CODEWITHSUNDEEP
pythonlistappendextend
ScottieB
ScottieB

Reputation: 4052

Append N >= 1 elements to a list

I am building a list in python by looping through some JSON blobs and appending elements. Sometimes the elements are single, sometimes double (or more).

my_list = []    
for j in jsons:
  my_list.append(j['foo'])

my_list ends up being ['a1', 'b1', ['c1', 'c2']]

If I use extend instead I get ['a', '1', 'b', '1', 'c1', 'c2'].

Do I have to first check if what I'm appending is a list, and then append it element-wise? Or is there a better function that already does this?

Upvotes: 0

Views: 169

Answers (2)

chepner
chepner

Reputation: 531430

You can use the singledispatch decorator to move some of the boilerplate out of your main loop. The decorator is available from the functools module in the standard library starting in Python 3.4, or via the singledispatch module on PyPi.

This defines a function adder which behaves differently depending on the type of its (first) argument.

@singledispatch
def adder(item):
    mylist.append(item)

@adder.register(list)
def _(item):
    mylist.extend(item)

mylist = []
for json in jsons:
    adder(json['foo'])

Upvotes: 2

Laurent LAPORTE
Laurent LAPORTE

Reputation: 22962

Yes, you need to explicitly check each item type.

For instance, you can write:

# sample jsons
jsons = [{'foo': 'a1'},
         {'foo': 'b1'},
         {'foo': ['c1', 'c2']}]

my_list = []
for json in jsons:
    item = json['foo']
    if isinstance(item, list):
        my_list.extend(item)
    else:
        my_list.append(item)

You get:

['a1', 'b1', 'c1', 'c2']

But, with Python you can use a ternary conditional expression to simplify:

my_list = []
for json in jsons:
    item = json['foo']
    my_list.extend(item if isinstance(item, list) else [item])

Upvotes: 3

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