JSONDecodeError: Expecting value: line 1 column 1 (char 0)

I am getting error Expecting value: line 1 column 1 (char 0) when trying to decode JSON.

The URL I use for the API call works fine in the browser, but gives this error when done through a curl request. The following is the code I use for the curl request.

The error happens at return simplejson.loads(response_json)

response_json = self.web_fetch(url)
response_json = response_json.decode('utf-8')
return json.loads(response_json)


def web_fetch(self, url):
    buffer = StringIO()
    curl = pycurl.Curl()
    curl.setopt(curl.URL, url)
    curl.setopt(curl.TIMEOUT, self.timeout)
    curl.setopt(curl.WRITEFUNCTION, buffer.write)
    curl.perform()
    curl.close()
    response = buffer.getvalue().strip()
    return response

Traceback:

File "/Users/nab/Desktop/myenv2/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  111.                         response = callback(request, *callback_args, **callback_kwargs)
File "/Users/nab/Desktop/pricestore/pricemodels/views.py" in view_category
  620.     apicall=api.API().search_parts(category_id= str(categoryofpart.api_id), manufacturer = manufacturer, filter = filters, start=(catpage-1)*20, limit=20, sort_by='[["mpn","asc"]]')
File "/Users/nab/Desktop/pricestore/pricemodels/api.py" in search_parts
  176.         return simplejson.loads(response_json)
File "/Users/nab/Desktop/myenv2/lib/python2.7/site-packages/simplejson/__init__.py" in loads
  455.         return _default_decoder.decode(s)
File "/Users/nab/Desktop/myenv2/lib/python2.7/site-packages/simplejson/decoder.py" in decode
  374.         obj, end = self.raw_decode(s)
File "/Users/nab/Desktop/myenv2/lib/python2.7/site-packages/simplejson/decoder.py" in raw_decode
  393.         return self.scan_once(s, idx=_w(s, idx).end())

Exception Type: JSONDecodeError at /pricemodels/2/dir/
Exception Value: Expecting value: line 1 column 1 (char 0)

Solution

Your code produced an empty response body; you'd want to check for that or catch the exception raised. It is possible the server responded with a 204 No Content response, or a non-200-range status code was returned (404 Not Found, etc.). Check for this.

Note:

There is no need to use simplejson library, the same library is included with Python as the json module. (This note referred to the question as it was originally formulated).
There is no need to decode a response from UTF8 to Unicode, the simplejson / json .loads() method can handle UTF8-encoded data natively.
pycurl has a very archaic API. Unless you have a specific requirement for using it, there are better choices.

Either requests or httpx offer much friendlier APIs, including JSON support.

Example using the Requests package

If you can, replace your call with:

import requests

response = requests.get(url)
response.raise_for_status()  # raises exception when not a 2xx response
if response.status_code != 204:
    return response.json()

Of course, this won't protect you from a URL that doesn't comply with HTTP standards; when using arbitrary URLs where this is a possibility, check if the server intended to give you JSON by checking the Content-Type header, and for good measure catch the exception:

if (
    response.status_code != 204 and
    response.headers["content-type"].strip().startswith("application/json")
):
    try:
        return response.json()
    except ValueError:
        # decide how to handle a server that's misbehaving to this extent