[Tutor] Simultaneous read and write on file

[Martin A. Brown]

Hello,

>> I have read in documentation that wb+ mode is for writing and 
>> reading. Am i using wrong mode, should i use rb+ ?
>
>Use w+ to create a new file, opened with read and write access. Use 
>r+ to open an existing file with read and write access. Unlike w+, 
>r+ does not truncate the file and will not create a new file.

[snip]

>FYI, in terms of POSIX open flags [1], the file mode gets mapped as follows:
>
>      |       flags       |   no +   |   +
>    -----------------------------------------
>    r |                   | O_RDONLY | O_RDWR
>    w | O_CREAT, O_TRUNC  | O_WRONLY | O_RDWR
>    a | O_CREAT, O_APPEND | O_WRONLY | O_RDWR
>
>Python 3 only:
>
>    x | O_CREAT, O_EXCL   | O_WRONLY | O_RDWR
>
>In terms of Windows open dispositions and access modes [2], the file
>mode gets mapped as follows:
>
>      |  disposition  |      no +     |            +
>    ---------------------------------------------------------------
>    r | OPEN_EXISTING | GENERIC_READ  | GENERIC_READ, GENERIC_WRITE
>    w | CREATE_ALWAYS | GENERIC_WRITE | GENERIC_READ, GENERIC_WRITE
>    a | OPEN_ALWAYS   | GENERIC_WRITE | GENERIC_READ, GENERIC_WRITE
>
>Python 3 only:
>
>    x | CREATE_NEW    | GENERIC_WRITE | GENERIC_READ, GENERIC_WRITE

The above is a very handy chart.  Did you find this somewhere, eryk 
sun, or is this from your own knowledge and experience?  This might 
benefit an experienced Windows/POSIX user trying to understand 
open() in Python if it were available in the standard documentation.

Thanks for mapping this to common operating systems.  I had inferred 
this already, but this is a great summary.

-Martin

-- 
Martin A. Brown
http://linux-ip.net/