On Sun, Jan 24, 2021 at 10:43:54PM -0500, Matt Wozniski wrote:
> On Sun, Jan 24, 2021 at 9:53 AM <2qdxy4rzwzuui...@potatochowder.com> wrote:
>
> > On 2021-01-25 at 00:29:41 +1100,
> > Steven D'Aprano <st...@pearwood.info> wrote:
> >
> > > On Sat, Jan 23, 2021 at 03:24:12PM +0000, Barry Scott wrote:
> > > > First problem I see is that the file may be a pipe and then you will
> > block
> > > > until you have enough data to do the auto detect.
> > >
> > > Can you use `open('filename')` to read a pipe?
> >
> > Yes. Named pipes are files, at least on POSIX.
> >
> > And no. Unnamed pipes are identified by OS-level file descriptors, so
> > you can't open them with open('filename'),
> >
>
> The `open` function takes either a file path as a string, or a file
> descriptor as an integer. So you can use `open` to read an unnamed pipe or
> a socket.
Okay, but I was asking about using open with a filename string. In any
case, the existence of named pipes answers my question.
[...]
> It's possible to do a `f.read(1)` on a file opened in text mode. If the
> first two bytes of the file are 0xC2 0x99, that's either ™ if the file is
> UTF-8, or 슙 if the file is UTF-16BE, or 駂 if the file is UTF-16LE.
Or  followed by the SGC control code in Latin-1. Or ™ in Windows-1252,
or ¬ô in MacRoman. Etc.
> And
> `f.read(1)` needs to pick one of those and return it immediately. It can't
> wait for more information. The contract of `read` is "Read from underlying
> buffer until we have n characters or we hit EOF."
In text mode, reads are always buffered:
https://docs.python.org/3/library/functions.html#open
so `f.read(1)` will read as much as needed, so long as it only returns a
single character.
A typical buffer size is 4096 bytes, or more.
In any case, I believe the intention of this proposal is for *open*, not
read, to perform the detection.
--
Steve
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