Add slides for traits exercises

Author: siddhantwahal

slides/02_traits.md

@@ -543,8 +543,75 @@ f.data_changed = True
 <!-- #region slideshow={"slide_type": "slide"} -->
 ## Exercise time!
 
-- Take the simple example without traits
-- Create a simple Traits model for it
+- From the starting script (`stage1_starting_script/face_detect.py`), extract
+  an object that represents an image file.
+- The class for the object should:
+    - Be a traits model, i.e., inherit from `HasStrictTraits`, and, expose
+    - Attributes:
+        - `filepath`: the absolute path to the image file
+        - `metadata`: a dictionary storing EXIF data
+        - `data` a numpy array containing the RGB data
+        - `faces`: a list containing detected faces
+    - Methods:
+        `detect_faces`: returns the list of detected faces
+    - Be reactive:
+        - Ensure `metadata` and `data` are updated with `filepath` is modified
+- Copy `stage1_starting_script/face_detect.py` to `stage2.1_traited_script` and
+  work there
 - *Do not do any plotting in the model!*
 
+- Hint for computing RGB data:
+
+```python
+import numpy as np
+import PIL.Image
+
+with PIL.Image.open(filepath) as img:
+    data = np.asarray(img)
+```
+
+<!-- #endregion -->
+
+<!-- #region slideshow={"slide_type": "slide"} -->
+### Solution
+`stage2.1_traited_script/traited_face_detect.py`
+<!-- #endregion -->
+
+<!-- #region slideshow={"slide_type": "slide"} -->
+## Exercise time!
+
+- Develop another traits model, one that represents a folder containing several
+  image files
+- The class for the object should expose:
+    - Attributes:
+        - `directory`: the absolute path to the folder
+        - `images`: a list of `ImageFile` instances from the previous exercise
+        - `data`: a pandas `DataFrame` to store metadata for each file in the folder
+    - Be reactive:
+        - Ensure `images` and `data` are updated when `directory` is modified
+    - Override `__init__` to ensure directory exists at object initialization
+    - Save work in `stage2.1_traited_script/image_folder.py`
+
+- Hints:
+    - Create a `DataFrame` from `List(Dict)`:
+    ```python
+    import pandas as pd
+    >>> records = [
+        {'A': 5, 'B': 0, 'C': 3, 'D': 3},
+        {'A': 7, 'B': 9, 'C': 3, 'D': 5},
+        {'A': 2, 'B': 4, 'C': 7, 'D': 6}
+    ]
+    >>> df = pd.DataFrame(records)
+       A  B  C  D
+    0  5  0  3  3
+    1  7  9  3  5
+    2  2  4  7  6
+    ```
+    - `os.path.isdir(directory)` to determine if `directory` is valid
+
+<!-- #endregion -->
+
+<!-- #region slideshow={"slide_type": "slide"} -->
+### Solution
+`stage2.1_traited_script/image_folder.py`
 <!-- #endregion -->