Using a list of lists to create objects

Question

I want to keep a list of temperature probes that will be taking temperature readings periodically. I want to store the arguments needed to create each instance of the temperature probe object in a list of lists. I then want to create each instance from this list of lists and name each object using index 0 of each nested list.

For example, I want the instances Probe1, Probe2, and Probe3 created with their corresponding arguments. I then want to take a temperature reading from each probe in the list.

I want to be able to add unlimited probes without having to change the code down the line.

The issue I'm running into is when I try and do anything with Probe1, Probe2, or Probe3 python tells me they don't exist. I'm new to programming and I'm sure I'm missing something obvious.

class max31865(object):
    def __init__(self, name, R_REF, csPin):
        self.name = name
        self.R_REF = R_REF
        self.csPin = csPin

    def readTemp(self):
        #code here to check temp


probe_list=[["Probe1", 430, 8],["Probe2", 430, 9],["Probe3", 430, 10]]

for probe in probe_list:
    x = str(probe[0])
    x = max31865(*probe)

for probe in probe_list:
    readTemp(probe[0])

There are many problems here. First of all, after each iteration, you are overwriting your x variable, so you basically lose all previously instantiated objects of max31865. Second, you are trying to call readTemp, which is a method defined in your max31865 class, from an outer scope, which is not possible. I am not sure what you are trying to accomplish here — rafaelc
– rafaelc, Commented Jul 6, 2018 at 3:02
If you have an error message please include it in the question. It's a great way to give specific information about your error, that you may not have realised was helpful or relevant. — Paul Rooney
– Paul Rooney, Commented Jul 6, 2018 at 3:07

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

I'm not sure what you want exactly but here are two likely usecases based on your question:

You want a simple list of probe objects, generated from a list of initialization arguments:

The most straightforward way to do this is with the iterable unpacking operator (*) in combination with list comprehension:

probe_list = [["Probe1", 430, 8],["Probe2", 430, 9],["Probe3", 430, 10]]
probe_obj_list = [max31865(*probe) for probe in probe_list]

Now you can call readTemp() on each object in the list, like so:

probe_obj_list[1].readTemp() # Read the temperature of the second object

Or do it in a loop:

for probe in probe_obj_list:
    probe.readTemp()

You want to be able to find probe objects by name:

Consider using a dictionary (also known as a map).

probe_list = [["Probe1", 430, 8],["Probe2", 430, 9],["Probe3", 430, 10]]
probe_obj_map = {probe[0] : max31865(*probe) for probe in probe_list} # Dict comprehension

Now you can access the probe objects by name like so:

probe_obj_map["Probe1"].readTemp() # Accessing the object mapped to by the string "Probe1"

And if you needed to loop through probe_list and find objects by name, you can (although i'm not sure why you would need to do that):

for probe_args in probe_list:
    probe_obj_map[probe_args[0]].readTemp() # Access the object mapped to by the first argument of the nested list (i.e. the name)

edited Jun 20, 2020 at 9:12

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answered Jul 6, 2018 at 9:02

Tomforge

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1 Comment

Jason Arsenault Over a year ago

Hi Tomforge, thanks for the answer. This is what I was looking for. Much appreciated!

YaOzI · Accepted Answer · 2018-07-06 03:19:27Z

Code corrections:

class Max31865(object):
    def __init__(self, name, R_REF, csPin): # missing `:` here
        self.name = name
        self.R_REF = R_REF
        self.csPin = csPin

    def read_temp(self):
        # code here to check temp
        # print the object's attributes or do anything you want
        print('Printing in the method: ', self.name, self.R_REF, self.csPin)


probe_list=[["Probe1", 430, 8],["Probe2", 430, 9],["Probe3", 430, 10]]

for probe in probe_list:
    # x = str(probe[0]) # probe[0] already is str
    x = Max31865(*probe) # Here x is instantiated as `Max31865` object
    print('Printing in the loop: ', x.name, x.R_REF, x.csPin)
    x.read_temp() # Then call the `read_temp()` method.

# for probe in probe_list:
#     readTemp(probe[0])
# This loop is confusing, just as @RafaelC noted in comment,
# 1. `readTemp` is a *method* of `Max31865` object, not a function you can call directly.
# 2. `readTemp` has no argument in it's definition, and you are giving one.

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Using a list of lists to create objects

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You want a simple list of probe objects, generated from a list of initialization arguments:

You want to be able to find probe objects by name:

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Collectives™ on Stack Overflow

2 Answers 2

You want a simple list of probe objects, generated from a list of initialization arguments:

You want to be able to find probe objects by name:

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