Understand workflow about reading and writing files in Python

[DL Neil]

Yes, better to reply to list - others may 'jump in'...


On 20/06/19 5:37 PM, Windson Yang wrote:
> Thank you so much for you review DL Neil, it really helps :D. However, 
> there are some parts still confused me, I replyed as below.

It's not a particularly easy topic...


> DL Neil <PythonList at danceswithmice.info 
> <mailto:PythonList at danceswithmice.info>> 于2019年6月19日周三 下午2:03写道：
> 
>     I've not gone 'back' to refer to any ComSc theory on buffer-management.
>     Perhaps you might benefit from such?
> 
> I just take a crash course on it so I want to know if I understand the 
> details correctly :D

...there are so many ways one can mess-up!


>     I like your use of the word "shift", so I'll continue to use it.
> 
>     There are three separate units of data to consider - each of which
>     could
>     be called a "buffer". To avoid confusing (myself) I'll only call the
>     'middle one' that:
>     1 the unit of data 'coming' from the data-source
>     2 the "buffer" you are implementing
>     3 the unit of data 'going' out to a data-destination.
> 
> Just to make it clear, when we use `f.write('abc')` in python, (1) means 
> 'abc', (2) means the buffer handle by Python (by default 8kb), (2) means 
> the file *f* we are writing to, right?

No! (sorry) f.write() is an output operation, thus nr3.

"f" is not a "buffer handle" but a "file handle" or more accurately a 
"file object".

When we:

	one_input = f.read( NRbytes )

(ignoring EOF/short file and other exceptions) that many bytes will 
'appear' in our program labelled as "one_input".

However, the OpSys may have read considerably more data, depending upon 
the device(s) involved, the application, etc; eg if we ask for 2 bytes 
the operating system will read a much larger block (or applicable unit) 
of data from a disk drive.

The same applies in reverse, with f.write( NRbytes/byte-object ), until 
we flush or close the file.

Those situations account for nr1 and nr3. In the usual case, we have no 
control over the size of these buffers - and it is best not to meddle!

Hence:-

>     1 and 3 may be dictated to you, eg hardware or file specifications,
>     code
>     requirements, etc.
> 
>     So, data is shifted into the (2) buffer in a unit-size decided by (1) -
>     in most use-cases each incoming unit will be the same size, but
>     remember
>     that the last 'unit' may/not be full-size. Similarly, data shifted out
>     from the (2) buffer to (3).
> 
>     The size of (1) is likely not that of (3) - otherwise why use a
>     "buffer"? The size of (2) must be larger than (1) and larger than (2) -
>     for reasons already illustrated.
> 
> Is this a typo? (2) larger than (1) larger than (2)?

Correct - well spotted! nr2 > nr1 and nr2 > nr3


>     I recall learning how to use buffers with a series of hand-drawn block
>     diagrams. Recommend you try similarly!

Try this!


>     Now, let's add a few critiques, as requested (interposed below):-
> 
> 
>     On 19/06/19 3:53 PM, Windson Yang wrote:t
>      > I'm trying to understand the workflow of how Python read/writes
>     data with
>      > buffer. I will be appreciated if someone can review it.
>      >
>      > ### Read n data
> 
>     - may need more than one read operation if the size of (3) "demands"
>     more data than the size of (1)/one "read".
> 
> 
> Looks like the size of len of one read() depends on 
> https://github.com/python/cpython/blob/master/Modules/_io/bufferedio.c#L1655 ?


You decide how many bytes should be read. That's how much will be 
transferred from the OpSys' I/O into the Python program's space. With 
the major exception, that if there is no (more) data available, it is 
defined as an exception (EOF = end of file) or if there are fewer bytes 
of data than requested (in which case you will be given only the number 
of bytes of data-available.


>      > 1. If the data already in the buffer, return data
> 
>     - this a data-transfer of size (3)
> 
>     For extra credit/an unnecessary complication (but probable speed-up!):
>     * if the data-remaining is less than size (3) consider a read-ahead
>     mechanism
> 
>      > 2. If the data not in the buffer:
> 
>     - if buffer's data-len < size (3)
> 
>      >      1. copy all the current data from the buffer
> 
>     * if "buffer" is my (2), then no-op
> 
> I don't understand your point here, when we read data we would copy some 
> data from the current buffer from python, right? 
> (https://github.com/python/cpython/blob/master/Modules/_io/bufferedio.c#L1638), 
> we use `out` (which point to res) to store the data here.

We're becoming confused: the original heading 'here' was "### Read n 
data" which is inconsistent with "out" and "from python".


If the read operation is set to transfer (say) 2KB into the program at a 
time, but the code processes it in 100B units, then it would seem that 
after the first read, twenty process loops will run before it is 
necessary to issue another input request.

