add a new challenge, generators

challenges/Readme.md

@@ -11,7 +11,9 @@ features of Python, and help you to write more "Pythonic" code...
 
 [decorators](decorators.py) - decorators, a pretty way of transforming functions using other functions. functional programming!  functions.
 
-[context managers](context_managers.py) - aka the "with" statement.  What's it for, and how does it help to make your code look nicer?
+[context managers](context_managers.py) - aka the `with` statement.  What's it for, and how does it help to make your code look nicer?
+
+[generators](generators.py) - more efficient iterators with python and the `yield` statement.
 
 [The Game of Life](life.md) - a programming classic.  Try it out using OO and functional paradigms in Python.
 

challenges/generators.py

@@ -0,0 +1,37 @@
+"""
+Imagine you wanted to rimplement "head" and "grep" in python.  Here's a first cut:
+"""
+
+def head(max_lines, filenames):
+    for fn in filenames:
+        with open(fn) as f:
+            for ix, line in f:
+                if ix >= max_lines:
+                    return
+                print(line, end='')
+
+
+def grep(needle, filenames):
+    for fn in filenames:
+        with open(fn) as f:
+            for line in f:
+                if needle in line:
+                    print(line, end='')
+
+"""
+There's a lot of duplicated code there!  Now we could build a helper function like this:
+"""
+
+def getlines(filenames):
+    for fn in filenames:
+        with open(fn) as f:
+            return f.readlines()
+
+"""
+but then head would be inefficient, because getlines always reads every single line in the file.
+
+Find out how to use a generator to make a version of getlines that is "lazy"
+
+More info here: http://anandology.com/python-practice-book/iterators.html
+"""
+