File write, weird behaviour

[Mats Wichmann]

On 2/19/23 14:06, Dieter Maurer wrote:
> Azizbek Khamdamov wrote at 2023-2-19 19:03 +0500:
>> ...
>> Example 2 (weird behaviour)
>>
>> file = open("D:\Programming\Python\working_with_files\cities.txt",
>> 'r+') ## contains list cities
>> # the following code DOES NOT add new record TO THE BEGINNING of the
>> file IF FOLLOWED BY readline() and readlines()# Expected behaviour:
>> new content should be added to the beginning of the file (as in
>> Example 1)
>> file.write("new city\n")
>>
>> file.readlines()
>> file.close()
>>
>> I could not find anything in documentation to explain this strange
>> behaviour. Why is this happening?
> 
> The effect of "r+" (and friends) is specified by the C standard.
> 
> The Linux doc (of `fopen`) tells us that ANSI C requires that
> a file positioning command (e.g. `seek`) must intervene
> between input and output operations. Your example above violates
> this condition. Therefore, weird behavior is to be expected.

If this isn't sufficiently described, someone should raise an issue 
against the Python docs.  I know that many concepts are "inherited from" 
environments generally in the POSIX space and the C language, because 
that's where Python was hatched (all of which makes perfect sense to me, 
who's been working in those spaces for...ever), but a Python programmer 
shouldn't have to read the ISO C standard (which is not free, although 
you can find copies on-line), or the POSIX standard (which also is not 
free, though manpages for systems like Linux cover the same material), 
in order to figure out how Python is going to work.