An Overview Of Python With Functional Programming

An Overview of Python with
Functional Programming
Adam Getchell
PHY 210

Attributes
• Portable, interpreted, object-oriented, strongly- and dynamically-typed, extensible
– Can write extensions to Python in C/C++
– Links to operating system functions using modules
• Open sourced, developed and supported by the Python Organization (http://www.python.org)
• Freely available on most platforms (Windows, Unix, Linux, *BSD, MacOS)
• Supports procedural, object-oriented, and functional programming
• Huge number of add-on modules available
– Numarray provides capabilities similar to MatLab, IDL, or Octave
• http://www.stsci.edu/resources/software_hardware/numarray
– SciPy implements plotting, parallel computation, FFT, ODE solvers, and linear algebra/LAPACK/BLAS routines from
http://www.netlib.org using thin python wrappers over C or FORTRAN code (and using WEAVE allows direct inlining
of C/C++ code within Python)
• http://www.scipy.org/
– mxODBC provides ODBC database connectivity (e.g. Oracle, SQLServer, mySQL, PostgreSQL, DB2, Sybase)
• http://www.egenix.com/files/python/mxODBC.html
– ReportLab Open Source PDF Library generates PDF files
• http://www.reportlab.org/
– PyXML parses/handles XML with DOM, SAX, XPath/XPointer
• http://pyxml.sourceforge.net/topics/download.html
– Jython implements Python on top of Java, gaining access to the Java platform with Python syntax
• http://www.jython.org
– wxPython implements a cross-platform GUI library for GUI applications
• http://www.wxpython.org/
– MatPlotLib is a cross platform Python 2D plotting library which produces publication quality figures in a variety of
hardcopy formats and interactive GUI environments
• http://matplotlib.sourceforge.net
• Runs large-scale software projects
– Google
– Hubble Space Telescope data processing pipeline
– Zope and Plone

Tokens and Structure
Code Pattern C Python
Statements Unlimited length, terminated with “;”
Code blocks delineated by { }
One statement per line, unless explicitly joined by or implicitly joined across (), [], or {}
Code blocks delineated by indentation level (tabs)
Comment /* Comment */ # Comment to end of line unless line joined implicitly
Variables Explicitly declared (statically typed)
int, long int, double, float, etc
Objects have identity, type, and value. Identity is memory address, type determines
supported methods and whether the object can be changed (mutable).
Variables dynamically typed at runtime
Boolean, int (equivalent to long), decimal, float, imaginary, Iterators, strings, Unicode
strings, tuples, lists, buffers, xrange, set, frozenset, dictionaries,
Scoping Global variables declared prior to main()
Local variables declared in each procedure
Statically scoped block structure (local, global, built-in) with static lexical scoping. Lexical
scoping defaults to namespace of enclosing function or class
Functions Type and arguments declared Name and parameters must be declared; can take variable list of arguments using varargs()
Functions are first-class elements of language supporting Functional Programming using List
Comprehensions, lambda, map(), reduce(), filter() and Generators with yield()
Matrices A[i][j] Vector = array([1,2,3,4,5]) # from numarray
Matrix = array([1,2],[3,4]) # from numarray
Tuple = ( 1, 2, “three” ) # immutable, ordered, faster than list
List = [ 4, 5, Tuple ] # mutable, ordered, expands dynamically as needed
Dictionary = { “key1”: “value1”, “key2”: “value2” } # mutable, unordered
Open Files Input_file=fopen(filename, “r”)
Output_file=fopen(filename, “w”)
f = open("/music/_singles/kairo.mp3", "rb") # read binary mode
g = open(“/temp/outputfile.txt”, “a”) # append mode
h = open(“/temp/outputfile.txt”, “w”) # overwrite mode
Input scanf(stdin, “ %lf “, &var)
fscanf(input_file, “ %lf “, &var)
s = raw_input(“Who goes there?”) or sys.stdin.readline()
s = f.read() or f.readlines()
Output printf(stdout, “ %lf “, var)
printf(stdout, “ %lf “, var)
print s
Relational
Operators
==, >=, <=, !=, <, > Python supports operator overloading
==,>=,<=,!=,<>, <, >, is, is not
Logical Operators &&, ||, ! and, or, not
Bitwise Operators &, |, ^, ~, >>, << &, |, ^, ~, >>, <<

