Appending an item to a python list in the declaration statement list = [].append(val) is a NoneType

Question

I am running into this situation while using chained methods in python. Suppose, I have the following code

hash = {}
key = 'a'
val = 'A'
hash[key] = hash.get(key, []).append(val)

The hash.get(key, []) returns a [] and I was expecting that dictionary would be {'a': ['A']}. However the dictionary is set as {'a': None}. On looking this up further, I realised that this was occurring due to python lists.

list_variable = []
list_variable.append(val)

sets the list_variable as ['A'] However, setting a list in the initial declaration

list_variable = [].append(val)
type(list_variable)
<type 'NoneType'>

What is wrong with my understanding and expectation that list_variable should contain ['A'] Why are the statements behaving differently?

Avoid using list as a variable name, it shadows the built-in list() method. — Burhan Khalid
– Burhan Khalid, Commented Jun 5, 2013 at 9:04
The problem occurred originally in the first code segment and there wasn't any variable name that can shadow the built in methods. Anyway, let me correct the question. — Ashish Narmen
– Ashish Narmen, Commented Jun 5, 2013 at 9:18

Martijn Pieters · Accepted Answer · 2013-06-05 08:55:33Z

6

The .append() function alters the list in place and thus always returns None. This is normal and expected behaviour. You do not need to have a return value as the list itself has already been updated.

Use the dict.setdefault() method to set a default empty list object:

>>> hash = {}
>>> hash.setdefault('a', []).append('A')
>>> hash
{'a': ['A']}

You may also be interested in the collections.defaultdict class:

>>> from collections import defaultdict
>>> hash = defaultdict(list)
>>> hash['a'].append('A')
>>> hash
defaultdict(<type 'list'>, {'a': ['A']})

If you want to return a new list with the extra item added, use concatenation:

lst = lst + ['val']

edited Jun 5, 2013 at 8:55

answered Jun 5, 2013 at 8:49

Martijn Pieters

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Comments

yedpodtrzitko · Accepted Answer · 2013-06-05 08:56:21Z

0

append operates in-place. However you can utilize setdefault in this case:

hash.setdefault(key, []).append(val)

answered Jun 5, 2013 at 8:56

yedpodtrzitko

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