How C trace functions really work

The heart of is a trace function, a callback that the Python interpreter will invoke for every function call and line of Python executed. The docs tell the basics, but it can get a little involved once you get into more advanced techniques. I just closed a bug in that was due to an incomplete implementation of a trace function, and I learned some more details along the way, including finding a bug in Python itself.

The basics

At its simplest, a trace function is a Python function like this:

def my_py_trace_fn(frame, event, arg):
    # .. do something useful ..
    return my_py_trace_fn


Once your trace function is registered, it will be invoked on every function call or return, every exception, and every line executed. The frame argument is the current execution frame, event is a string indicating what kind of event it is, and arg is something useful like the value returned from the function, or the exception being raised.

Notice that the function returns itself. In every frame, your function is invoked, and whatever it returns is used as the trace function for the next event in that frame. This can be used to change how frames are traced depending on dynamic conditions, but usually your trace function simply returns itself so that the same function continues to be invoked. I’ll call this behavior trace chaining. We’ll be coming back to it, because it’s central to the issues here.

Down to C

At the lowest level, Python’s trace facility is implemented in C, of course, just like the rest of the interpreter. Every thread of execution has two global pointers. The first is called c_tracefunc, and points to a C trace function to invoke, with this signature:

int my_trace(PyObject *obj, PyFrameObject *frame, int event, PyObject *arg)

The second pointer is called c_traceobj, and points to an arbitrary Python object that will be passed as the first argument to the function. The remaining arguments are analogous to the ones in the Python function, except that here event is an int instead of a string.

You register your function and your object by passing them to PyEval_SetTrace(), and then they are invoked for all of the events described above.

At the C level, there is no chaining, the trace function stored in the thread is always invoked.

Implementing the Python level on the C level

Naturally, the Python sys.settrace() function is implemented on top of PyEval_SetTrace(). Rather than show the exact C code, with all of its error handling and reference counting, I’ve only included the bare bones to show how it works:

PyObject *
sys_settrace(PyObject * py_fn)
    if (py_fn == Py_None) {
        PyEval_SetTrace(NULL, NULL);
    else {
        PyEval_SetTrace(trace_trampoline, py_fn);
    return Py_None;

Simply put, sys.settrace(None) will clear the trace function, setting both C pointers to NULL. Calling sys.settrace(my_py_trace_fn) will install a C function called trace_trampoline as the C function to call, with your Python function as the object to pass to it.

Here’s a sketch of trace_trampoline:

    PyObject *obj, PyFrameObject *frame, int event, PyObject *arg
    PyObject *callback;

    if (event == PyTrace_CALL) {
        /* Remember obj is really my_py_trace_fn */
        callback = obj;
    else {
        callback = frame->f_trace;
    if (callback == NULL) {
        return 0;
    result = /* Call callback(frame, event as str, arg) */;
    if (/* error occurred in callback */) {
        PyEval_SetTrace(NULL, NULL);
        frame->f_trace = NULL;
        return -1;
    if (result != Py_None) {
        frame->f_trace = result;
    return 0;

Now we see where the chaining happens: trace_trampoline uses a member of the frame object called f_trace to record the last value returned by the trace function in this frame. When entering a new frame, the event is CALL, and the registered trace function is called. But for all other events, f_trace is used.

After calling the trace function, the return value is examined. If an error occurred, the trace function is unregistered and never called again. Otherwise, the return value is recorded in f_trace, to be used for the next event in this frame.


As a companion to settrace(), sys.gettrace() returns the currently registered Python trace function. Now that we know that settrace() stores its argument in the c_traceobj pointer, it’s clear that gettrace() simply returns whatever is in c_traceobj.

An obvious goal for settrace() and gettrace() is that getting the trace function from gettrace() and then passing it to settrace() should be a no-op:

# This should be as if nothing happened:

When the trace function is written in Python, this invariant is true, gettrace() and settrace() round-trip it quite nicely. When the trace function is written in C, it doesn’t just happen, and making it work is kind of a pain.

C trace functions

To make program measurement faster, uses a trace function written in C. Writing a trace function in C starts out simply enough:

Your function will be invoked on calls, lines, and returns, and all is well.

What happens now when someone calls sys.gettrace()? As we saw, sys.gettrace() returns whatever Python object is stored in c_traceobj. In the case of a Python trace function, that is the function itself. But with a C trace function, it’s whatever object you registered with PyEval_SetTrace(). Using the description I just gave above, there’s no reason for your object to be callable, it simply maintains state for your trace function.

But anyone calling sys.gettrace() will expect a callable Python object, in fact, will expect that it can be passed to sys.settrace() to reinstate it as the trace function.

Re-installable C trace functions

To keep sys.gettrace() working properly, the Python object you create must be callable. Luckily, that is easy to do, you can add the equivalent of a __call__ method to your object. That method will be invoked as a Python trace function, so it should expect the three arguments a Python trace function takes: a frame object, an event string, and an arbitrary Python value. You’ll need to convert those arguments to the form your C function expects, and call it.

Here’s a sketch based on’s C tracer:

static PyObject *
Tracer_call(Tracer *self, PyObject *args, PyObject *kwds_unused)
    PyFrameObject *frame;
    PyObject *what_str;
    PyObject *arg;
    int what;
    static char *what_names[] = {
        "call", "exception", "line", "return",

    if (!PyArg_ParseTuple(args, "O!O!O:Tracer_call",
            &PyFrame_Type, &frame, &PyString_Type, &what_str, &arg)) {
        goto done;

    /* In Python, the what argument is a string, we need to find an int
       for the C function. */
    for (what = 0; what_names[what]; what++) {
        if (!strcmp(MyText_AS_STRING(what_str), what_names[what])) {

    /* Invoke the C function, and return ourselves. */
    if (Tracer_trace(self, frame, what, arg) == RET_OK) {
        return (PyObject *)self;

    return NULL;

Here my C class is called Tracer, and my C function is Tracer_trace. All that happens here is unpacking the Python arguments and converting the string event into an int.

