python language programming presentation

Python Programming
D. Sheefa Ruby Grace
Department of Computer Science
Sarah Tucker College

Uses of Python
 shell tools
 system admin tools, command line programs
 extension-language work
 rapid prototyping and development
 language-based modules
 instead of special-purpose parsers
 graphical user interfaces
 database access
 distributed programming
 Internet scripting

Basic operations
 Assignment:
 size = 40
 a = b = c = 3
 Numbers
 integer, float
 complex numbers: 1j+3, abs(z)
 Strings
 'hello world', 'it's hot'
 "bye world"
 continuation via or use """ long text """"

String operations
 concatenate with + or neighbors
 word = 'Help' + x
 word = 'Help' 'a'
 subscripting of strings
 'Hello'[2]  'l'
 slice: 'Hello'[1:2]  'el'
 word[-1]  last character
 len(word)  5
 immutable: cannot assign to subscript

Lists
 lists can be heterogeneous
 a = ['spam', 'eggs', 100, 1234, 2*2]
 Lists can be indexed and sliced:
 a[0]  spam
 a[:2]  ['spam', 'eggs']
 Lists can be manipulated
 a[2] = a[2] + 23
 a[0:2] = [1,12]
 a[0:0] = []
 len(a)  5

Basic programming
a,b = 0, 1
# non-zero = true
while b < 10:
# formatted output, without
n
print b,
# multiple assignment
a,b = b, a+b

Control flow: if
x = int(raw_input("Please enter #:"))
if x < 0:
x = 0
print 'Negative changed to zero'
elif x == 0:
print 'Zero'
elif x == 1:
print 'Single'
else:
print 'More'
 no case statement

Control flow: for
a = ['cat', 'window', 'defenestrate']
for x in a:
print x, len(x)
 no arithmetic progression, but
 range(10)  [0, 1, 2, 3, 4, 5, 6, 7, 8,
9]
 for i in range(len(a)):
print i, a[i]
 do not modify the sequence being
iterated over

Loops: break, continue, else
 break and continue like C
 else after loop exhaustion
for n in range(2,10):
for x in range(2,n):
if n % x == 0:
print n, 'equals', x, '*', n/x
break
else:
# loop fell through without finding a
factor
print n, 'is prime'

Defining functions
def fib(n):
"""Print a Fibonacci series up to n."""
a, b = 0, 1
while b < n:
print b,
a, b = b, a+b
>>> fib(2000)
 First line is docstring
 first look for variables in local, then global
 need global to assign global variables

Functions: default argument
values
def ask_ok(prompt, retries=4,
complaint='Yes or no, please!'):
while 1:
ok = raw_input(prompt)
if ok in ('y', 'ye', 'yes'): return 1
if ok in ('n', 'no'): return 0
retries = retries - 1
if retries < 0: raise IOError,
'refusenik error'
print complaint
>>> ask_ok('Really?')

Keyword arguments
 last arguments can be given as keywords
def parrot(voltage, state='a stiff', action='voom',
type='Norwegian blue'):
print "-- This parrot wouldn't", action,
print "if you put", voltage, "Volts through it."
print "Lovely plumage, the ", type
print "-- It's", state, "!"
parrot(1000)
parrot(action='VOOOM', voltage=100000)

Lambda forms
 anonymous functions
 may not work in older versions
def make_incrementor(n):
return lambda x: x + n
f = make_incrementor(42)
f(0)
f(1)

List methods
 append(x)
 extend(L)
 append all items in list (like Tcl lappend)
 insert(i,x)
 remove(x)
 pop([i]), pop()
 create stack (FIFO), or queue (LIFO)  pop(0)
 index(x)
 return the index for value x

List methods
 count(x)
 how many times x appears in list
 sort()
 sort items in place
 reverse()
 reverse list

Functional programming tools
 filter(function, sequence)
def f(x): return x%2 != 0 and x%3 0
filter(f, range(2,25))
 map(function, sequence)
 call function for each item
 return list of return values
 reduce(function, sequence)
 return a single value
 call binary function on the first two items
 then on the result and next item
 iterate

