How should I convert a list of tuples into separate strings?

Question

import obd
connection = obd.OBD()
r = connection.query(obd.commands.GET_DTC)
print(r.value)


example output:
[
  ("P0030", "HO2S Heater Control Circuit"),
  ("P1367", "Unknown error code")
]

I would like to store/access the second value in each outputted item (e.g. "HO2S Heater Control Circuit") as its own variable. Do I need to decode this output as a list or tuple or both?

What do you mean, "as its own variable"? Do you want to flatten the `list`? Or are you asking how to access nested items? Or do you just need mutable containers? Please be more specific. — TigerhawkT3, Jan 23 '16 at 12:33

score 1 · Answer 1 · edited May 23 '17 at 11:44

From the page where this code seems to be from:

The value field of the response object will contain a list of tuples, where each tuple contains the DTC, and a string description of that DTC (if available).

So if r = connection.query(obd.commands.GET_DTC), then r.value is a "list of tuples". You can use the zip() function (as described in this question) to transpose the structure with zip(*r.value) which gives

    [('P0030', 'P1367'), ('HO2S Heater Control Circuit', 'Unknown error code')]

You just want the second element of this list, so

    zip(*r.value)[1]

gives you the tuple

    ('HO2S Heater Control Circuit', 'Unknown error code')

You could then use this as you wish. Notice that this gives you all of the "second values in each outputted item". You could iterate through all them (and, say print each one) with:

    for description in zip(*r.value)[1]:
        print description

It may be a good idea to assign zip(*r.value)[1] to a variable if you want to use it more than once.

Padraic Cunningham · Answer 2 · 2016-01-23T13:10:27.250

If you want to use each second element you can unpack the tuples in a for loop:

for _, var in r.value:
    # use var

i.e:

In [4]: l = [                                                                                           
  ("P0030", "HO2S Heater Control Circuit"),
  ("P1367", "Unknown error code")
]

In [5]: for _, var in l:
          print(var)
   ...:     
HO2S Heater Control Circuit
Unknown error code

If you wanted them all for some reason could also use a list comp with the logic above or operator.itemgetter:

In [7]: list(map(itemgetter(1), l))
Out[7]: ['HO2S Heater Control Circuit', 'Unknown error code']

In [8]: from operator import itemgetter

In [9]: list(map(itemgetter(1), l))
Out[9]: ['HO2S Heater Control Circuit', 'Unknown error code']

You could also use itemgetter(-1) to get the last elements.

This is much simpler than my answer. – e0k Jan 23 '16 at 13:10 — e0k, Jan 23 '16 at 13:10

score 0 · Answer 3 · answered Jan 23 '16 at 13:01

You could "unpack" a tuple. So, from your example, r.value[0] is the tuple ("P0030", "HO2S Heater Control Circuit") Then,

id, desc = r.value[0]

would unpack the tuple, r.value[0] into varaiables id and desc so that P0030 is stored in id and HO2S Heater Control Circuit is stored in desc

How should I convert a list of tuples into separate strings?

3 Answers3