Note that once your function has been round-tripped through sys.settrace(sys.gettrace()), you end up with a convoluted configuration. At first, right after registering your C trace function, the two per-thread pointers look like this:

When a line of code is executed, your Tracer_trace function is invoked, with your Tracer instance as its first argument.

After you execute sys.settrace(sys.gettrace()), the pointers look like this:

Now when a line of Python is executed, trace_trampoline is invoked. It will transform its C-friendly arguments into Python-friendly ones, and call the c_traceobj object, that is, your object. That invokes Tracer_call, which takes the Python values and converts them back into C-friendly values, then calls your original C trace code at Tracer_trace.

The result is a complicated detour through a few levels of adapter code, but your C trace function ends up getting called as it should.

The one remaining problem

OK, now we’re all set, right? Not quite yet. Here’s a simple test of our trace function:

import sys

def foo():
    a = 4   # line number 4, etc.
    a = 6

def bar():
    a = 9
    a = 11

a = 13
a = 15

Here we invoke a function which invokes a function which fiddles with the trace function, and then we return back to the top level. Line 5 should be a no-op, as discussed earlier. If it is, then our trace function will get events for all of the lines in the file.

If we run this program with a Python-based trace function, it works exactly as you’d expect, and all the lines are recorded. But if a C-based trace function is used, lines 6, 11, and 15 are missing from the trace (the values assigned to a are the line numbers, by the way.)

What’s going on? We went through a lot of trouble to make our C trace function round-trip properly, why isn’t it invoked? In fact, the trace function is properly registered, and if another function call followed bar(), it would be properly traced.

The problem is our old friend trace chaining. Remember that although our trace function is in C, it’s been reinstated as a Python function in disguise, so it’s now running under the Python trace function facility trace_trampoline. That code uses the f_trace member of the frame object to keep a “local trace function”. On entering a new frame, the registered trace function is called, and from then on, the value returned by the trace function is stored in the f_trace member of the frame to be used as the trace function for the next event.

The first half of our program was traced by a C trace function, which didn’t touch f_trace. At line 5, our trace function is round-tripped, and as a result, the Python trace implementation layer trace_trampoline is now in place as the C trace function, with our object standing in as a Python callable function. When line 6 is executed, the trace_trampoline code looks at f_trace, and sees a NULL pointer, because it’s never been updated, and so it doesn’t invoke a function at all.

The same thing will happen as each frame finishes execution and returns. The tracing doesn’t work properly until the next function call when the registered function is called again, and trace_trampoline starts to record return values in f_trace.

The last (?) fix

The only way I could see to fix this was to have my C function keep f_trace up-to-date just in case someone round-trips us with gettrace() and settrace(). To do this, every time we are invoked for a new frame (that is, whenever the event is CALL), we set ourselves into the frame’s f_trace member. This way, when the round-trip happens and we’re under the trace_trampoline regime, the frames will have their f_trace members pre-populated with the proper value to invoke us.

It’s unfortunate to have to do work in the C function that’s only to anticipate the possibility of a settrace round-trip. But assigning one pointer (and incrementing the reference count) is a very slight amount of work, and it makes everything work properly.

If you want to see real code that implements all this, tracer.c in the repo has all of these twists and turns.

The biggest surprise: a 10-year bug

As much head-scratching as this bug took, and as long as it took me to understand the issues and settle on a solution, the biggest surprise didn’t come until I began this writeup. When I started to describe the trace_trampoline code, I was struck by this clause:

if (result != Py_None) {
    frame->f_trace = result;

I didn’t understand why it skipped the assignment to f_trace if the result was None. The docs for sys.settrace state very clearly (emphasis mine):

The local trace function should return a reference to itself (or to another function for further tracing in that scope), or None to turn off tracing in that scope.

Yet the C code seems very clearly to avoid chaining if the return value is None. How is it that returning None “turns off tracing in that scope”? The answer is surprising: it doesn’t, the trace_trampoline code has a bug! Returning None from a local trace function has no effect, and tracing continues anyway, in direct contradiction of the docs. Apparently no one knew this, and it’s been like this ever since trace_trampoline was first written almost 10 years ago.

So now I own Python issue 11992 to fix...

See also

If you liked this, then you’ll might also enjoy:


Great post. Both answered my need for a simple example for a trace function to start exploring from, and had the indepth implementation details and interesting anecdotes.

Hi there Ned,

Really appreciate your post and the great insight that it covers.

I am looking into creating a general-purpose tool to trace program execution across multiple interacting processes (some C++, some python & perhaps collated across distributed devices) and gather it all in one place to obtain a chronologically-correlated dataset that, when visualized with correlation to source, can really help to pinpoint all of the interactions that take place in a complex system.

I was curious about the Python component since it was the one shrouded in the most mystery for me, and some light digging on led me here.

Now I understand that instead of trying to fork, I should be able to implement a lightweight trace function in C modeled after (or identical to) the one used by and all I’d need to add to it then would be the timekeeping component. I believe by prioritizing the role of the time dimension, all the complexity around dealing with “branch coverage” become moot. I expect also that I will gain a lot of insight from how you implement the bit where “[] takes care to not trace code that you aren’t interested in.”

Thank you.

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