How to catch exception output from Python subprocess.check_output()?

Question

I'm trying to do a Bitcoin payment from within Python. In bash I would normally do this:

bitcoin sendtoaddress <bitcoin address> <amount>

So for example:

bitcoin sendtoaddress 1HoCUcbK9RbVnuaGQwiyaJGGAG6xrTPC9y 1.4214

If it is successful I get a transaction id as output, but if I try to transfer an amount larger than my bitcoin balance, I get the following output:

error: {"code":-4,"message":"Insufficient funds"}

In my Python program I now try to do the payment as follows:

import subprocess

try:
    output = subprocess.check_output(['bitcoin', 'sendtoaddress', address, str(amount)])
except:
    print "Unexpected error:", sys.exc_info()

If there's enough balance it works fine, but if there's not enough balance sys.exc_info() prints out this:

(<class 'subprocess.CalledProcessError'>, CalledProcessError(), <traceback object at 0x7f339599ac68>)

It doesn't include the error which I get on the command line though. So my question is; how can I get the outputted error ({"code":-4,"message":"Insufficient funds"}) from within Python?

Answer 1

According to the subprocess.check_output() docs, the exception raised on error has an output attribute that you can use to access the error details:

try:
    subprocess.check_output(...)
except subprocess.CalledProcessError as e:
    print(e.output)

You should then be able to analyse this string and parse the error details with the json module:

if e.output.startswith('error: {'):
    error = json.loads(e.output[7:]) # Skip "error: "
    print(error['code'])
    print(error['message'])