List comprehensions (2.0)
 Create lists without map(), filter(), lambda
 = expression followed by for clause + zero or more for or
of clauses
>>> vec = [2,4,6]
>>> [3*x for x in vec]
[6, 12, 18]
>>> [{x: x**2} for x in vec}
[{2: 4}, {4: 16}, {6: 36}]

List comprehensions
 cross products:
>>> vec1 = [2,4,6]
>>> vec2 = [4,3,-9]
>>> [x*y for x in vec1 for y in vec2]
[8,6,-18, 16,12,-36, 24,18,-54]
>>> [x+y for x in vec1 and y in vec2]
[6,5,-7,8,7,-5,10,9,-3]
>>> [vec1[i]*vec2[i] for i in
range(len(vec1))]
[8,12,-54]

List comprehensions
 can also use if:
>>> [3*x for x in vec if x >
3]
[12, 18]
>>> [3*x for x in vec if x <
2]
[]

del – removing list items
 remove by index, not value
 remove slices from list (rather than by
assigning an empty list)
>>> a = [-1,1,66.6,333,333,1234.5]
>>> del a[0]
>>> a
[1,66.6,333,333,1234.5]
>>> del a[2:4]
>>> a
[1,66.6,1234.5]

Tuples and sequences
 lists, strings, tuples: examples of sequence type
 tuple = values separated by commas
>>> t = 123, 543, 'bar'
>>> t[0]
123
>>> t
(123, 543, 'bar')

Tuples
 Tuples may be nested
>>> u = t, (1,2)
>>> u
((123, 542, 'bar'), (1,2))
 kind of like structs, but no element
names:
 (x,y) coordinates
 database records
 like strings, immutable  can't assign to
individual items

Tuples
 Empty tuples: ()
>>> empty = ()
>>> len(empty)
0
 one item  trailing comma
>>> singleton = 'foo',

Tuples
 sequence unpacking  distribute elements across
variables
>>> t = 123, 543, 'bar'
>>> x, y, z = t
>>> x
123
 packing always creates tuple
 unpacking works for any sequence

Dictionaries
 like Tcl or awk associative arrays
 indexed by keys
 keys are any immutable type: e.g., tuples
 but not lists (mutable!)
 uses 'key: value' notation
>>> tel = {'hgs' : 7042, 'lennox': 7018}
>>> tel['cs'] = 7000
>>> tel

Dictionaries
 no particular order
 delete elements with del
>>> del tel['foo']
 keys() method  unsorted list of keys
>>> tel.keys()
['cs', 'lennox', 'hgs']
 use has_key() to check for existence
>>> tel.has_key('foo')
0

Conditions
 can check for sequence membership
with is and is not:
>>> if (4 in vec):
... print '4 is'
 chained comparisons: a less than b
AND b equals c:
a < b == c
 and and or are short-circuit operators:
 evaluated from left to right
 stop evaluation as soon as outcome clear

Conditions
 Can assign comparison to variable:
>>> s1,s2,s3='', 'foo', 'bar'
>>> non_null = s1 or s2 or s3
>>> non_null
foo
 Unlike C, no assignment within expression

Comparing sequences
(1,2,3) < (1,2,4)
[1,2,3] < [1,2,4]
'ABC' < 'C' < 'Pascal' < 'Python'
(1,2,3) == (1.0,2.0,3.0)
(1,2) < (1,2,-1)

Modules
 collection of functions and variables, typically in scripts
 definitions can be imported
 file name is module name + .py
 e.g., create module fibo.py
def fib(n): # write Fib. series up to n
...
def fib2(n): # return Fib. series up to n

Modules
 import module:
import fibo
 Use modules via "name space":
>>> fibo.fib(1000)
>>> fibo.__name__
'fibo'
 can give it a local name:
>>> fib = fibo.fib
>>> fib(500)

