adds list comprehesion

learning_python3.md

@@ -1473,6 +1473,39 @@ Plus you could add a prompt that asks how big the shift should be (ROT13, ROT16,
 # List comprehension
 
 This is a bit of an advanced topic but I'm putting it here to show you a very unique and powerful feature of python.
+It's an *in-line* combination of `list`, `for` and conditional logic which allows us to make lists of lists which obey certain conditions.
+You can [learn](https://12ft.io/proxy?q=https%3A%2F%2Frealpython.com%2Flist-comprehension-python%2F) about it online.
+
+```python3
+mixed_list = ["one", "2", "three", "4", "5", "six6", "se7en", "8"]
+
+digits = [d for d in mixed_list if d.isdigit()]
+
+print(digits)
+```
+
+The code above can be recreated **without** list comprehension as well, it will just be *a lot* longer.
+
+```python3
+mixed_list = ["one", "2", "three", "4", "5", "six6", "se7en", "8"]
+
+digits = []
+for d in mixed_list:
+    if d.isdigit():
+        digits.append(d)
+
+print(digits)
+```
+
+As the `d` in the list comprehension is **always** a digit, we can convert it to an integer on the spot!
+
+```python3
+mixed_list = ["one", "2", "three", "4", "5", "six6", "se7en", "8"]
+
+digits = [int(d) for d in mixed_list if d.isdigit()]
+
+print(digits)
+```
 
 # Handling files