Allows images to be callable

README.md

@@ -20,6 +20,7 @@ pip install mpl-image-labeller
 - Smart interactions with default Matplotlib keymap
 
 ![gif of usage for labelling images of cats and dogs](example.gif)
+
 ## Usage
 
 ```python
@@ -35,9 +36,15 @@ labeller = image_labeller(
 plt.show()
 ```
 
+**accessing the axis**
 You can further modify the image (e.g. add masks over them) by using the plotting methods on
 axis object accessible by `labeller.ax`.
 
+**Lazy Loading Images**
+If you want to lazy load your images you can provide a function to give the images. This function should take
+the integer `idx` as an argument and return the image that corresponds to that index. If you do this then you
+must also provide `N_images` in the constructor to let the object know how many images it should expect. See `examples/lazy_loading.py` for an example.
+
 ### Controls
 
 - `<-` move one image back

examples/lazy_loading.py

@@ -0,0 +1,19 @@
+# You can lazy load images by providing a function instead of a list for *images*
+# if you do this then you must also provide *N_images* in the labeller constructor
+
+
+from mpl_image_labeller import image_labeller
+import matplotlib.pyplot as plt
+
+from numpy.random import default_rng
+
+
+def lazy_image_generator(idx):
+    rng = default_rng(idx)
+    return rng.random((10, 10))
+
+
+labeller = image_labeller(
+    lazy_image_generator, classes=["cool", "rad", "lame"], N_images=57
+)
+plt.show()

mpl_image_labeller/_labeller.py

@@ -23,6 +23,7 @@ def __init__(
         init_labels=None,
         label_keymap: Union[List[str], str] = "1234",
         labelling_advances_image: bool = True,
+        N_images=None,
         fig: Figure = None,
     ):
         """
@@ -42,15 +43,34 @@ def __init__(
             longer perform savefig.
         labelling_advances_image : bool, default: True
             Whether labelling an image should advance to the next image.
+        N_images : int or None
+            The number of images. Required if passing a Callable for images, otherwise
+            ignored.
         fig : Figure
             An empty figure to build the UI in. Use this to embed image_labeller into
             a gui framework.
         """
         self._images = images
+        if callable(images):
+            if not isinstance(N_images, int):
+                raise TypeError(
+                    "If images is a callable then N_images must be provided"
+                )
+            self._N_images = N_images
+            def _get_image(i):
+                return self._images(i)
+        else:
+            self._N_images = len(images)
+            def _get_image(i):
+                return self._images[i]
+
+        self._get_image = _get_image
+
         self._label_advances = labelling_advances_image
+
         if init_labels is None:
-            self._labels = [None] * len(images)
-        elif len(init_labels) != len(images):
+            self._labels = [None] * self._N_images
+        elif len(init_labels) != self._N_images:
             raise ValueError("init_labels must have the same length as images")
         else:
             self._labels = init_labels
@@ -91,7 +111,7 @@ def __init__(
 
         self._image_index = 0
         self._ax = self._fig.add_subplot(111)
-        self._im = self._ax.imshow(images[0])
+        self._im = self._ax.imshow(self._get_image(0))
 
         # shift axis to make room for list of keybindings
         box = self._ax.get_position()
@@ -144,7 +164,7 @@ def labels(self):
 
     @labels.setter
     def labels(self, value):
-        if len(value) != len(self._images):
+        if len(value) != self._N_images:
             raise ValueError(
                 "Length of labels must be the same as the number of images"
             )
@@ -156,15 +176,14 @@ def image_index(self):
 
     @image_index.setter
     def image_index(self, value):
-        N = len(self._images)
         if value == self._image_index:
             # quick return to avoid unnecessary draw
             return
-        elif value >= N:
-            if self._image_index == N - 1:
+        elif value >= self._N_images:
+            if self._image_index == self._N_images - 1:
                 # quick return to avoid unnecessary draw
                 return
-            self._image_index = N - 1
+            self._image_index = self._N_images - 1
         elif value < 0:
             if self._image_index == 0:
                 # quick return to avoid unnecessary draw
@@ -180,7 +199,7 @@ def _update_title(self):
         )
 
     def _update_displayed(self):
-        self._im.set_data(self._images[self._image_index])
+        self._im.set_data(self._get_image(self._image_index))
         self._update_title()
         self._fig.canvas.draw_idle()
 
@@ -194,7 +213,7 @@ def _key_press(self, event):
                 self._label_keymap[event.key]
             ]
             if self._label_advances:
-                if self.image_index == len(self._images) - 1:
+                if self.image_index == self._N_images - 1:
                     # make sure we update the title we are on the last image
                     self._update_title()
                     self._fig.canvas.draw_idle()