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devel / comp.lang.python / Re: File write, weird behaviour

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o Re: File write, weird behaviourThomas Passin

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Re: File write, weird behaviour

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 by: Thomas Passin - Sun, 19 Feb 2023 17:59 UTC

On 2/19/2023 11:57 AM, Axy via Python-list wrote:
> Looks like the data to be written is buffered, so actual write takes
> place after readlines(), when close() flushes buffers.
>
> See io package documentation, BufferedIOBase.
>
> The solution is file.flush() after file.write()

Another possibility, from the Python docs:

"...TextIOWrapper (i.e., files opened with mode='r+') ... To disable
buffering in TextIOWrapper, consider using the write_through flag for
io.TextIOWrapper.reconfigure()"

Also from the docs:

"Warning: Calling f.write() without using the with keyword or calling
f.close() might result in the arguments of f.write() not being
completely written to the disk, even if the program exits successfully."

I realize that in the example, close() was called ... but not
immediately after the write().

> Can't deny, such a behaviour looks utterly weird. Try to avoid designing
> your software this way.
>
> Axy.
>
> On 19/02/2023 14:03, Azizbek Khamdamov wrote:
>> Example 1 (works as expected)
>>
>> file = open("D:\Programming\Python\working_with_files\cities.txt",
>> 'r+') ## contains list cities
>> # the following code adds new record to the beginning of the file,
>> expected behaviour
>> file.write("new city\n")
>>
>> file.close()
>>
>>
>> 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?

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