Following script:

surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, 10, 10)
context = cairo.Context(surface)
source_surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, 10, 10)
context.set_source_surface(source_surface)
context.get_source().get_surface()

fails with:

Traceback (most recent call last):
  File "bug.py", line 7, in <module>
    context.get_source().get_surface()
SystemError: NULL object passed to Py_BuildValue

Reason:

get_source passes NULL to PycairoPattern_FromPattern:

static PyObject *
pycairo_get_source (PycairoContext *o) {
  return PycairoPattern_FromPattern (
           cairo_pattern_reference (cairo_get_source (o->ctx)), NULL);
}

PycairoPattern_FromPattern stores NULL to "base" field:

PyObject *
PycairoPattern_FromPattern (cairo_pattern_t *pattern, PyObject *base) {
  ...
  Py_XINCREF(base);
  ((PycairoPattern *)o)->base = base;
  ...
}

get_surface returns "base" field:

static PyObject *
surface_pattern_get_surface (PycairoSurfacePattern *o) {
  return Py_BuildValue("O", o->base);
}