:
        import urlparse

        parsed_url = urlparse.urlparse(url)
        return urlparse.parse_qs(parsed_url.query)[key][0]
    else:
        from urllib.parse import parse_qs, urlparse

        parsed_url = urlparse(url)
        return parse_qs(parsed_url.query)[key][0]

Parameters

Parameter Details

url: A string containing the full URL to parse, including the query string (e.g., 'http://example.com/path?key1=value1&key2=value2'). Must be a valid URL format.

key: A string representing the query parameter name to extract from the URL. This should match exactly with a parameter name in the query string (case-sensitive).

Return Value

Returns a string containing the first value associated with the specified query parameter key. If the key has multiple values, only the first one is returned (index [0]). Will raise a KeyError if the key doesn't exist in the query string, or an IndexError if the key exists but has no values.

Dependencies

• urlparse (Python 2 only)
• urllib.parse (Python 3 only)

Required Imports

No imports needed at the call site - the function handles imports internally based on Python version

Conditional/Optional Imports

These imports are only needed under specific conditions:

import urlparse

from urllib.parse import parse_qs, urlparse

Usage Example

# Assuming is_py2 is defined in the module
# is_py2 = sys.version_info[0] == 2

url = 'https://example.com/search?query=python&page=1&sort=desc'

# Extract the 'query' parameter
query_value = parse_query_string(url, 'query')
print(query_value)  # Output: 'python'

# Extract the 'page' parameter
page_value = parse_query_string(url, 'page')
print(page_value)  # Output: '1'

# Note: Will raise KeyError if key doesn't exist
try:
    missing = parse_query_string(url, 'nonexistent')
except KeyError:
    print('Key not found in query string')

Best Practices

• Always wrap calls in try-except blocks to handle KeyError when the query parameter doesn't exist
• Be aware that this function only returns the first value if a parameter appears multiple times in the query string
• Ensure the 'is_py2' variable is properly defined in the module scope before calling this function
• Consider URL-decoding the returned value if it contains encoded characters
• This function is primarily for legacy codebases supporting both Python 2 and 3; for Python 3-only code, use urllib.parse directly
• The function assumes the query parameter has at least one value; it will raise IndexError if the key exists but has an empty list of values

Tags

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