How to implement for loop while opening text file

Question

I am trying to get each value from data.txt into my list. Unfortunately, I do not understand why For Loop does not work in the following case:

#(data length is 8)

example_list = []

with open("/path/data.txt") as example:

    for number in range(example.readlines())):
        example_list.append(example.readlines()[number])

print(example_list)

Error list index out of range appears.

When in stead of For Loop I manually insert "number" (from 0 to 8) it does work, therefore I do not understand why it does not work with for loop

Appreciate any help on that

Does this answer your question? Using "readlines()" twice in a row — Ahmed AEK
– Ahmed AEK, Commented Feb 7, 2023 at 20:17
example.readlines() already gives you a list of the lines, you don't need to loop over and create a new list. You can just do example_list = example.readlines(). — Henry
– Henry, Commented Feb 7, 2023 at 20:17

liquid_snake · Accepted Answer · 2023-02-07 21:11:48Z

1

If you are going to iterate over the contents of a file, you first have to check what file structure the method you use returns. In this case, readlines() returns a list. Link.

You can not use a list as an argument for range() because it expects an integer.

So you need to iterate over that list returned by readlines(), not over the file itself.

example_list = []

with open("file.txt") as example:
    # lines is a list
    lines = example.readlines()
    for item in lines:
        # add each element to the example_list
        example_list.append(item)

print("example list: ", example_list)

You can also use example_list.append(item.strip()) to get cleaner data. Link

answered Feb 7, 2023 at 21:11

liquid_snake

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2 Comments

shadowtalker Over a year ago

Note that over a file-like object does already iterate over lines. The .readlines() method simply collects all lines into a list. See e.g. here: "IOBase (and its subclasses) supports the iterator protocol, meaning that an IOBase object can be iterated over yielding the lines in a stream. Lines are defined slightly differently depending on whether the stream is a binary stream (yielding bytes), or a text stream (yielding character strings). See readline() below."

shadowtalker Over a year ago

I offer that the problem is more that OP fundamentally misunderstands loops in Python, moreso than their lack of reading the docs for .readline().

maciek97x · Accepted Answer · 2023-02-07 20:18:30Z

0

You can iterate over file's lines like this:

example_list = []

with open("/path/data.txt") as example:
    for number in example:
        example_list.append(number)

print(example_list)

Or simplier

example_list = open("/path/data.txt").readlines()

answered Feb 7, 2023 at 20:18

maciek97x

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Comments

rocha-p · Accepted Answer · 2023-02-07 20:19:11Z

0

Try this instead:

example_list = []

with open("test.txt") as f:
    for line in f:
        example_list.append(line.strip())

print(example_list)

answered Feb 7, 2023 at 20:19

rocha-p

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Comments

treuss · Accepted Answer · 2023-02-07 21:18:00Z

0

Shortest (and imho best) version:

with open("test.txt") as f:
    example_list = [line.strip() for line in f]

answered Feb 7, 2023 at 21:18

treuss

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