Modules
 function definition + executable
statements
 executed only when module is imported
 modules have private symbol tables
 avoids name clash for global variables
 accessible as module.globalname
 can import into name space:
>>> from fibo import fib, fib2
>>> fib(500)
 can import all names defined by module:
>>> from fibo import *

Module search path
 current directory
 list of directories specified in PYTHONPATH
environment variable
 uses installation-default if not defined,
e.g., .:/usr/local/lib/python
 uses sys.path
>>> import sys
>>> sys.path
['', 'C:PROGRA~1Python2.2', 'C:Program Files
Python2.2DLLs', 'C:Program FilesPython2.2lib', 'C:
Program FilesPython2.2liblib-tk', 'C:Program Files
Python2.2', 'C:Program FilesPython2.2libsite-
packages']

Classes
 mixture of C++ and Modula-3
 multiple base classes
 derived class can override any methods of
its base class(es)
 method can call the method of a base
class with the same name
 objects have private data
 C++ terms:
 all class members are public
 all member functions are virtual
 no constructors or destructors (not needed)

Classes
 classes (and data types) are objects
 built-in types cannot be used as base classes by user
 arithmetic operators, subscripting can be redefined for
class instances (like C++, unlike Java)

Class definitions
Class ClassName:
<statement-1>
...
<statement-N>
 must be executed
 can be executed conditionally (see Tcl)
 creates new namespace

Namespaces
 mapping from name to object:
 built-in names (abs())
 global names in module
 local names in function invocation
 attributes = any following a dot
 z.real, z.imag
 attributes read-only or writable
 module attributes are writeable

Namespaces
 scope = textual region of Python program
where a namespace is directly accessible
(without dot)
 innermost scope (first) = local names
 middle scope = current module's global names
 outermost scope (last) = built-in names
 assignments always affect innermost
scope
 don't copy, just create name bindings to
objects
 global indicates name is in global scope

Class objects
 obj.name references (plus module!):
class MyClass:
"A simple example class"
i = 123
def f(self):
return 'hello world'
>>> MyClass.i
123
 MyClass.f is method object

Class objects
 class instantiation:
>>> x = MyClass()
>>> x.f()
'hello world'
 creates new instance of class
 note x = MyClass vs. x = MyClass()
 ___init__() special method for
initialization of object
def __init__(self,realpart,imagpart):
self.r = realpart
self.i = imagpart

Instance objects
 attribute references
 data attributes (C++/Java data members)
 created dynamically
x.counter = 1
while x.counter < 10:
x.counter = x.counter * 2
print x.counter
del x.counter

Method objects
 Called immediately:
x.f()
 can be referenced:
xf = x.f
while 1:
print xf()
 object is passed as first argument of function  'self'
 x.f() is equivalent to MyClass.f(x)

Inheritance
class
DerivedClassName(BaseClassName)
<statement-1>
...
<statement-N>
 search class attribute, descending chain of base classes
 may override methods in the base class
 call directly via
BaseClassName.method

Multiple inheritance
class
DerivedClass(Base1,Base2,Base3)
:
<statement>
 depth-first, left-to-right
 problem: class derived from two classes with a common
base class

Private variables
 No real support, but textual replacement (name
mangling)
 __var is replaced by _classname_var
 prevents only accidental modification, not true
protection

Exceptions
 syntax (parsing) errors
while 1 print 'Hello World'
File "<stdin>", line 1
while 1 print 'Hello World'
^
SyntaxError: invalid syntax
 exceptions
 run-time errors
 e.g., ZeroDivisionError, NameError, TypeError

Handling exceptions
while 1:
try:
x = int(raw_input("Please enter a number: "))
break
except ValueError:
print "Not a valid number"
 First, execute try clause
 if no exception, skip except clause
 if exception, skip rest of try clause and use
except clause
 if no matching exception, attempt outer try
statement

Handling exceptions
 try.py
import sys
for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except IOError:
print 'cannot open', arg
else:
print arg, 'lines:',
len(f.readlines())
f.close
 e.g., as python try.py *.py

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