Transcript - Cal State LA - Instructional Web Server

An Introduction to Python – Part IV
Dr. Nancy Warter-Perez
Overview
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Creating 2-D Lists (already covered)
Scopes
Modules
Doc Strings
Debugging
Project Development Time
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Creating 2-D Lists
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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)]
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To create a 2-D list initialized to 0 based on the length
of strings c and r:
L = [[0 for col in c] for row in r]
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To assign the value 5 to the element at the 2nd row
and 3rd column of L
L[2][3] = 5
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Mutable and Immutable Objects
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Immutable objects can not be changed
in place. They must be reassigned.
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Immutable objects: simple variables,
tuples, strings
Ex:
>>>T = (1,2,3)
>>>T [2] = 5 # error!
>>>T = T[0:2] + (5,) # (1,2,5)
Note: there are no constant variables in
python
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Mutable objects
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Mutable objects such as lists and
dictionaries can be changed in place
Ex:
>>> L = [1, 2, 3]
>>> L[2] = 88 # [1,2,88]
>>> D = {‘a’:1, ‘b’:2}
>>> D[‘c’] = 3 # {‘a’:1,’b’:2,’c’:3}
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Functions are passed by assignment
– Call by value or call by reference?
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Immutable objects are similar to call by value.
Call by value will pass a copy of a variable a the
parameter and thus, will not change the variable
Example:
def fn(x):
x=x+1
y=5
fn(y)
print y
--- prints 5
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Functions are passed by assignment
– Call by value or call by reference?
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Mutable objects are similar to call by reference
Call by reference – will pass the location of the
parameter and thus, will change the parameter
Example:
def fn(x):
x[1] = 77
y = [1, 2, 3]
fn(y)
print y
--- prints [1, 77, 3]
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Scopes
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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:
Ex 2:
x=5
x=5
def fnc():
def fnc():
x=2
global x
print x,
x=2
fnc()
print x,
print x
fnc()
>>> 2 5
print x
Introduction to Python – Part IV
>>> 2 2
8
Modules
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Why use?
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Code reuse
System namespace partitioning (avoid name
clashes)
Implementing shared services or data
How to structure a Program
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One top-level file
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Main control flow of program
Zero or more supplemental files known as
modules
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Libraries of tools
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Modules - Import
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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')
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Using Global Variables with
Modules
Contents of aa.py
y=5
def x():
global y
y=y+1
def z():
global y
y=y+1
return y
Contents of bb.py
import aa
aa.x()
print aa.z()
print aa.y #ref global var
Output after running bb:
7
7
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Import happens only once!
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Imports load and run a module’s code only the first time
Later imports use the already loaded module object
So, if you make changes to a module that you’ve already
imported, make sure to reload the module in the
interactive window
Ex:
After making changes to module aa (previous slide)
>>> reload (aa) #reload module aa
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Python Documentation
Sources
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#comments
The dir function
In-file documentation
Lists of attributes
available on objects
Docstrings:__doc__ In-file documentation
attached to objects
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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
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Debugging
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Two types of bugs
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Syntax errors – “easy” to find
Logical errors – harder to find
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Can be a problem in your algorithm
Can be a problem in your coding
Debugging a program is like solving a puzzle
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Must first understand what your program is
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Logically trace what is happening in your program
supposed to do
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What do you expect to happen versus what happened
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Follow the flow of data
Follow the flow of control
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Debugging Tools/Methods
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Can use print statements to “manually”
debug
Can use debugger in PythonWin
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In Class Example
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Workshop
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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
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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
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