Pxwebpy `code_list` Troubleshooting With SSB API

by Alex Johnson 49 views

Introduction

This article addresses a common issue encountered when using the pxwebpy library to interact with the SSB (Statistics Norway) API, specifically concerning the code_list functionality. Many users, especially newcomers to the library, may struggle to understand how to correctly implement code_list for data aggregation. This article aims to provide a comprehensive guide to troubleshooting code_list usage, understanding bad requests, and ensuring proper data aggregation when using pxwebpy with the SSB API. Let's dive in and explore the intricacies of using code_list effectively with pxwebpy for the SSB API.

Understanding the Problem: code_list and Bad Requests

The core issue revolves around constructing the correct API request using pxwebpy to leverage the code_list parameter. The code_list parameter is crucial for specifying aggregations directly within the API request, which can significantly simplify data retrieval and manipulation. When using pxwebpy, a common scenario involves attempting to replicate a working URL request directly in Python, only to encounter a bad request error. This often stems from a misunderstanding of how pxwebpy translates the code_list parameter into the final URL.

One example of this problem can be seen in the user's attempt to translate the following URL:

https://data.ssb.no/api/pxwebapi/v2/tables/05889/data?lang=no&valueCodes[Region]=K-3305&valueCodes[ContentsCode]=Godkjente&valueCodes[Tid]=2020K1,2020K2,2020K3,2020K4&codelist[Region]=agg_KommSummer

into pxwebpy code:

api = PxApi("ssb")
api.language = "no"
data = api.get_table_data(
 "05889", 
 value_codes={
 "Region": "3305",
 "ContentsCode": "Godkjente",
 "Tid": "2020*",
 }, 
 code_list={
 "Region": "agg_KommSummer"
 }
)

When this code results in a bad request, it indicates a discrepancy between the intended API call and the actual request generated by pxwebpy. To effectively troubleshoot this, it’s important to understand how pxwebpy handles the code_list parameter and how it differs from a direct URL construction.

Common Pitfalls

  • Incorrect Parameter Names: One frequent mistake is using incorrect parameter names. The SSB API is particular about the names used for dimensions and codes. If there's a slight variation, the API might not recognize the request, leading to a bad request error. For example, mistaking "Region" for "Aggregation" can alter the API's interpretation of the request.
  • Misunderstanding of Aggregation: Another pitfall is misunderstanding how aggregation works within the SSB API. Aggregation, facilitated by code_list, combines data based on predefined hierarchies. If the aggregation doesn't yield the expected results, it could be due to an incorrect code_list specification or an issue with the underlying data structure.
  • Missing or Incorrect Wildcards: When specifying time periods or other codes, the use of wildcards (e.g., "2020*") is critical. An incorrect or missing wildcard can lead to incomplete data retrieval or errors.

Diagnosing Bad Requests and Aggregation Issues

To diagnose bad requests and aggregation issues, you can take several steps:

  1. Inspect the Generated URL: A crucial step in debugging is to inspect the actual URL generated by pxwebpy. Unfortunately, as the user pointed out, there isn't a built-in way to directly inspect the request URL in pxwebpy. However, you can use logging or other debugging techniques to capture the parameters being sent.
  2. Simplify the Request: Start with a minimal request and gradually add parameters. This helps isolate the parameter causing the issue. For example, remove the code_list parameter and check if the request works. If it does, the problem likely lies in the code_list configuration.
  3. Consult the API Documentation: The SSB API documentation is the ultimate source of truth. Review the documentation for the specific table you're querying, paying close attention to the dimension names, code lists, and valid values.
  4. Test with Direct URL Requests: Construct the URL manually and test it using a tool like curl or Postman. This helps verify that the API endpoint and parameters are correct.
  5. Check for Typos: A simple typo in a parameter name or value can lead to a bad request. Double-check all spellings and casing.

Troubleshooting Steps and Solutions

Let's walk through a systematic approach to troubleshooting the example provided by the user:

Step 1: Verify Parameter Names

First, ensure that all parameter names are correct. In the example, the user initially tried changing "Region" to "Aggregation." While this prevented a bad request, it didn't produce the desired aggregation. The key is to consult the SSB API documentation for the specific table (05889) to identify the correct dimension names. It's possible that the dimension is indeed named "Region," but the code_list might be expecting a different name or structure.

Step 2: Simplify the Request

Try removing the code_list parameter and making the request. This helps determine if the issue is specifically with the code_list or if there's a more general problem. Here’s how you can modify the code:

api = PxApi("ssb")
api.language = "no"
data = api.get_table_data(
 "05889", 
 value_codes={
 "Region": "3305",
 "ContentsCode": "Godkjente",
 "Tid": "2020*",
 }
)
print(data)

If this request works, the issue is likely related to the code_list parameter. If it still fails, there might be a problem with the value_codes or the table identifier.

