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I'm having trouble understanding how methods work when creating a class of my own.

this is my class method: 

def compare_coll(self,coll_1, coll_2) -> None:
    '''  compares two collections of key words which are dictionaries, 
    <coll_1> and <coll_2>, and removes from both collections every word 
    that appears in both collection'''


    overlap = set(coll_1).intersection(set(coll_2))
    for key in overlap:
        del coll_1[key], coll_2[key]

My class 'Collection_of_word_counts' has one instance variable, called 'counts' which stores a dictionary where the keys are words and the values are their number of occurrences. the initializer will read in the words from a txt file, and store them in self.counts.

I don't know how to use this method. I know that I implemented it correctly.

>>> c1.compare_coll(c2)

produces:

Traceback (most recent call last):
File "<string>", line 1, in <fragment>
builtins.TypeError: compare_coll() missing 1 required positional argument: 'coll_2'

and

>>> Collection_of_word_counts.compare_coll(c1,c2)

produced the same error.

I'm not exactly sure how to use this in the shell.

Professor Scientist
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1 Answers1

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Your compare_coll method is an instance method, not a class method. The first argument to an instance method, called self by convention, is always the instance of the class the method is being called from (e.g. self == c1 for c1.compare_coll(c2)), so you only need to provide one other argument. Try:

def compare_coll(self, other) -> None:
    overlap = set(self.counts).intersection(set(other.counts))
    for key in overlap:
        del self.counts[key], other.counts[key]

This will work for either c1.compare_coll(c2) or Collection_of_word_counts.compare_coll(c1, c2).

Note that I have referenced counts directly for each of the two instances within the method.

jonrsharpe
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  • I suppose i'm a little confused about the difference between a class method and an instance method, but i will do some further research. – Professor Scientist Feb 08 '14 at 23:01
  • For this and [your previous question](http://stackoverflow.com/questions/21560404/understanding-list-comprehension): http://stackoverflow.com/help/someone-answers – jonrsharpe Feb 08 '14 at 23:05