Modify nested dictionaries via variables

Question

I am relatively new to programming and am trying to better understand how to update values within dictionaries. A problem that I keep coming across is that when I set a dictionary's value to a variable and try to update it within a function, the value is not updated properly.

test_dict = {
    'medals': {
    'bronze': 0,
    'silver': 0,
    'gold': 0,
    },
}

def add_medals_1(test_dict):
    test_dict['medals']['bronze'] += 10
    print(test_dict['medals']['bronze'])

add_medals_1(test_dict) # Updates value of bronze to 10
add_medals_1(test_dict) # Updates value of bronze to 20

def add_medals_2(test_dict):
    silver_medals = test_dict['medals']['silver']
    silver_medals += 10
    print(silver_medals)

add_medals_2(test_dict) # Updates value of silver to 10
add_medals_2(test_dict) # Value of silver remains at 10

In the function add_medals_1, the value of 'bronze' is properly updated and increments each time the function is called. In the function add_medals_2, the value of 'silver' is not properly updated and does not increment. I am confused by this because both functions are similar but do not produce the output I expected.

@JackMoody Please post this as an answer, not a comment.

Code-Apprentice
– Code-Apprentice

2019-01-25 22:29:16 +00:00
Commented Jan 25, 2019 at 22:29 — Code-Apprentice
– Code-Apprentice, Commented Jan 25, 2019 at 22:29

Pablo Santa Cruz · Accepted Answer · 2019-01-25 22:26:55Z

2

The thing is, in add_medals_2 you are not updating the dictionary, you are update a copy you took from the dictionary.

Like this:

def add_medals_2(test_dict):
    # 1) HERE, you are copying test_dict['medals']['silver']
    # to another memory location (variable) called silver_medals
    silver_medals = test_dict['medals']['silver']
    # 2) THEN, you update variable's value to += 10
    silver_medals += 10
    # You print the updated value
    print(test_dict)
    print(silver_medals)
    # BUT, test_dict was never updated in add_medals_2

edited Jan 25, 2019 at 22:26

answered Jan 25, 2019 at 22:24

Pablo Santa Cruz

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2 Comments

Code-Apprentice Over a year ago

I added print(test_dict) to show that it is never updated.

Christian Kilian Over a year ago

Thank you for your response, that makes sense to me.

Code-Apprentice · Accepted Answer · 2019-01-25 22:28:39Z

1

silver_medals = test_dict['medals']['silver']

This copies the value on the left and assigns it to the name on the right.

silver_medals += 10

Now you assign the name on the left to a new value. This will not change the value in the dictionary because there is no knowledge of where the original value came from.

answered Jan 25, 2019 at 22:28

Code-Apprentice

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