Outline for May 22, 2014

1. Examples
   1. Put lines in a file in random order [randlines.py]
   2. Read in a list of words from a file, then search it as requested; similar to linear search program [search-1.py]
   3. Now see how many words you checked total [search-1c.py]
2. Gotchas!
   1. What to do when a function returns a value
   2. if x == ’a’ or ’A’
   3. Scope, especially defining functions within functions
3. Namespaces
4. Importing modules
   1. import math
   2. from math import sin, cos, sqrt
   3. from math import *
5. time module
   1. time.clock()
   2. How to time a function call
6. Debugging
   1. Syntax errors: where Python notices it, not necessarily where it is
   2. It (seems to do) nothing: usually forgot to call something
   3. It hangs: look for an infinite loop or a recursion with no base case (or one that may not be reached) — print is your friend!
   4. Tracing flow of execution: put print statements at beginning, end of each function saying which function you are entering and leaving; printing parameters, return values can be very useful
   5. Exceptions
      1. NameError: variable doesn’t exist in local environment
      2. TypeError: using a value improperly (like indexing a string with a non-integer); item in format string doesn’t match item, or conversion is invalid; passing wrong number of arguments to a function or method
      3. IndexError: index of string or list element is out of bounds
      4. AttributeError: referencing a method that doesn’t exist
   6. Semantic errors: try to figure out where the error occurs (hand-running a simple case, or using pythontutor.org, is very helpful)
   7. Simplify — complex expressions sometimes need to be written as two or more statements
   8. Still can’t get the bug: take a walk outside, get away from the program for a bit, ask a friend to look at the program