Issue in parsing the string output from the command using "subprocess"

[MRAB]

On 2018-11-18 14:59, srinivasan wrote:
> Dear Python Experts Team,
> 
> As am newbie to python and learning python, working on embedded linux
> platform, my intention is to delete all the SSID's before connecting my
> Wi-Fi module to specific SSID., I am trying to parse command output using
> the "subprocess"  with wrapper "execute_cmd_output_string" written on
> it described
> as below,  using the nmcli commands "*nmcli -t -f TYPE,UUID con" and "**"nmcli
> connection delete uuid ".*
> 
> Could you please help me, what could be the bug in the below method "*def
> wifi_disconnect_reconnect(self, ssid, pw):"  *using the method
> "execute_cmd_output_string" (in-turn uses "subprocess") which is failing to
> give correct output of., UUID's  for *"nmcli connection delete uuid "*  ?
> 
> But which works fine with "commands.getstatusoutput" (res =
> commands.getstatusoutput("nmcli -t -f TYPE,UUID con")), but I dont want to
> include "commands.getstatusoutput" which costs me for including one more
> python module in my rootfs
> 
> Typical output for "nmcli -t -f TYPE,UUID con"
> 
> ~$ nmcli -t -f TYPE,UUID con
> 802-11-wireless:3f5011d4-5681-4fed-8dea-95dee790e9e2
> 802-11-wireless:bfb93be0-740d-426e-b215-0fdc2f652877
> 802-11-wireless:69b60cf4-65aa-442b-a54c-fb82905adb0d
> 802-11-wireless:dc2a15ec-d3da-491e-9c8f-cb054f375837
> 802-11-wireless:5164e7f2-4489-462e-b093-76bc51bf1303
> 802-11-wireless:d462e8c4-fac7-42f2-8f9a-6846f52d4e8c
> 802-11-wireless:e5020744-5c9c-453c-92ec-7a854fc893e6
> 802-11-wireless:eed3358e-8635-471d-b7e9-5c2973a05128
> 802-11-wireless:ceea6707-a929-4941-9047-a75e061914b6
> 802-11-wireless:dd6b9c83-db7b-42b9-99c0-14a04f6f35f5
> 802-11-wireless:9f764fff-6288-49c4-9412-902e89230136
> 802-11-wireless:72c627cd-77a3-4d16-bb2c-058040d8e4fc
> ~$
> 
> *Python Code Snipeet:*
> *------------------------------*
> *    def wifi_disconnect_reconnect(self, ssid, pw):*
> *        """*
> *            Connect to Access point using SSID and PW.*
> 
> *            :param ssid: SSID of the ACCESS POINT.*
> *            :param pw: password for connecting to the access point.*
> *            :return: command output as True or False.*
> *        """*
> 
> *        cmd = "nmcli -t -f TYPE,UUID con"*
> *        res = self._helper.execute_cmd_output_string(cmd)*
> *        print(res)*
> *        lines = res[1].split('\n') ------------------------> I suspect the
> issue might be here*
> *        print(lines)*
> 
> *        for line in lines:*
> *            parts = line.split(":")*
> *            print(parts)*
> *            print(parts[1])*
> *            if (parts[0] == "802-11-wireless"):*
> *                print("nmcli connection delete uuid "+ parts[1])*
> *                os.system("nmcli connection delete uuid "+ parts[1])*
> 
> *        cmd = "nmcli device wifi connect '%s' password %s" % (ssid, pw)*
> *        exit_code = self._helper.execute_cmd_return_code(cmd)*
> 
> *        return True if exit_code == 0 else False*
> 
> 
>      def execute_cmd_output_string(self, cmd, enable_shell=False):
>          """
>              Execute a command and return its output as a string.
> 
>              :param cmd: abs path of the command with arguments
>              :param enable_shell : force the cmd to be run as shell script
>              :return: a string.
>          """
> 
>          try:
>              result = subprocess.check_output(split(cmd),
>                                               stderr=subprocess.STDOUT,
>                                               shell=enable_shell)
> 
>          except subprocess.CalledProcessError as e:
>              s = """While executing '{}' something went wrong.
>                  Return code == '{}'
>                  Return output:\n'{}'
>                  """.format(cmd, e.returncode, e.output, shell=enable_shell)
>              raise AssertionError(s)
> 
>          return result.strip().decode("utf-8")
> 
> 
> if __name__ == "__main__":
>      m = wifi()
>      print("disconnect and reconnect")
>      print(m.wifi_disconnect_reconnect("NaWiFi", "abcds"))
> 
> 
> *Errors:*
> -----------
> Traceback (most recent call last):
> 802-11-wireless:3f5011d4-5681-4fed-8dea-95dee790e9e2
> 802-11-wireless:dc2a15ec-d3da-491e-9c8f-cb054f375837
> 802-11-wireless:5164e7f2-4489-462e-b093-76bc51bf1303
>    File
> "/home/srinivasan/Downloads/qa_wifi_nov15_after_incorporating_thilo_comments_zip/qa/test_library/wifi.py",
> line 153, in <module>
> 802-11-wireless:d462e8c4-fac7-42f2-8f9a-6846f52d4e8c
>      print(m.wifi_connect("NI WiFi", "T.f.o.s.1996!"))
> 802-11-wireless:e5020744-5c9c-453c-92ec-7a854fc893e6
> 802-11-wireless:eed3358e-8635-471d-b7e9-5c2973a05128
>    File
> "/home/srinivasan/Downloads/qa_wifi_nov15_after_incorporating_thilo_comments_zip/qa/test_library/wifi.py",
> line 77, in wifi_connect
> 802-11-wireless:ceea6707-a929-4941-9047-a75e061914b6
>      print(parts[1])
> 802-11-wireless:dd6b9c83-db7b-42b9-99c0-14a04f6f35f5
> *IndexError: list index out of range*
> 802-11-wireless:9f764fff-6288-49c4-9412-902e89230136
> 802-11-wireless:72c627cd-77a3-4d16-bb2c-058040d8e4fc
> [u'0']
> [u'0']
> 
[snip]
execute_cmd_output_string returns a string, so 'res' will be a string.

For example, res == u'802-11-wireless:3f5011d4-5681-4fed-8dea-95dee790e9e2'.

That means that res[1] == u'0'.

So res[1].split('\n') == [u'0'].

If you follow the code from there, so you'll see that it'll eventually 
raise IndexError.