File write, weird behaviour

[Thomas Passin]

On 2/19/2023 6:10 PM, Mats Wichmann wrote:
> 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.

The Python docs say that text file behavior is done by Python, not C, 
and so its behavior will be consistent across all platforms.  I didn't 
find any words about this case, though.