Bug 33013

Summary:	context.get_source().get_surface() fails with "SystemError: NULL object passed to Py_BuildValue"
Product:	pycairo	Reporter:	kriomant
Component:	general	Assignee:	Steve Chaplin <d74n5pohf9>
Status:	RESOLVED FIXED	QA Contact:
Severity:	major
Priority:	medium
Version:	unspecified
Hardware:	x86 (IA32)
OS:	Linux (All)
Whiteboard:
i915 platform:		i915 features:

Description kriomant 2011-01-12 00:08:02 UTC

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);
}

Comment 1 Steve Chaplin 2011-03-27 20:40:08 UTC

Thanks for the bug report, its now fixed in git.
I had assumed the only way to create a surface pattern was using cairo.SurfacePattern() but as your example shows you can bypass this by calling
context.set_source_surface() and
pattern = context.get_source()

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