1. An Introduction to Python – Part IV Dr. Nancy Warter-Perez June 23, 2005

2. 6/23/05Introduction to Python – Part IV2 Overview Creating 2-D Lists (already covered) Scopes Modules Doc Strings Debugging Project Development Time

3. 6/23/05Introduction to Python – Part IV3 Creating 2-D Lists To create a 2-D list L, with C columns and R rows initialized to 0: L = [[]] #empty 2-Dlist L = [[0 for col in range(C)] for row in range(R)] To assign the value 5 to the element at the 2 nd row and 3 rd column of L L[2][3] = 5

4. 6/23/05Introduction to Python – Part IV4 Scopes Scopes divine the “visibility” of a variable Variables defined outside of a function are visible to all of the functions within a module (file) Variables defined within a function are local to that function To make a variable that is defined within a function global, use the global keyword Ex 1: x = 5 def fnc(): x = 2 print x, fnc() print x >>> 2 5 Ex 2: x = 5 def fnc(): global x x = 2 print x, fnc() print x >>> 2 2

5. 6/23/05Introduction to Python – Part IV5 Modules Why use? Code reuse System namespace partitioning (avoid name clashes) Implementing shared services or data How to structure a Program One top-level file Main control flow of program Zero or more supplemental files known as modules Libraries of tools

6. 6/23/05Introduction to Python – Part IV6 Modules - Import Import – used to gain access to tools in modules Ex: contents of file b.py def spam(text): print text, 'spam' contents of file a.py import b b.spam('gumby')

7. 6/23/05Introduction to Python – Part IV7 Python Documentation Sources #comments In-file documentation The dir function Lists of attributes available on objects Docstrings:__doc__In-file documentation attached to objects

8. 6/23/05Introduction to Python – Part IV8 Dir and DocString Example Ex: b.py # Internal comment """Module Docstring comment """ def fn(): """Function Docstring comment """ >>> import b >>> dir(b) ['__builtins__', '__doc__', '__file__', '__name__', 'fn'] >>> print b.__doc__ Module Docstring comment >>> print b.fn.__doc__ Function Doctring comment

9. 6/23/05Introduction to Python – Part IV9 Debugging Two types of bugs Syntax errors – “easy” to find Logical errors – harder to find Can be a problem in your algorithm Can be a problem in your coding Debugging a program is like solving a puzzle Must first understand what your program is supposed to do Logically trace what is happening in your program What do you expect to happen versus what happened Follow the flow of data Follow the flow of control

10. 6/23/05Introduction to Python – Part IV10 Debugging Tools/Methods Can use print statements to “manually” debug Can use debugger in PythonWin In Class Example

11. 6/23/05Introduction to Python – Part IV11 Workshop Separate your LCS functions into a different module Import the module and call the functions from within your while loop (continually prompting the user if they want to continue) Trace through your program Use print statements to view the score and trace back matrices as they are formed Using the debugger to view the score and trace back matrices