$ gdb --args ./python -c '
import io
class R(io.RawIOBase):
    def writable(self): return True
    def write(self, b):
        print("About to evaluate {!r}.format".format(b))
        b.format
        print("Never reached")
                           
b = io.BufferedWriter(R())
b.write(b"x")
b.flush()
'
[. . .]
(gdb) run
[. . .]
About to evaluate <memory at 0x7ffff6ee4380>.format
python: Objects/stringobject.c:120: PyString_FromString: Assertion `str != NULL' failed.

Program received signal SIGABRT, Aborted.
0x00007ffff71422a8 in raise () from /usr/lib/libc.so.6
(gdb) bt full
#0  0x00007ffff71422a8 in raise () from /usr/lib/libc.so.6
No symbol table info available.
#1  0x00007ffff714372a in abort () from /usr/lib/libc.so.6
No symbol table info available.
#2  0x00007ffff713b1b7 in __assert_fail_base () from /usr/lib/libc.so.6
No symbol table info available.
#3  0x00007ffff713b262 in __assert_fail () from /usr/lib/libc.so.6
No symbol table info available.
#4  0x000000000046f7b8 in PyString_FromString (str=0x0)
    at Objects/stringobject.c:120
        size = 0
        op = 0x7ffff7f9d160
        __PRETTY_FUNCTION__ = "PyString_FromString"
#5  0x0000000000562fec in memory_format_get (self=0x7ffff6ee4380)
    at Objects/memoryobject.c:330
No locals.
#6  0x00000000005587d5 in getset_get (descr=0x7ffff7fb93a8, 
    obj=0x7ffff6ee4380, type=0x81da40 <PyMemoryView_Type>)
    at Objects/descrobject.c:146
        res = 0x7ffff7fb5580
#7  0x00000000004611e6 in _PyObject_GenericGetAttrWithDict (
    obj=0x7ffff6ee4380, name=0x7ffff7f9d160, dict=0x0) at Objects/object.c:1399
        tp = 0x81da40 <PyMemoryView_Type>
        descr = 0x7ffff7fb93a8
        res = 0x0
        f = 0x55876a <getset_get>
        dictoffset = 8675760
        dictptr = 0xf74ab600
        __PRETTY_FUNCTION__ = "_PyObject_GenericGetAttrWithDict"
#8  0x000000000046164a in PyObject_GenericGetAttr (obj=0x7ffff6ee4380, 
    name=0x7ffff7f9d160) at Objects/object.c:1461
No locals.
#9  0x0000000000460ab0 in PyObject_GetAttr (v=0x7ffff6ee4380, 
    name=0x7ffff7f9d160) at Objects/object.c:1196
        tp = 0x81da40 <PyMemoryView_Type>
#10 0x00000000004d0dae in PyEval_EvalFrameEx (f=0x7ffff7e31c00, throwflag=0)
    at Python/ceval.c:2541
        [. . .]
#16 0x0000000000421b0e in PyObject_CallMethodObjArgs (callable=0x7ffff7e601e0, 
    name=0x7ffff7fc3820) at Objects/abstract.c:2751
        args = 0x7ffff6cb94c0
        tmp = 0x7ffff6ee4350
        vargs = {{gp_offset = 24, fp_offset = 48, 
            overflow_arg_area = 0x7fffffffddb0, 
            reg_save_area = 0x7fffffffdcf0}}
#17 0x00007ffff6a83283 in _bufferedwriter_raw_write (self=0x7ffff6ee1318, 
    start=0x8ff710 "x", '\313' <repeats 198 times>, <incomplete sequence \313>..., len=1) at /media/disk/home/proj/python/cpython/Modules/_io/bufferedio.c:1726
        buf = {buf = 0x8ff710, obj = 0x0, len = 1, itemsize = 1, readonly = 1, 
          ndim = 1, format = 0x0, shape = 0x7fffffffdde0, strides = 0x0, 
          suboffsets = 0x0, smalltable = {9432288, 140737488346688}, 
          internal = 0x0}
        memobj = 0x7ffff6ee4380
        res = 0x7fffffffde60
        n = 5605226
        errnum = 0
#18 0x00007ffff6a8360d in _bufferedwriter_flush_unlocked (self=0x7ffff6ee1318)
    at /media/disk/home/proj/python/cpython/Modules/_io/bufferedio.c:1773
        written = 0
        n = 1
        rewind = 0
#19 0x00007ffff6a7faf6 in buffered_flush_and_rewind_unlocked (
    self=0x7ffff6ee1318)
    at /media/disk/home/proj/python/cpython/Modules/_io/bufferedio.c:756
        res = 0x7ffff7fc8700
#20 0x00007ffff6a7fd52 in buffered_flush (self=0x7ffff6ee1318, args=0x0)
    at /media/disk/home/proj/python/cpython/Modules/_io/bufferedio.c:783
        res = 0x7ffff6ee1318
#21 0x00000000004d824c in call_function (pp_stack=0x7fffffffe078, oparg=0)
    at Python/ceval.c:4334
        meth = 0x7ffff6a7fc4f <buffered_flush>
        self = 0x7ffff6ee1318
        flags = 4
        tstate = 0x8461b0
        na = 0
        nk = 0
        n = 0
        pfunc = 0x7ffff7f151e8
        func = 0x7ffff6cb86f0
        x = 0x7ffff6ee1318
        w = 0x7ffff7f151e8
[. . .]