The `readline.set_completer_delims` doesn't play well with others because it assumes that only it ever allocates or modifies the rl_completer_word_break_characters buffer. If other programs modify this value, for example changing it from heap allocated space to stack allocated space, the results can be catastrophic. To remind you, the function essentially works as: set_completer_delims(PyObject *self, PyObject *args) { // ... free((void*) rl_completer_word_break_characters; rl_completer_word_break_characters = strdup(break_chars); // ... } where `break_chars` is the user provided string. Take, for example, R as another programs which changes the readline completer strings. When an embedded R instance is initialized (say, using `r2py`) something similar to the following takes place:: static void set_rl_completer_word_break_characters(const char *new) { static char[201] buffer; strncpy(buffer, new, 200); rl_completer_word_break_characters = buffer; } static void initialize_embedded_R(...) { // ... set_rl_completer_word_break_characters(...); } As you might expect the next trip through `readline.set_completer_delims` after initializing R will be catastrophic when we attempt to free a stack allocate buffer. I think we should consider modifying the `readline.set_completer_delims` to store the allocated buffers in the module state:: set_completer_delims(PyObject *self, PyObject *args) { // ... free(_readlinestate_global->break_chars); rl_completer_word_break_characters = strdup(break_chars); _readlinestate_global->break_chars = rl_completer_word_break_characters; // ... } This would prevent the segfault and memory leaks, and would render weird hacks (like https://bitbucket.org/lgautier/rpy2/commits/408bae913653 in the r2py code) unnecessary.