diff -r 6e781cf3d459 -r baf3fe7c6354 python/pngtile.pyx
--- a/python/pngtile.pyx	Tue Dec 29 15:38:31 2009 +0200
+++ b/python/pngtile.pyx	Tue Dec 29 16:44:48 2009 +0200
@@ -2,6 +2,12 @@
     struct FILE :
         pass
 
+cdef extern from "errno.h" :
+    extern int errno
+
+cdef extern from "string.h" :
+    char* strerror (int err)
+
 cdef extern from "Python.h" :
     int PyFile_Check (object p)
     FILE* PyFile_AsFile (object p)
@@ -34,9 +40,12 @@
     int pt_image_open (pt_image **image_ptr, pt_ctx *ctx, char *png_path, int cache_mode)
     int pt_image_info_func "pt_image_info" (pt_image *image, pt_image_info **info_ptr)
     int pt_image_status (pt_image *image)
+    int pt_image_update (pt_image *image)
     int pt_image_tile (pt_image *image, pt_tile_info *info, FILE *out)
     void pt_image_destroy (pt_image *image)
 
+    char* pt_strerror (int err)
+
 OPEN_UPDATE = PT_OPEN_UPDATE
 CACHE_ERROR = PT_CACHE_ERROR
 CACHE_FRESH = PT_CACHE_FRESH
@@ -46,29 +55,39 @@
 class Error (BaseException) :
     pass
 
+cdef int trap_err (char *op, int ret) except -1 :
+    if ret < 0 :
+        raise Error("%s: %s: %s" % (op, pt_strerror(ret), strerror(errno)))
+
+    else :
+        return ret
+
 cdef class Image :
     cdef pt_image *image
 
     def __cinit__ (self, char *png_path, int cache_mode = 0) :
-        if pt_image_open(&self.image, NULL, png_path, cache_mode) < 0 :
-            raise Error("pt_image_open: " + png_path)
+        trap_err("pt_image_open", 
+            pt_image_open(&self.image, NULL, png_path, cache_mode)
+        )
     
     def info (self) :
         cdef pt_image_info *image_info
-
-        if pt_image_info_func(self.image, &image_info) < 0 :
-            raise Error("pt_image_info")
+        
+        trap_err("pt_image_info",
+            pt_image_info_func(self.image, &image_info)
+        )
 
         return (image_info.width, image_info.height)
     
     def status (self) :
-        cdef int status = pt_image_status(self.image)
-        
-        if status < 0 :
-            raise Error("pt_image_status")
-
-        else :
-            return status
+        return trap_err("pt_image_status", 
+            pt_image_status(self.image)
+        )
+    
+    def update (self) :
+        trap_err("pt_image_update", 
+            pt_image_update(self.image)
+        )
 
     def tile (self, size_t width, size_t height, size_t x, size_t y, object out) :
         cdef FILE *outf
@@ -87,8 +106,9 @@
         ti.x = x
         ti.y = y
         
-        if pt_image_tile(self.image, &ti, outf) < 0 :
-            raise Error("pt_image_tile")
+        trap_err("pt_image_tile", 
+            pt_image_tile(self.image, &ti, outf)
+        )
 
     def __dealloc__ (self) :
         if self.image :