adding session 7 into the index!

Author: PythonCHB

materials/source/index.rst

@@ -64,7 +64,11 @@ Week 6: November 7
 Week 7: November 14
 ...................
 
-November 21: Thanksgiving week -- no class
+:ref:`session_1_07`
+
+:ref:`notes_session07`
+
+**November 21: Thanksgiving week -- no class**
 
 Week 8: November 28
 ...................

materials/source/notes/session07.rst

@@ -22,3 +22,41 @@ Zandra Eng
 Issues that came up during the week.
 ====================================
 
+Mutating vs. re-assigning
+-------------------------
+
+I've seen code like this in a few trigram solutions::
+
+  output = output + [follower]
+
+(``output`` is a list of strings, follower is a single string)
+
+What it does is add a new item to a list.
+
+But is that an efficient way to do that?
+
+If you are adding one element to a list -- append() is the way to go. This works fine, but it's creating a whole new list just to throw it away again:
+
+output_list.append (random_trigram_followers)
+
+and if you are adding another list of objects, you want to use extend(). The way it is now, you are actually doing:
+
+1) create a list with random_trigram_followers in it.
+2) create a new list with the contents of output_list the new list.
+3) re-assign the name output_list to that new list.
+4) throw away the original output_list and the temporary list you created for random_trigram_followers
+
+That's a LOT of overhead!
+
+Be cognizant of when you are mutating (changing) an object vs creating a new one and assigning it to the same name. When you do assignment (=) you are probably creating a new object.
+
+
++= is different -- it is the "in_place" operator, so:
+
+a_list += another_list
+
+does not create an new lists -- it adds to the original list "in place" -- it is identical to:
+
+a_list.extend(another_list)
+
+And is an efficient operation.