why writing list to file puts each item from list on seperate line?

[Steven D'Aprano]

On Fri, 30 Dec 2005 20:22:52 -0800, homepricemaps wrote:

> if i use the code below to write a list to a file
> 
> list = (food, price, store)

Why are you shadowing the built in type list? This is bad practice. Sooner
or later you will do this:

list = [1, 2, 3]
something = process(list)
... lots of code ...
# try to convert to a list
myList = list(something)

and then you'll spend ages trying to work out why list() raises an
exception.



> data.append(list)
> f = open(r"test.txt", 'a')
> f.write ( os.linesep.join( list ) )
> 
> 
> it outputs to a file like this
> 
> apple
> .49
> star market

That's what you told it to do. Walk through the code:

>>> data = []
>>> L = ("apple", "0.49", "market")
>>> data.append(L)
>>> data
[("apple", "0.49", "market")]  # a list with one tuple

So far so good. But now watch:

>>> f = open(r"test.txt", 'a')  

I hope you aren't opening the file EVERY time you want 
to write a single line

>>> f.write(os.linesep.join(L))

Remember what L is: ("apple", "0.49", "market"). You now join that list
(actually a tuple) into a single string: "apple\n0.49\nmarket\n" and write
that string to the file.

What happened to data? It never gets used after you append to it.


> and i want it to do
> 
> apple, .49. star market

Then what you want to do is change data to a list of strings rather than
a list of tuples. Before appending to data, you join the tuple ("apple",
"0.49", "market") like so:

data.append(", ".join(L) + "\n")  # note newline at the end of each line

Then, after you have appended ALL the lines, you open your file once for
writing, and write data in one go:

f.writelines(data)


Hope this helps.



-- 
Steven.