file.read Method Documentation (Python 2.7.10)

[Stephen Tucker]

Chris,

Thanks for your reply.

I hope the evidence below (taken from IDLE) clarifies my issue:

Stephen.

======================


1. Create BOM.txt
-----------------

>>> myfil = open ("BOM.txt", "wb")
>>> myfil.write ("\xef" + "\xbb" + "\xbf")
>>> myfil.close()

2. Input three bytes at once from BOM.txt and print them
--------------------------------------------------------

>>> myfil = open ("BOM.txt", "rb")
>>> myBOM = myfil.read (3)
>>> myBOM
'\xef\xbb\xbf'
>>> myfil.close()

3. Input three bytes one at a time from BOM.txt and print them
--------------------------------------------------------------

>>> myfil = open ("BOM.txt", "rb")
>>> myBOM_1 = myfil.read (1)
>>> myBOM_2 = myfil.read (1)
>>> myBOM_3 = myfil.read (1)
>>> myBOM_1
'\xef'
>>> myBOM_2
'\xbb'
>>> myBOM_3
'\xbf'
>>> myfil.close()

4. Input three bytes at once from BOM.txt and print them
--------------------------------------------------------

>>> import codecs
>>> myfil = codecs.open ("BOM.txt", mode="rb", encoding="UTF-8")
>>> myBOM = unicode (myfil.read (3))
>>> myBOM
u'\ufeff'
>>> myfil.close ()

5. Attempt to input three bytes one at a time from BOM.txt and print them
-------------------------------------------------------------------------

>>> myfil = codecs.open ("BOM.txt", mode="rb", encoding="UTF-8")
>>> myBOM_4 = myfil.read (1)
>>> myBOM_5 = myfil.read (1)
>>> myBOM_6 = myfil.read (1)
>>> myBOM_4
u'\ufeff'
>>> myBOM_5
u''
>>> myBOM_6
u''
>>> myfil.close()

Notes

A. The attempt at Part 5 actually inputs all three bytes when we ask it to
input just the first one!

B. The outcome from Part 5 shows that, actually, the request to input text
in Part 4 brought about a response from the program something like this:

   Input the UTF-8-encoded character as the first "byte";
   As expected, after reaching the end of the file, continue supplying an
empty string for each of the requested extra bytes.

======================


On Wed, Jan 11, 2023 at 11:00 AM Chris Angelico <rosuav at gmail.com> wrote:

> On Wed, 11 Jan 2023 at 21:31, Stephen Tucker <stephen_tucker at sil.org>
> wrote:
> >
> > Chris -
> >
> > In the Python 2.7.10 documentation, I am referring to section 5.
> Built-in Types, subsection 5.9 File Objects.
> >
> > In that subsection, I have the following paragraph:
> >
> > file.read([size])
> >
> > Read at most size bytes from the file (less if the read hits EOF before
> obtaining size bytes). If the size argument is negative or omitted, read
> all data until EOF is reached. The bytes are returned as a string object.
> An empty string is returned when EOF is encountered immediately. (For
> certain files, like ttys, it makes sense to continue reading after an EOF
> is hit.) Note that this method may call the underlying C function fread()
> more than once in an effort to acquire as close to size bytes as possible.
> Also note that when in non-blocking mode, less data than was requested may
> be returned, even if no size parameter was given.
> >
>
> Yes, so it should be that number of bytes, which is what it does, isn't it?
>
> ChrisA
> --
> https://mail.python.org/mailman/listinfo/python-list
>