Python cURL command via os.system() fails with JSON parse error

Question

Hello I'am tring to send a cURL command to a Orion Context Broker via a Python script. I am running the script on OpenWRT, so i am not able to install the requests or urllib2 library, for memory issues, additionally libraries like subprocess fail to compile. So I am using os.system() to execute the cURL command. This is the code of the script:

import sys
import os
from urllib import urlencode
sys.path.insert(0, '/usr/lib/python2.7/bridge')
from bridgeclient import BridgeClient as bridgeclient

value = bridgeclient()


header="(curl 10.130.1.228:1026/v1/updateContext -s -S --header 'Content-Type: application/json' \
--header 'Accept: application/json'     -d @- | python -mjson.tool) <<EOF"

json_string="""
{
    "contextElements": [
        {
            "type": "Room",
            "isPattern": "false",
            "id": "R1",
            "attributes": [
                {
                    "name": "temperature",
                    "type": "float",
                    "value": "firstValue"
                },
                {
                    "name": "pressure",
                    "type": "float",
                    "value": "secondValue"
                }
            ]    
        }
    ],
    "updateAction": "UPDATE"
}
EOF""" 


while(True):

    all=value.getall()
    sentValue1=""
    sentValue2=""
    if all['Tmp'] is None:
        sentValue1=all['Tmp']
    else:
        sentValue1="NoValue"
    if all['Prs'] is None:
        sentValue2=all['Prs']
    else:
        sentValue2="NoValue"
    json_string=json_string.replace("firstValue",sentValue1)
    json_string=json_string.replace("secondValue",sentValue2)   
    os.system(header+json_string)

If I copy and paste the command i give to os.system(), as it is in the terminal window, everything works smoothly and my Orion instance gets updated. But if i run the same command via the said script i get this response from the server:

{
    "errorCode": {
        "code": "400",
        "details": "JSON Parse Error",
        "reasonPhrase": "Bad Request"
    }
}

I think is some formatting issue and i have tried everything to make it work but with no luck.

UPDATE:

I found in the contextBroker log this message:

from=10.130.1.1 | srv=pending | subsrv=<defalut> | comp=Orion | 
op=AlarmMenager.cpp[405]:badInput | msg=Releasing alarm BadInput 
10.130.1.1: JSON Parse Error: unspecified file(1):expected end of input

and this one:

from=10.130.1.1 | srv=pending | subsrv=<defalut> | comp=Orion | 
op=AlarmMenager.cpp[405]:badInput | msg=Releasing alarm BadInput 
10.130.1.1: JSON Parse Error: unspecified file(1):expected object

Both repeated for each cURL request I made.

UPDATE 2:

I managed to make subprocess.call() to work, but it gives the exact same response.

would it be worth if you specified /usr/bin/python in your header variable? — Shan-Desai
– Shan-Desai, Commented Nov 9, 2017 at 15:34
@Shan-Desai I tried but it didn't work. Also as I said the string header+json_string copied and paste in the terminal works with no problems. — Gianmarco Biscini
– Gianmarco Biscini, Commented Nov 9, 2017 at 15:55
Try using pycurl if you can. However to address the issue. Use import json and then instead of writing a string create your json_string as a python dictionary and use json.dumps(json_string) — Shan-Desai
– Shan-Desai, Commented Nov 9, 2017 at 16:01
@Shan-Desai Thank you very much for your help. But how can i make a python dictionary if i need a nested json? — Gianmarco Biscini
– Gianmarco Biscini, Commented Nov 9, 2017 at 16:07

Gianmarco Biscini · Accepted Answer · 2017-11-10 15:39:15Z

Thanks to @Shan-Desai I solved the problem.

I built the json string using json.dump and used pycurl.

this is the working code:

import sys
import os
import subprocess
import json
from urllib import urlencode
from collections import OrderedDict
import StringIO
import pycurl

sys.path.insert(0, '/usr/lib/python2.7/bridge')
from bridgeclient import BridgeClient as bridgeclient


fout = StringIO.StringIO()
value = bridgeclient()
apiurl = '10.130.1.228:1026/v1/updateContext'
headers=['Content-Type: application/json','Accept: application/json']

firstValue = 'firstValue'

secondValue = 'secondValue'

d_attributes = [{'name': 'temperature', 'type': 'float', 'value': firstValue},
            {'name': 'pressure', 'type': 'float', 'value': secondValue}]

d_context = [{'type': 'Room', 'isPattern': 'false', 'id': 'R1', 'attributes': d_attributes}]

d_json = {'contextElements': d_context, 'updateAction': 'UPDATE'}

c = pycurl.Curl()

while (True):

        all = value.getall()

        if all['Tmp'] is not None:
            firstValue = all['Tmp']
        else:
            firstValue = "NoValue"
        if all['Prs'] is not None:
            secondValue = all['Prs']
        else:
            secondValue = "NoValue"
        d_json["contextElements"][0]["attributes"][0]["value"]=firstValue
        d_json['contextElements'][0]['attributes'][1]['value']=secondValue
        c.setopt(pycurl.WRITEFUNCTION, fout.write)
        c.setopt(pycurl.URL, apiurl)
        c.setopt(pycurl.HTTPHEADER, headers)
        c.setopt(pycurl.POST, 1)
    s_json=json.dumps(d_json)
        c.setopt(pycurl.POSTFIELDS,s_json)
        c.perform()
        c.getinfo(pycurl.RESPONSE_CODE)
        print(json.dumps(OrderedDict(d_json)))
        print(fout.getvalue())

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