Parsing a text file into a list in python

When you write the code lines = f.readlines() a list of lines is being return to you. When you then say lines[3], you're getting the 3rd line. Thats why you're ending up with individual characters.

All you need to do is say

files = open("Your File.txt")

file_list =  files.readlines()

first_twelve = file_list[0:12] #returns a list with the first 12 lines

Once you've got the first_twelve array you can do whatever you want with it.

To print each line you would do:

for each_line in first_twelve:
    print each_line

That should work for you.

You have the line list=lines[3] in your source code.

Two issues here.

1. Don't use list as a variable name. You silently overwrote the built-in list constructor when you did that.
2. When you take one item from a list lines[3] now you only have that object -- in this case a string. When you try to append to it you can't -- it isn't a list.

You can demonstrate your bug easily in the console:

>>> li=['1']
>>> li.append('2')
>>> li
['1', '2']
>>> st='1'
>>> st.append('2')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'append'

Other comments, in general, on your code.

Assume you have a text file called '/tmp/test/txt' that contains this text:

Line 1
Line 2
...
Line 19

Reading the contents of that file is a simple as this:

with open('/tmp/test.txt', 'r') as fin:
    lines=fin.readlines()

If you want a subset of the lines, you can use a slice:

subset=lines[3:12]

If you want to process each line for something, like strip the carriage return, use the file object as an iterator:

with open('/tmp/test.txt', 'r') as fin:
    lines=[]
    for line in fin:
        lines.append(line.strip()) 

For your specific problem of having NULs in the data, perhaps you are reading a binary file masquerading as text? You need to post an example of the file.

Edit

Your file contains Unicode characters. (right after 'Temperature') which may be some of the odd characters you are seeing. If you are only interested in the lines with numbers, you can ignore them.

You do not YET have a list of lists, but it easy to get:

data=[]                               # will hold the lines of the file
with open(ur_file,'rU') as fin:       
    for line in fin:                  # for each line of the file
        line=line.strip()             # remove CR/LF
        if line:                      # skip blank lines
            data.append(line)

print data                            # list of STRINGS separated by spaces
matrix=[map(float,line.split()) for line in data[3:10]]  # convert the strings..
print matrix                          # NOW you have a list of list of floats...

The tweak below might help you to get rid of the \00 character embedded in your data

f = open("/Users/Jennifer/Desktop/test.text", "r")

lines = f.readlines()
lines = [x.replace('\x00','') for x in lines]

for i in range(3,12):
    l = []
    l.append(lines[i])

I am not sure if your data has other delimiters (say comma or space) to separate the numbers. If so, a simple split will help to convert the line into a list:

line = '123.00,456.00,789.00'

l = line.split(',')  # list will become ['123.00','456.00','789.00']

Edit

Continue from Rachel's updated code:

f= open("/Users/Jennifer/Desktop/test.txt", "r")
file_list = f.readlines()

first_twelve = file_list[3:11]

data = [x.replace('\t',' ') for x in first_twelve]
data = [x.replace('\x00','') for x in data]
data = [x.replace(' \r\n','') for x in data]

items = []
for dataline in data:
    items += dataline.split(' ')
items = [float(x) for x in items if len(x) > 0]  # remove dummy items left in the list

print items

Using readLines() is memory-inefficient. It takes the whole file into memory. Instead, do this:

[i.split() for i in open('filename.txt')]