Take the content of a list and append it to another list

Question

I am trying to understand if it makes sense to take the content of a list and append it to another list.

I have the first list created through a loop function, that

Answer 1

That seems fairly reasonable for what you're trying to do.

A slightly shorter version which leans on Python to do more of the heavy lifting might be:

for logs in mydir:

    for line in mylog:
        #...if the conditions are met
        list1.append(line)

    if any(True for line in list1 if "string" in line):
        list2.extend(list1)
    del list1

    ....

The (True for line in list1 if "string" in line) iterates over list and emits True whenever a match is found. any() uses short-circuit evaluation to return True as soon as the first True element is found. list2.extend() appends the contents of list1 to the end.

Answer 2

Take a look at itertools.chain for a fast way to treat many small lists as a single big list (or at least as a single big iterable) without copying the smaller lists:

>>> import itertools
>>> p = ['a', 'b', 'c']
>>> q = ['d', 'e', 'f']
>>> r = ['g', 'h', 'i']
>>> for x in itertools.chain(p, q, r):
        print x.upper()

Answer 3

You can also combine two lists (say a,b) using the '+' operator. For example,

a = [1,2,3,4]
b = [4,5,6,7]
c = a + b

Output:
>>> c
[1, 2, 3, 4, 4, 5, 6, 7]

Answer 4

Using the map() and reduce() built-in functions

def file_to_list(file):
     #stuff to parse file to a list
     return list

files = [...list of files...]

L = map(file_to_list, files)

flat_L = reduce(lambda x,y:x+y, L)

Minimal "for looping" and elegant coding pattern :)

Answer 5

If we have list like below:

list  = [2,2,3,4]

two ways to copy it into another list.

1.

x = [list]  # x =[] x.append(list) same 
print("length is {}".format(len(x)))
for i in x:
    print(i)

length is 1
[2, 2, 3, 4]

2.

x = [l for l in list]
print("length is {}".format(len(x)))
for i in x:
    print(i)

length is 4
2
2
3
4

Answer 6

You probably want

list2.extend(list1)

instead of

list2.append(list1)

Here's the difference:

>>> a = range(5)
>>> b = range(3)
>>> c = range(2)
>>> b.append(a)
>>> b
[0, 1, 2, [0, 1, 2, 3, 4]]
>>> c.extend(a)
>>> c
[0, 1, 0, 1, 2, 3, 4]

Since list.extend() accepts an arbitrary iterable, you can also replace

for line in mylog:
    list1.append(line)

by

list1.extend(mylog)