Iterating through list of list in Python


Question

I want to iterate through list of list.
I want to iterate through irregularly nested lists inside list also.
Can anyone let me know how can I do that?

x = [u'sam', [['Test', [['one', [], []]], [(u'file.txt', ['id', 1, 0])]], ['Test2', [], [(u'file2.txt', ['id', 1, 2])]]], []]
1
39
5/16/2018 12:04:08 AM

Accepted Answer

This traverse generator function can be used to iterate over all the values:

def traverse(o, tree_types=(list, tuple)):
    if isinstance(o, tree_types):
        for value in o:
            for subvalue in traverse(value, tree_types):
                yield subvalue
    else:
        yield o

data = [(1,1,(1,1,(1,"1"))),(1,1,1),(1,),1,(1,(1,("1",)))]
print list(traverse(data))
# prints [1, 1, 1, 1, 1, '1', 1, 1, 1, 1, 1, 1, 1, '1']

for value in traverse(data):
    print repr(value)
# prints
# 1
# 1
# 1
# 1
# 1
# '1'
# 1
# 1
# 1
# 1
# 1
# 1
# 1
# '1'
47
8/9/2015 7:06:06 AM

So wait, this is just a list-within-a-list?

The easiest way is probably just to use nested for loops:

>>> a = [[1, 3, 4], [2, 4, 4], [3, 4, 5]]
>>> a
[[1, 3, 4], [2, 4, 4], [3, 4, 5]]
>>> for list in a:
...     for number in list:
...         print number
...
1
3
4
2
4
4
3
4
5

Or is it something more complicated than that? Arbitrary nesting or something? Let us know if there's something else as well.

Also, for performance reasons, you might want to look at using list comprehensions to do this:

http://docs.python.org/tutorial/datastructures.html#nested-list-comprehensions


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