An Introduction to Python Part IV

1
An Introduction to Python Part IV

• Dr. Nancy Warter-Perez
• June 23, 2005

2
Overview

• Creating 2-D Lists (already covered)
• Scopes
• Modules
• Doc Strings
• Debugging
• Project Development Time

3
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 2nd
  row and 3rd column of L
• L23 5

4
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 2 x 5 def fnc() global x x 2
print x, fnc() print x gtgtgt 2 2
Ex 1 x 5 def fnc() x 2 print
x, fnc() print x gtgtgt 2 5
5
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
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
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
Dir and DocString Example

• Ex b.py
• Internal comment
• """Module Docstring comment """
• def fn()
• """Function Docstring comment """
• gtgtgt import b
• gtgtgt dir(b)
• '__builtins__', '__doc__', '__file__',
  '__name__', 'fn'
• gtgtgt print b.__doc__
• Module Docstring comment
• gtgtgt print b.fn.__doc__
• Function Doctring comment

9
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
Debugging Tools/Methods

• Can use print statements to manually debug
• Can use debugger in PythonWin
• In Class Example

11
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