Merging a list of lists

Question

How do I merge a list of lists?

[['A', 'B', 'C'], ['D', 'E', 'F'], ['G', 'H', 'I']]

into

['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I']

Even better if I can add a value on the beginning and end of each item before merging the lists, like html tags.

ie, the end result would be:

['<tr>A</tr>', '<tr>B</tr>', '<tr>C</tr>', '<tr>D</tr>', '<tr>E</tr>', '<tr>F</tr>', '<tr>G</tr>', '<tr>H</tr>', '<tr>I</tr>']

Answer 1

Don't use sum(), it is slow for joining lists.

Instead a nested list comprehension will work:

>>> x = [['A', 'B', 'C'], ['D', 'E', 'F'], ['G', 'H', 'I']]
>>> [elem for sublist in x for elem in sublist]
['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I']
>>> ['<tr>' + elem + '</tr>' for elem in _]

The advice to use itertools.chain was also good.

Answer 2

To concatenate the lists, you can use sum

values = sum([['A', 'B', 'C'], ['D', 'E', 'F'], ['G', 'H', 'I']], [])

To add the HTML tags, you can use a list comprehension.

html_values = ['<tr>' + i + '</tr>' for i in values]

Answer 3

import itertools

print [('<tr>%s</tr>' % x) for x in itertools.chain.from_iterable(l)]

You can use sum, but I think that is kinda ugly because you have to pass the [] parameter. As Raymond points out, it will also be expensive. So don't use sum.

Answer 4

Use itertools.chain :

>>> import itertools
>>> list(itertools.chain(*mylist))
['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I']

Wrapping the elements in HTML can be done afterwards.

>>> ['<tr>' + x + '</tr>' for x in itertools.chain(*mylist)]
['<tr>A</tr>', '<tr>B</tr>', '<tr>C</tr>', '<tr>D</tr>', '<tr>E</tr>', '<tr>F</tr>',
'<tr>G</tr>', '<tr>H</tr>', '<tr>I</tr>']

Note that if you are trying to generate valid HTML you may also need to HTML escape some of the content in your strings.