In that example, the buffer (nr2) is twenty-times the length of the 
input 'buffer' (nr1).

So, from the second to the twentieth iteration of the process, your 
step-1 "1. If the data already in the buffer, return data" (and thus my 
"no-op) applies!

This is a major advantage of having a buffer in the first place - 
transfers within RAM are significantly faster than I/O operations!


>      >      2. create a new buffer object, fill the new buffer with raw
>     read which
>      > read data from disk.
> 
>     * this becomes: perform read operation and append incoming data (size
>     (1)) to "buffer" - hence why "buffer" is larger than (1), by definition.
>     NB if size (1) is smaller than size (3), multiple read operations
>     may be
>     necessary. Thus a read-loop!?
> 
> Yes, you are right, here is a while loop 
> (https://github.com/python/cpython/blob/master/Modules/_io/bufferedio.c#L1652) 
> 
> 
> 
>      >      3. concat the data in the old buffer and new buffer.
> 
>     = now no-op. Hopefully the description of 'three buffers' removes this
>     confusion of/between buffers.
> 
>   I don't get it. When we call the function like seek(0) then 
> read(1000), we can still use the data from buffer from python, right?

I fear that we are having terminology issues - see the original 
description of three 'buffers'. Which "buffer" are you talking about?
1 the seek/read are carried-out against a file object, which will indeed 
have its own buffer, size unknown to Python. (buffer 1)
2 the read(1000) operation will (on its own) allow you to populate a 
buffer within your code, 1000-bytes in length. (buffer 2)


>      >      4. return the data
> 
>     * make the above steps into a while-loop and there won't be a separate
>     step here (it is the existing step 1!)
> 
> 
>     * build all of the above into a function/method, so that the 'mainline'
>     only has to say 'give me data'!
> 
> 
>      > ### Write n data
>      > 1. If data small enough to fill into the buffer, write data to
>     the buffer
> 
>     =yes, the data coming from source (1), which in this case is 'your'
>     code
>     may/not be sufficient to fill the output size (3). So, load it into the
>     "buffer" (2).
> 
>      > 2. If data can't fill into the buffer
>      >      1. flush the data in the buffer
> 
>     =This statement seems to suggest that if there is already some data in
>     the buffer, it will be wiped. Not recommended!
> 
> We check if any data in the buffer if it does, we flush them to the disk 
> (https://github.com/python/cpython/blob/master/Modules/_io/bufferedio.c#L1948) 
> 
> 
>     =Have replaced the next steps, see below for your consideration:-
> 
>      >          1. If succeed:
>      >              1. create a new buffer object.
>      >              2. fill the new buffer with data return from raw write
>      >          2. If failed:
>      >              1. Shifting the buffer to make room for writing data
>     to the
>      > buffer
>      >              2. Buffer as much writing data as possible (may raise
>      > BlockingIOError)
>      >      2. return the data
> 
>     After above transfer from data-source (1) to "buffer" (2):
> 
>     * if len( data in "buffer" ) >= size (3): output
>              else: keep going
> 
>     * output:
>              shift size(3) from "buffer" to output
>              retain 'the rest' in/as "buffer"
> 
>     NB if the size (2) of data in "buffer" is/could be multiples of size
>     (3), then the "output" function should/could become a loop, ie keep
>     emptying the "buffer" until size (2) < size (3).
> 
> 
>     Finally, don't forget the special cases:
>     What happens if we reach 'the end' (of 'input' or 'output' phase), and
>     there is still data in (1) or (2)?
>     Presumably, in "Read" we would discard (1), but in the case of "Write"
>     we MUST empty "buffer" (2), even if it means the last write is of less
>     than size (3).
> 
> Yes, you are right, when we are writing data to the buffer and the 
> buffer is full, we have to flush it.
> 
>     NB The 'rules' for the latter may vary between use-cases, eg add
>     'stuffing' if the output record MUST be x-bytes long.
> 
> 
>     Hope this helps.
>     Do you need to hand-code this stuff though, or is there a better way?
> 
> I'm trying to write an article for it :D


Perhaps it would help to discuss the use-case you will use as the 
article's example.

"I take a crash course" cf "write an article"???


Web-Refs:

Wikipedia: https://en.wikipedia.org/wiki/Data_buffer

The PSL's IO library (?the code you've been reading): 
https://docs.python.org/3.6/library/io.html?highlight=buffer#io.TextIOBase.buffer

The PSL's Readline library (which may be easier to visualise for 
desktop-type users/coders - unless you're into IoT applications and 
similar) https://docs.python.org/3.6/library/readline.html?highlight=buffer

PSL's Buffer protocol, in case you really want to 're-invent the wheel', 
but with some possibly-helpful explanation: 
https://docs.python.org/3.6/c-api/buffer.html?highlight=buffer


-- 
Regards =dn