Is this a dictionary & lists bug?

Question

Can anybody make sense of this:

Method 1

In[25]: dico = {'A': [], 'B': []}
In[26]: dico['A'].append(1)
In[27]: print(dico)
{'B': [], 'A': [1]}

Method 2

In[28]: letters = ['A', 'B']
In[29]: dico = dict.fromkeys(letters, [])
In[30]: print(dico)
{'B': [], 'A': []}
In[31]: dico['A'].append(1)
In[32]: print(dico)
{'B': [1], 'A': [1]}

I cannot figure out why in the 2nd case the value is appended in each key.... this is a serious bug or I am seriously tired.....

Because `dict.fromkeys` will *not* create copies of the fillvalue implicitly, and the *same list object* is used as the value for every key. This is usually how things in Python work. Certainly, all built-in functions/methods will work like this. — juanpa.arrivillaga, Jun 01 '17 at 22:32
Yes sorry, I use it just after in my script... I am building a dataframe from a dictionary, the issue I have is to populate the lists included in the dictionary.... — Baka, Jun 01 '17 at 22:35
OK I found a solution, simply: dico['A'] = dico['A'] + ['1']. — Baka, Jun 01 '17 at 22:39

score 0 · Answer 1 · answered Jun 01 '17 at 22:40

0

You can do this nicely with a dictionary comprehension:

dico = {k: [] for k in letters}

answered Jun 01 '17 at 22:40

mgilson

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Is this a dictionary & lists bug?

1 Answers1