Changes according to the review of PR timhoffm#6:

Raising an error if `shapes_to_replace` is empty.
Wrapping `open()`into a `with` statement.

Author: m-entrup

pptx_blueprint/__init__.py

@@ -43,37 +43,37 @@ def replace_picture(self, label: str, filename: _Pathlike, *, do_not_scale_up: b
         """
         shapes_to_replace = self._find_shapes(label)
         if not shapes_to_replace:
-            return
+            raise ValueError(f"The label '{label}' can't be found in the template.")
         if isinstance(filename, str):
             filename = pathlib.Path(filename)
         if not filename.is_file():
             raise FileNotFoundError(f"The file does not exist: {filename}")
-        img_file = open(filename, "rb")
-        old_shape: BaseShape
-        for old_shape in shapes_to_replace:
-            slide_shapes = old_shape._parent
-            img_shape = slide_shapes.add_picture(
-                image_file=img_file,
-                left=old_shape.left,
-                top=old_shape.top,
-            )
-            # Scaling the image if `do_not_scale == False`:
-            if img_shape.height <= old_shape.height and img_shape.width <= old_shape.width and not do_not_scale_up:
-                old_aspect_ratio = old_shape.width / old_shape.height
-                new_aspect_ratio = img_shape.width / img_shape.height
-                if old_aspect_ratio >= new_aspect_ratio:
-                    img_shape.width = old_shape.width
-                    img_shape.height = int(img_shape.width / new_aspect_ratio)
-                else:
-                    img_shape.height = old_shape.height
-                    img_shape.width = int(img_shape.height * new_aspect_ratio)
-            # Centering the image at the extent of the placeholder:
-            img_shape.top += int((old_shape.height - img_shape.height) / 2)
-            img_shape.left += int((old_shape.width - img_shape.width) / 2)
-            del slide_shapes[slide_shapes.index(old_shape)]
-            # Removing shapes is performed at the lxml level.
-            # The `element` attribute contains an instance of `lxml.etree._Element`.
-            slide_shapes.element.remove(old_shape.element)
+        with open(filename, "rb") as img_file:
+            old_shape: BaseShape
+            for old_shape in shapes_to_replace:
+                slide_shapes = old_shape._parent
+                img_shape = slide_shapes.add_picture(
+                    image_file=img_file,
+                    left=old_shape.left,
+                    top=old_shape.top,
+                )
+                # Scaling the image if `do_not_scale == False`:
+                if img_shape.height <= old_shape.height and img_shape.width <= old_shape.width and not do_not_scale_up:
+                    old_aspect_ratio = old_shape.width / old_shape.height
+                    new_aspect_ratio = img_shape.width / img_shape.height
+                    if old_aspect_ratio >= new_aspect_ratio:
+                        img_shape.width = old_shape.width
+                        img_shape.height = int(img_shape.width / new_aspect_ratio)
+                    else:
+                        img_shape.height = old_shape.height
+                        img_shape.width = int(img_shape.height * new_aspect_ratio)
+                # Centering the image at the extent of the placeholder:
+                img_shape.top += int((old_shape.height - img_shape.height) / 2)
+                img_shape.left += int((old_shape.width - img_shape.width) / 2)
+                del slide_shapes[slide_shapes.index(old_shape)]
+                # Removing shapes is performed at the lxml level.
+                # The `element` attribute contains an instance of `lxml.etree._Element`.
+                slide_shapes.element.remove(old_shape.element)
 
     def replace_table(self, label: str, data) -> None:
         """Replaces rectangle placeholders on one or many slides.