Dictionary with Lists

[Shaun]

Hi,

I'm trying to create a dictionary with lists as the value for each
key. I was looking for the most elegant way of doing it... I thought
this would work:

testDict = {}
....
testDict [1] = testDict.get (1, []).append ("Test0") # 1 does not
exist, create empty array
print testDict
testDict [1] = testDict.get (1, []).append ("Test1")
print testDict

(Obviously I wouldn't normally write code like this.. but this is how
it would unfold in a loop)
However, the first printout gives {1: None} instead of the desired
{1: ['test']}. What's wrong with this syntax?

 

[Mick Krippendorf]

Hi,

I'm trying to create a dictionary with lists as the value for each
key. I was looking for the most elegant way of doing it...

from collections import defaultdict

d = defaultdict(list)

d["joe"].append("something")
d["joe"].append("another")
d["jim"].append("slow down, grasshopper")

print d


HTH,
mick.

 

[Mel]

testDict = {}
...
testDict [1] = testDict.get (1, []).append ("Test0") # 1 does not
exist, create empty array
print testDict
testDict [1] = testDict.get (1, []).append ("Test1")
print testDict
[ ... ]
However, the first printout gives {1: None} instead of the desired
{1: ['test']}. What's wrong with this syntax?

The trouble is that the list.append method returns None, after modifying the
value of the parent list in place. So of course, None gets assigned to
testDict[1] after the valuable work has been done.

The old way to do what you want is

testDict.setdefault (1, []).append ("Test0")

The new way is to inherit from defaultdict.

Mel.

 

[Shaun]

Okay that makes sense. I was assuming that list.append returned the
new list.

thanks

 

[John Nagle]

Hi,

I'm trying to create a dictionary with lists as the value for each
key. I was looking for the most elegant way of doing it...

Try using a tuple, instead of a list, for each key. Tuples
are immutable, so there's no issue about a key changing while
being used in a dictionary.

John Nagle

 

[Mick Krippendorf]

Try using a tuple, instead of a list, for each key. Tuples
are immutable, so there's no issue about a key changing while
being used in a dictionary.

Only if Shaun wanted to use lists as keys (which of course doesn't
work). Luckily he just wants them as values.

Mick.