io.FileIO.mode doesn't reflect "The mode as given in the constructor." as per the documentation(https://docs.python.org/3/library/io.html#io.FileIO.mode).
Was noted in issue4362, but seems to be still relevant.

Example:
>>> f = open("t.tmp", "w+b")
>>> f.mode
'rb+'

The mode string is calculated here: https://github.com/python/cpython/blob/master/Modules/_io/fileio.c#L1055

and is based on these four values in the fileio struct:

    unsigned int created : 1;
    unsigned int readable : 1;
    unsigned int writable : 1;
    unsigned int appending : 1;

This is not enough information to distinguish between r+ and w+. That distinction was lost in _io_FileIO___init___impl, where it specified the flags that went into creating fd, but these flags are not saved.

The bit fields in this struct add up to 7 bits, so it seems possible to fix this if another bit is allocated for this purpose.

If it's not worth it, then the documentation needs to change a little.

It is a duplicate of issue25341.