dictionary - Python: How to extend the content of a list store in a dict? -

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• most pythonic way build dictionary single list 2 answers

i facing simple , strange problem python.

i have dict several keys, contain empty lists. want append/extend content of lists of specific keys. code therefore think of looks this

d = dict.fromkeys(['a','b','c'],[]) d['a'].append('hello') 

however, result not expect. value of each key list containing 1 element: 'hello'

>>> d {'a': ['hello'], 'c': ['hello'], 'b': ['hello']} 

if try follows, results in same thing

d.get('a').append('bye') 

what operations have obtain this?

>>> d {'a': ['hello'], 'c': [], 'b': []} 

can explains goes wrong syntax?

your syntax fine, you're using same list on , on again. since lists mutable, when append, you're adding 'hello' same list, happens referenced in 3 places. want instead create dict little more manually:

d = {k:[] k in ['a', 'b', 'c']} 

(the above python 2.7 or later. older python can use: dict((k,[]) k in ['a', 'b', 'c']))