python-将项目追加到列表中始终作为子列表

Question

What would be the most efficient way to append an item into an existing list as another sub-list. for instance, considering data['rows'] is a list, the following:

 for indx, row in enumerate(my_data):
    data['rows'].append(row) 

does the job in when row is already a list.

> 'Rows': [['150', '01'], ['10', '02'], ['22', '03'], ['33', '4'] ...

however if the value in row is not a list, this will produce:

> 'Rows': ['150', '10', '22', '33', '44', '15']

I've a condition that in case a row is not a list it uses data['Rows'].append(list(row)) but there must be a better way..

Answer 1

If I understand your question, something like

I have a list of elements, these elements may be lists already, they may not be. If they are lists, I want them appended as such. If they aren't lists, I want them wrapped in a list, and then I want the resulting single-element list appended.

This is something I too have struggled with, and the best I could come up with is something like the following:

elems = [1,2,[3,4],5]

data1 = []
for e in elems:
    data1.append(e if isinstance(e, list) else [e,])
print data1

# Or with a lambda function that does the work for you
wrap = lambda x: x if isinstance(x, list) else [x,]

data2 = []
for e in elems:
    data2.append(wrap(e))
print data2

Both output:

[[1], [2], [3, 4], [5]]

Answer 2

If I understood your question correctly, you could do something like this:

data['Rows']=data.get('Rows', []).append(list(row))

This will create the list if it isn't present in the dictionary already.

You might consider rephrasing your question to make it more clear. You say "if it's not a list", but it isn't clear what it is then. Since you are working with dicts I assumed it meant that the key is not present.