FAQ: Collections - Counter

Frequently Asked Questions Python FAQ
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This community-built FAQ covers the “Counter” exercise from the lesson “Collections”.

Paths and Courses
This exercise can be found in the following Codecademy content:

Learn Intermediate Python 3

FAQs on the exercise Counter

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2

I went to the last exercise and it didn’t pass. Here is the code:

opening_inventory = ['shoes', 'shoes', 'skirt', 'jeans', 'blouse', 'shoes', 't-shirt', 'dress', 'jeans', 'blouse', 'skirt', 'skirt', 'shorts', 'jeans', 'dress', 't-shirt', 'dress', 'blouse', 't-shirt', 'dress', 'dress', 'dress', 'jeans', 'dress', 'blouse']

closing_inventory = ['shoes', 'skirt', 'jeans', 'blouse', 'dress', 'skirt', 'shorts', 'jeans', 'dress', 'dress', 'jeans', 'dress', 'blouse']

# Write your code below!
from collections import Counter

def find_amount_sold(opening, closing, item):
  opening_count = Counter(opening)
  closing_count = Counter(closing)
  opening_count.subtract(closing_count)
  return opening_count[item]

tshirts_sold = find_amount_sold(opening_inventory, closing_inventory, "t-shirt")
print(tshirts_sold)

I knew that there was nothing wrong. I tried to switch the "t-shirt" with 't-shirt' and it passed. How did Codecademy check that I used single quotes? It must be because it didn’t let me pass with double quotes. It might not be checking for equality because in Python "t-shirt" and 't-shirt' are equal. How do you check if a string is in single quotes and double quotes? Do most Python Learning Platforms suggest using double quotes or single quotes? Maybe I should get used to using single quotes also because I’m so used to double quotes.

3

Python translates double quotes to single quotes. It’s not an issue which we prefer to use. The occasional SCT might be matching literal expressions and not have a pattern match for the variations.

("|'|'''|""")

Such is the case in an environment with hundreds of authors and virtually nothing for a style guide. They write according to their individual styles and comprehension level. That leads to the diverse lesson checks that are the one irritant most common. It’s a MOOC, warts and all. Make the most of it.

4

Normally this doesn’t happen, the exercises don’t care if you use single quotes or double quotes.

What do those acronyms mean?

5

Submission Correctness Test

Massive Open Online Course

The latter should be followed up for accuracy. It might be ‘online open’.

6

I understand MOOC which means that the course in a course that is open to many people and is online which means that a lot of people with different abilities contribute to this course. But I don’t quite understand the S in SCT.

7

When we click Save or Run in an exercise (in the LE) it invokes the Submission Correctness Test. Our code in the editor is the submission to be tested.

8

Sometimes I view the test files of exercises. Like in this exercise which is the last one of the lesson: https://www.codecademy.com/courses/learn-intermediate-python-3/lessons/collections-python/exercises/review-of-specialized-containers-python.
The _test.py file:

load_file_in_context("clothes_app.py")

try:
  if len(promoted_bundles) != 2:
    fail_tests("`promoted_bundles` did not have the correct number of elements. Expected 2 but got " + str(len(promoted_bundles)))

except NameError:
  fail_tests("`Did you create a `list` called `promoted_bundles`?")
  
try:
  if round(promoted_bundles[0].bundle_price, 2) != 106.95:
    fail_tests("`promoted_bundles` did not contain the correct data. Make sure that the sum of each bundle is calculated correctly and that only bundles with a price of over 100 dollars are included.")
  if round(promoted_bundles[1].bundle_price, 2) != 103.95:
    fail_tests("`promoted_bundles` did not contain the correct data. Make sure that the sum of each bundle is calculated correctly and that only bundles with a price of over 100 dollars are included.")


except NameError:
  fail_tests("Did you create a `list` called `promoted_bundles`?")
    
pass_tests()

Where is load_file_in_context(), pass_tests() and fail_tests() defined?

Is the _test.py file a protected file like in a encapsulation exercise in the course? It taught me that to signal to other people that an attribute is protected you use one _ before the attribute.

9

An underscore in the name implies that the attribute is only ever accessed from a method.

10

What attribute? :thinking: :thinking: :thinking:

11

The one with the underscore in the name.

12

That’s the _test.py file, that’s not an attribute in the the _test.py file.

13

So the lesson allowed single quotes instead of double quotes thus Python didn’t recognize the difference between single quotes and double?

14

These last exercises are a nightmare for learners. The directions are unclear and badly written.

15 
opening_inventory = ['shoes', 'shoes', 'skirt', 'jeans', 'blouse', 'shoes', 't-shirt', 'dress', 'jeans', 'blouse', 'skirt', 'skirt', 'shorts', 'jeans', 'dress', 't-shirt', 'dress', 'blouse', 't-shirt', 'dress', 'dress', 'dress', 'jeans', 'dress', 'blouse']

closing_inventory = ['shoes', 'skirt', 'jeans', 'blouse', 'dress', 'skirt', 'shorts', 'jeans', 'dress', 'dress', 'jeans', 'dress', 'blouse']

# Write your code below!
from collections import Counter

def find_amount_sold(opening, closing, item):
  opening_count = Counter(opening)
  closing_count = Counter(closing)
  opening_count.subtract(closing_count)
  return opening_count[item]

tshirts_sold = find_amount_sold(opening_inventory, closing_inventory, "t-shirt")
print(tshirts_sold)

Checkpoiint 4 Instructions

Awesome! Now we have our Counter object with the difference in item inventory. You may have noticed earlier we defined a parameter named item in our function declaration. This is because the goal of our function is to return the difference in inventory for a specific item rather than all of them!

Using the parameter item, access the item’s inventory from the opening_count Counter object and return it.

Checkpoint 4 Hint

Don’t forget to return only the result of the subtraction for the specific item . This will look like: opening_count[item] .

My Question

Something is not making sense to me – from the Hint it seems that the function’s goal is to return the difference between opening_count and closing_count (both of which are Count objects) for a specific item. But the correct answer is just returning the value for opening_count(item).

I tried inserting a print command into the function and found that the opening_count.subtract(closing_count) just returns None.

Please help me understand. Thanks!