python file API

[Chris Angelico]

On Tue, Sep 25, 2012 at 7:49 AM, Dave Angel <d at davea.name> wrote:
> On 09/24/2012 05:35 PM, zipher wrote:
>> Let file-type have an attribute .pos for position.   Now you can get rid of the seek() and tell() methods and manipulate the file pointer more easily with standard arithmetic operations.
>>
>>>>> file.pos = x0ae1      #move file pointer to an absolute address
>>>>> file.pos +=1            #increment the file pointer one byte
>>>>> curr_pos = file.pos  #read current file pointer
>
> And what approach would you use for positioning relative to
> end-of-file?  That's currently done with an optional second parameter to
> seek() method.

Presumably the same way you reference a list element relative to
end-of-list: negative numbers. However, this starts to feel like magic
rather than attribute assignment - it's like manipulating the DOM in
JavaScript, you set an attribute and stuff happens. Sure it's legal,
but is it right? Also, it makes bounds checking awkward:

file.pos = 42 # Okay, you're at position 42
file.pos -= 10 # That should put you at position 32
foo = file.pos # Presumably foo is the integer 32
file.pos -= 100 # What should this do?
foo -= 100 # But this sets foo to the integer -68
file.pos = foo # And this would set the file pointer 68 bytes from end-of-file.

I don't see it making sense for "file.pos -= 100" to suddenly put you
near the end of the file; it should either cap and put you at position
0, or do what file.seek(-100,1) would do and throw an exception. But
doing the exact same operation on a saved snapshot of the position and
reassigning it would then have quite different semantics in an unusual
case, while still appearing identical in the normal case.

ChrisA