Step 3: Inspect value_codes

Ensure that the value_codes are correctly specified. The user included "2020*" for the "Tid" (Time) dimension, which is a valid wildcard. However, the API might expect specific time periods (e.g., "2020K1", "2020K2", etc.). If the wildcard isn’t working as expected, try specifying the time periods explicitly.

Step 4: Reintroduce code_list

Once the basic request works, reintroduce the code_list parameter. Double-check the code_list value against the API documentation. The value "agg_KommSummer" might not be the correct code list for the "Region" dimension. There might be a different code list or a specific format required. Try the following code:

api = PxApi("ssb")
api.language = "no"
data = api.get_table_data(
 "05889", 
 value_codes={
 "Region": "3305",
 "ContentsCode": "Godkjente",
 "Tid": "2020*",
 }, 
 code_list={
 "Region": "agg_KommSummer" # Ensure this is the correct code_list value
 }
)
print(data)

Step 5: Verify Aggregation

If the request works but the aggregation is not as expected, verify the aggregation logic. The code_list specifies how the data should be aggregated. If the aggregation is missing, it could be due to:

  • An incorrect code_list value.
  • Data structure issues where the aggregation cannot be performed.
  • Missing data for the specified aggregation.

To ensure correct aggregation, you might need to experiment with different code_list values or adjust the value_codes to match the aggregation requirements.

Addressing the Bonus Question: Inspecting the Request URL

As the user pointed out, pxwebpy lacks a built-in way to directly inspect the generated request URL, which can be a significant hurdle for debugging. While there's no one-click solution within the library itself, there are several workarounds:

1. Logging

You can use Python’s built-in logging module to capture the parameters being sent in the request. Although this won't give you the exact URL, it provides insight into the parameters pxwebpy is using. Here’s an example of how to implement logging:

import logging

logging.basicConfig(level=logging.DEBUG)

api = PxApi("ssb")
api.language = "no"

# Monkey-patch the get method to log the URL and params
original_get = api._api.get

def logged_get(url, params=None, **kwargs):
 logging.debug(f"Request URL: {url}")
 if params:
 logging.debug(f"Request Params: {params}")
 return original_get(url, params, **kwargs)

api._api.get = logged_get

data = api.get_table_data(
 "05889", 
 value_codes={
 "Region": "3305",
 "ContentsCode": "Godkjente",
 "Tid": "2020*",
 }, 
 code_list={
 "Region": "agg_KommSummer"
 }
)

print(data)

This code snippet overrides the internal get method of the pxwebpy API client and logs the URL and parameters before making the request. While it requires a bit of advanced Python, it can be incredibly helpful for debugging.

2. Monkey-Patching with urllib.parse

Another approach is to use urllib.parse.urlencode to manually construct the URL from the parameters. This involves intercepting the parameters before they are sent and constructing the URL string. This method is more involved but provides the exact URL that would be sent.

3. Contributing to pxwebpy

Consider contributing to the pxwebpy library by suggesting the inclusion of a feature to inspect the request URL. This would benefit the entire community and make debugging much easier.

Best Practices for Using code_list

To effectively use code_list with pxwebpy and the SSB API, follow these best practices:

  • Thoroughly Review API Documentation: Always refer to the official SSB API documentation for the most accurate information on dimensions, code lists, and valid values. This is crucial for avoiding common errors.
  • Start Simple and Iterate: Begin with a basic request and incrementally add parameters. This approach helps isolate issues and ensures each part of the request works as expected.
  • Use Logging for Debugging: Implement logging to capture the parameters being sent in the request. This provides valuable insights into the API calls and helps identify discrepancies.
  • Test with Direct URL Requests: Construct and test URLs manually using tools like curl or Postman. This validates the API endpoint and parameter structure independently of pxwebpy.
  • Pay Attention to Parameter Names and Values: Double-check all parameter names and values for typos or incorrect specifications. Small errors can lead to significant issues.
  • Understand Aggregation Logic: Ensure you understand how aggregation works within the SSB API and how to correctly specify code_list for the desired aggregation. Experiment with different values and validate the results.

Conclusion

Troubleshooting code_list usage with pxwebpy and the SSB API can be challenging, but with a systematic approach, it becomes manageable. By verifying parameter names, simplifying requests, consulting API documentation, and using logging, you can diagnose and resolve bad requests and aggregation issues effectively. Remember to follow best practices and incrementally test your code to ensure accuracy. Understanding these steps will empower you to effectively use pxwebpy to extract and manipulate data from the SSB API. Happy coding!

For more information on the SSB API and pxwebpy, refer to the official documentation and community resources. To learn more about APIs and web requests, check out resources like Mozilla Developer Network's guide on HTTP.