How to determine number of appends when looping over list in Python?

Question

I have the following code:

lstcurrencies = ['btc', 'eth', 'usdt']
dict_lstlegacy_symbols = dict.fromkeys(lstcurrencies, []) # creates {'btc': [], 'eth': [], 'usdt': []}

for currency in lstcurrencies:
    print(currency)
    dict_lstlegacy_symbols[currency].append(1)

This creates the three lists:

dict_lstlegacy_symbols['btc']: [1, 1, 1]
dict_lstlegacy_symbols['eth']: [1, 1, 1]
dict_lstlegacy_symbols['usdt']: [1, 1, 1]

What I would actually like to get is:

dict_lstlegacy_symbols['btc']: [1]
dict_lstlegacy_symbols['eth']: [1]
dict_lstlegacy_symbols['usdt']: [1]

I don't understand why it adds the 1 three times instead of only one time. How can I obtain my desired result?

Because you have **exactly one** list in your dict, the list you passed to `dict.fromkeys` — juanpa.arrivillaga, May 26 '21 at 09:47
so just create your dict like this: `{curr:[] for curr in lstcurrencies}` — juanpa.arrivillaga, May 26 '21 at 09:49
I'd use [`dict_lstlegacy_symbols = defaultdict(list)`](https://docs.python.org/3/library/collections.html#collections.defaultdict) for this case. — Olvin Roght, May 26 '21 at 09:50

pjk · Answer 1 · 2021-05-26T11:42:21.293

0

Use defualt dict instead.

from collections import defaultdict

lstcurrencies = ['btc', 'eth', 'usdt']
dict_lstlegacy_symbols =defaultdict(list) 

for currency in lstcurrencies:
    print(currency)
    dict_lstlegacy_symbols[currency].append(1)

print(dict_lstlegacy_symbols)

edited May 26 '21 at 11:42

answered May 26 '21 at 09:47

pjk

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How to determine number of appends when looping over list in Python?

1 Answers1