[Tutor] creating files with python and a thanks

[Danny Yoo]

On Mon, 18 Feb 2002, Eve Kotyk wrote:

> > Let's look at that error message you showed before:
> > 
> > Traceback (innermost last):
> >   File "<pyshell#78>", line 1, in ?
> >     gather_data()
> >   File "<pyshell#75>", line 9, in gather_data
> >     f.write(output)
> > TypeError: read-only buffer, list
>
> No problem but could you explain the error message a little?  I
> assumed it meant that I had variable elements of a different type
> (such as a string and integers) but I tried the code again using only
> strings and I still got the same error.  What is a read-only buffer?


Yes, that error message was confusing, and according to:

    http://mail.python.org/pipermail/python-dev/2000-December/010845.html

the developers changed the message to make a little more sense.  *grin*
Python 2.2 gives a better error message, so let me get it to error out in
a similar way:


###
>>> f = open('foo.txt', 'w') 
>>> some_data = ['this is the first line', 'this is the second line']
>>> f.write(42)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: argument 1 must be string or read-only character buffer, not
int
>>> f.write(some_data)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: argument 1 must be string or read-only character buffer, not
list
>>> f.write("ok, ok")
>>> 
###


So f.write() likes to see only strings.  By the way, if we want to write
the contents of a bunch of strings in a list, we can use the
f.writelines() function:

    http://www.python.org/doc/current/lib/bltin-file-objects.html



'Character buffer' is a low level term that's roughly a synonym for
"string", but I believe it's actually a little broader.  It's somewhat
obscure: there's a mention of "buffers" in the documentation:

    http://www.python.org/doc/lib/built-in-funcs.html

with the "buffer()" function, as well as some reference material in the
Python/C API:

    http://www.python.org/doc/current/api/bufferObjects.html

>From what I can gather: a character buffer is any object in Python that
supports byte-oriented access, that is, stuff that's easy to write into a
file.  Buffers provide a way to get access to data in a very low-level,
efficient manner.  However, we probably won't ever need to manipulate
buffers by hand, as the majority of "buffer" objects we run into are
strings.  I guess I'm trying to say: don't worry about character buffers.  
*grin*



Hope this helps!