File still empty after call to f.write()

Question

This program (written in Python) is supposed to display only the first two lines of text written to a particular file. However when I run it, though it has no errors it doesn't display any output, in the IDE nor on the file itself.

def file_read_from_head(fname, nlines):
    from itertools import islice
    with open(fname) as f:
        for line in islice(f, nlines):
            print(line)


f = open('test.txt', 'w')
f.write = ("Hello welcome to Python \n"
           "THis is the second line \n"
           "This is the third line \n")


print(file_read_from_head('test.txt', 2))

That `=` should not be there in `f.write = (...)`. You want to call `f.write`, not assign to it. You should also close `f` after writing (before you try and read what you've written). — khelwood, Oct 18 '20 at 10:26
What @khelwood said. Also you might need to close the file (`f.close()`) before reading it, because the text is written to a buffer first. Closing the file guarantees that all text from the buffer is written to the actual file on disk. — Niklas Mertsch, Oct 18 '20 at 10:29

jwalton · Answer 1 · 2020-10-18T11:05:35.960

2

A few notes:

as indicated in the comments you should be calling f.write(), not assigning output to a variable called f.write
After your call to f.write() you also need to close the file. You can achieve this with f.close(). However, it is much better practice to use a context manager (see the comments below this answer). Using a context manager makes it easier to avoid mistakes (such as forgetting to close a file...). You actually already use a context manager in your file_read_from_head() function.
Your function file_read_from_head() calls print(line), and so print(file_read_from_head()) isn't needed (nor is doing what you intend)
It is generally considered bad practice to import within functions (see this question for a discussion). Instead it preferred to have all your import statements at the top of your file.

Considering all the above, we can rework your code as:

from itertools import islice

def file_read_from_head(fname, nlines):
    with open(fname) as f:
        for line in islice(f, nlines):
            print(line)


# Context managers make it easier to avoid forgetting to close files
with open('test.txt', 'w') as f:
    f.write("Hello welcome to Python \n"
            "This is the second line \n"
            "This is the third line \n")


file_read_from_head('test.txt', 2)

edited Oct 18 '20 at 11:05

answered Oct 18 '20 at 10:29

jwalton

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rather than suggesting to `close` the file, you should really be suggesting to use `with` instead – Tomerikoo Oct 18 '20 at 10:34
There's nothing *wrong* with opening and closing a file as OP did. But I agree, using a context manager is much cleaner (also I hinted at this in the answer) – jwalton Oct 18 '20 at 10:36
There a few things wrong with opening and closing files, one of them is presented above in the question.... I think you edited that in right when I was writing the comment. I would also the code to match that suggestion – Tomerikoo Oct 18 '20 at 10:37
Again, *it is not wrong*. Yes, it's easier to make mistakes, and is less pythonic, and I would always advocate using a context manager. but it is not "wrong" – jwalton Oct 18 '20 at 10:38
2

Yes I guess this argument can go on forever because it depends on how you define wrong... The idiomatic way and the *best* one to handle files is to use `with`. Which your answer suggests to do, which is perfect, I would just change the code sample as-well and avoid using `open/close` in my answers at all – Tomerikoo Oct 18 '20 at 10:40

score 0 · Answer 2 · answered Oct 18 '20 at 10:39

you've done a few things wrong. first f.write is a function not a variable. so replace it with:

f.write("Hello welcome to Python \n"
       "THis is the second line \n"
       "This is the third line \n")
f.close()

when you're opening the file in the file_read_from_head function, you're not providing a read/write flag so replace it with:

with open(fname, 'r') as f:

also you're printing the result of file_read_from_head, which would be none so you should remove the print() around the function call if you want to.

Your second suggestion is irrelevant as the default mode for opening files is `'r'`, so in that case `open(fname)` is perfectly fine — Tomerikoo, Oct 18 '20 at 10:42

File still empty after call to f.write()

2 Answers2