Move guard into Rust.

Author: tychedelia

crates/processing_pyo3/src/lib.rs

@@ -161,10 +161,11 @@ fn _present(module: &Bound<'_, PyModule>) -> PyResult<()> {
 
 #[pyfunction]
 #[pyo3(pass_module)]
-fn _readback_png(module: &Bound<'_, PyModule>) -> PyResult<Vec<u8>> {
-    let graphics =
-        get_graphics(module)?.ok_or_else(|| PyRuntimeError::new_err("call size() first"))?;
-    graphics.readback_png()
+fn _readback_png(module: &Bound<'_, PyModule>) -> PyResult<Option<Vec<u8>>> {
+    let Some(graphics) = get_graphics(module)? else {
+        return Ok(None);
+    };
+    graphics.readback_png().map(Some)
 }
 
 #[pyfunction]

crates/processing_pyo3/src/python/jupyter_post_execute.py

@@ -2,10 +2,9 @@
 import IPython.display as _ipy_display
 
 def _processing_post_execute(result):
-    if getattr(processing, '_graphics', None) is None:
-        return
     processing._present()
     png_data = processing._readback_png()
-    _ipy_display.display(_ipy_display.Image(data=bytes(png_data)))
+    if png_data is not None:
+        _ipy_display.display(_ipy_display.Image(data=bytes(png_data)))
 
 get_ipython().events.register('post_run_cell', _processing_post_execute)