Accessing methods from objects within a list

Question

I am new to Python (having come from C++) so I am still getting used to some things.

I have created a class called Sensor which I am using as a sensor that detects when an avatar or a player moves in to it. I want to have up to 20 sensors as one time so I would like to store them in a list and then iterate through the list, checking each one for a detected entry.

I have the following code:

class Sensor:

    def __init__(self, loc, size=1, enabled=True):
        self.enabled = enabled
        self.distOfBorderFrmCentre = size / 2.0
        self.location = loc

    # Additional Sensor code here. Just not implemented yet so would not
    # benefit the post.

    def position(self):
        # No real use. Using this function for demo reasons
        return self.position

self.sensors = [ Sensor(loc) for loc in self.route ]
for i in range(len(self.sensors)):
    print self.sensors[i].position()

# Also gives same output
#for sensor in self.sensors:
    #print sensor .position()

The output I am given is:

<bound method Sensor.position of <sensor.Sensor instance at 0x03628E40>>
<bound method Sensor.position of <sensor.Sensor instance at 0x03628DC8>>
<bound method Sensor.position of <sensor.Sensor instance at 0x03628DF0>>
<bound method Sensor.position of <sensor.Sensor instance at 0x03628E18>>
<bound method Sensor.position of <sensor.Sensor instance at 0x03628E40>>

So am I missing something? I suspect I probably am. I have searched and searched but every time I see examples of calling methods from objects within lists, the syntax I listed above is what is used.

Thanks

Is the indentation correct here? what is self? Also, generally loops like for i in range(len(...)) can be simplified. for sensor in self.sensors: # do something with a sensor — mgilson
– mgilson, Commented Dec 3, 2013 at 18:11
Well... that's the output I would expect... the return value of position is a reference to the position method... which you're then printing... When you ommited the () you were printing the reference directly, now that you're calling it, it's returning the self.position anyway... — Jon Clements
– Jon Clements, Commented Dec 3, 2013 at 18:13
Apologies, the indentation is correct. If it is incorrect then it is me pasting it wrongly in the question. Self explicitly shows that the variable is a member of the class. And I have also tried for sensor in self.sensors: #do something. I shall update my question to show this — Chris
– Chris, Commented Dec 3, 2013 at 18:14

Hyperboreus · Accepted Answer · 2013-12-03 18:17:40Z

2

You will need to invoke the methods:

for i in range(len(self.sensors)):
    print self.sensors[i].position()

Note the trailing parentheses.

I have changed a bit your example to make it runnable. This snippet works:

#! /usr/bin/python2.7

class Sensor:

    def __init__(self, loc, size=1, enabled=True):
        self.enabled = enabled
        self.distOfBorderFrmCentre = size / 2.0
        self.location = loc


    def position(self):
        return self.location

sensors = [ Sensor(loc) for loc in ('Loc 1', 'Loc 2', 'Loc 3') ]
for i in range(len(sensors)):
    print sensors[i].position()

#Previous two lines are a bit senseless, you can use:
for sensor in sensors:
    print sensor.position()

edited Dec 3, 2013 at 18:17

answered Dec 3, 2013 at 18:09

Hyperboreus

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8 Comments

Chris Over a year ago

Unfortunately, I still receive the same error after adding the parentheses. I have updated my original question nonetheless to add your change.

jpwagner Over a year ago

error? your current wording of the question contains no error

Chris Over a year ago

@ Hyperboreus - Seen and thank you. Combination of a long day and not spotting that I had incorrectly wrote "return self.position" instead of "return self.location"

Chris Over a year ago

@jpwagner - let's not be pernickety here. You know what I meant and so did others

Hyperboreus Over a year ago

@Chris Glad to help. Also note that everytime you write for i in range(len(someList)) a compiler commits suicide.

|

Jon Clements · Accepted Answer · 2013-12-03 18:16:19Z

1

When you had:

for i in range(len(self.sensors)):
    print self.sensors[i].position

You were printing a reference to the the position method of the class, but now you've changed it to be:

for i in range(len(self.sensors)):
    print self.sensors[i].position()

You're now calling the function each time, which is great, except that the return value of the function:

def position(self):
    # No real use. Using this function for demo reasons
    return self.position

Is to return a reference to the function itself... which is effectively a round-about method of what you did the first time... Make your position function do something more meaningful... (maybe: return self.location or something for testing)

answered Dec 3, 2013 at 18:16

Jon Clements

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2 Comments

Chris Over a year ago

Wow... you've just pointed out the most incredible oversight on my part ever! Of course that wouldn't work... I just didn't spot it! Feel dumb now. Thanks

jpwagner Over a year ago

as @Jon pointed out, you're calling the function correctly now. i've deleted my answer pertaining to that issue, but note that you can also use the @property decorator to call the method as if it were a property of the class (ie, no "()" )

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