2.x vs 3.x, lists vs. iterators

mpoulin · web-flow · commit e69d7e7657db · 2018-09-13T11:00:37.000-06:00
Describe the differences between 2.x and 3.x and suggest situations when iterators would be more appropriate than lists.
diff --git a/docs/writing/style.rst b/docs/writing/style.rst
@@ -581,21 +581,12 @@ provide a powerful, concise way to work with lists. Also, the :py:func:`map` and
 :py:func:`filter` functions can perform operations on lists using a different,
 more concise syntax.
 
-Starting with Python 3.0, the :py:func:`map` and :py:func:`filter` 
-functions return an iterator instead of a list. If you really need a list, you
-should wrap these functions in :py:func`list` like so
-
-.. code-block:: python
-
-    list(map(...))
-    list(filter(...))
-
 Filtering a list
 ~~~~~~~~~~~~~~~~
 
-**Very Bad**:
+**Bad**:
 
-Never remove items from a list that you are iterating over.
+Never remove items from list while you are iterating through it.
 Python will lose track of its current position.
 
 .. code-block:: python
@@ -606,28 +597,69 @@ Python will lose track of its current position.
         if i > 4:
             a.remove(i)
 
+Python has standard ways of filtering lists, but there are several things you need to consider
 
-**Bad**:
+* Python 2.x vs. 3.x
+* Lists vs. iterators
+* Creating a new list vs. modifying the original list
+
+Python 2.x vs. 3.x
+::::::::::::::::::
+
+* Starting with Python 3.0, the :py:func:`map` and :py:func:`filter` 
+functions return an iterator instead of a list. If you really need a list, you
+should wrap these functions in :py:func`list` like so
 
 .. code-block:: python
 
-    # Filter elements greater than 4
-    a = [3, 4, 5]
-    b = []
-    for i in a:
-        if i > 4:
-            b.append(i)
+    list(map(...))
+    list(filter(...))
 
-**Good**:
+* List comprehensions and generator expressions work the same in both 2.x and 3.x (except that comprehensions in 2.x "leak" variables into the enclosing namespace)
+
+    * comprehensions create a new list object 
+    * generators iterate over the original list
+
+* The filter function
+
+    * in 2.x returns a list (use itertools.ifilter if you want an iterator)
+    * in 3.x returns an iterator
+
+Lists vs. iterators
+:::::::::::::::::::
+
+Creating a new list requires more work and uses more memory. If you a just going to loop through the new list, consider using an iterator instead.
 
 .. code-block:: python
 
-    a = [3, 4, 5]
-    b = [i for i in a if i > 4]
-    # Or (Python 2.x):
-    b = filter(lambda x: x > 4, a)
-    # Or (Python 3.x)
-    b = list(filter(lambda x: x > 4, a))
+    # comprehensions create a new list object
+    filtered_values = [value for value in sequence if value != x]
+    # Or (2.x)
+    filtered_values = filter(lambda i: i != x, sequence) 
+
+    # generators are lazy
+    filtered_values = (value for value in sequence if value != x)
+    # Or (3.x)
+    filtered_values = filter(lambda i: i != x, sequence)
+    # Or (2.x)
+    filtered_values = itertools.ifilter(lambda i: i != x, sequence) 
+
+
+
+Creating a new list vs. modifying the original list
+:::::::::::::::::::::::::::::::::::::::::::::::::::
+
+Modifying the original list can be risky if there are other variables referencing it. But you can use *slice assignment* if you really want to do that.
+
+.. code-block:: python
+
+    # replace the contents of the original list
+    sequence[::] = [value for value in sequence if value != x]
+    # Or
+    sequence[::] = (value for value in sequence if value != x)
+    # Or
+    sequence[::] = filter(lambda value: value != x, sequence)
+
     
 Modifying the values in a list
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