Tokens and Structure part II
Pointers Declaration: int *p (p is the address of an
integer, it points to an integer)
int *p, value, var;
var = 10;
p = &var;
Value = *p (Value = 10)
Python does not directly have pointers
However, everything in Python is an object
Objects are passed by reference and can be compared using is and is not operators, which
tests for equality by looking at object identity (memory address)
Python modules and C-extensions can handle the encapsulation of C-pointers in Python objects
If Statements if (i<N)
statements;
else if (i>N)
statements;
else
statements;
if i<N:
statements
elif i>N:
statements
else:
statements
For Statements for (i=0; i < N; i++)
statements;
for i in range(N):
statements
While Statements while (i<N)
statements;
while i < N:
statements
Including files #include <stdio.h> import sys
Exception
Handling
None for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except IOError:
print 'cannot open', arg
else:
print arg, 'has', len(f.readlines()), 'lines‘
f.close()
Classes None class MyClass:
"A simple example class“
i = 12345
def f(self):
return 'hello world'

Tokens and Structure part III
Compound Data
Type Operations
None >>> a = ['spam', 'eggs', 100, 1234]
>>> a ['spam', 'eggs', 100, 1234]
>>> a[0]
'spam'
>>> a[3]
1234
>>> a[-2]
100
>>> a[1:-1]
['eggs', 100]
>>> a[:2] + ['bacon', 2*2]
['spam', 'eggs', 'bacon', 4]
>>> 3*a[:3] + ['Boe!']
['spam', 'eggs', 100, 'spam', 'eggs', 100, 'spam', 'eggs', 100, 'Boe!']
Multiple
Inheritance
None class DerivedClassName(Base1, Base2, Base3):
statements
Structs typedef struct
{
int x;
int array[100];
} Foo; /* note semi-colon here */
class Employee:
pass
john = Employee() # Create an empty employee record
# Fill the fields of the record
john.name = 'John Doe'
john.dept = 'computer lab‘
john.salary = 1000
Iterators None for element in [1, 2, 3]:
print element
The for loop calls iter() on element
Generators None def reverse(data):
for index in range(len(data)-1, -1, -1):
yield data[index]
>>> for char in reverse('golf'):
... print char
...
f
l
o
g

Script
# Need this to run console commands
import os
# Need this to send email
import smtplib
# Need this to process email
from email.MIMEText import MIMEText
# Need this for time
from time import strftime
# First, run the webchecker script and write results to file
os.system("webchecker.py -q http://169.237.48.10 > results.txt")
# Get local time
runtime = strftime('%a %d %b %Y %H:%M:%S')
# Open the file
fp = open("results.txt")
# Create message
msg = MIMEText(fp.read())
fp.close()
me = "Webnanny"
you = "AdamG@hrrm.ucdavis.edu"
msg['Subject'] = 'www.hr.ucdavis.edu link check %s' % runtime
msg['From'] = me
msg['To']= you
server = smtplib.SMTP('hrrm.ucdavis.edu')
server.set_debuglevel(1)
#server.connect()
server.sendmail(me, you, msg.as_string())
server.quit()

Procedural Program
"""Delete files older than AGING constant using os.path.walk"""
import sys, os, time
AGING = 172800 # Default 2 days
def lister(dummy, dirName, filesInDir):
print '[' + dirName + ']'
for fname in filesInDir:
path = os.path.join(dirName, fname)
if not os.path.isdir(path):
print path, time.ctime(os.stat(path).st_mtime), fileage(path),
prune(path)
purge(path)
def fileage(file):
curtime = time.time()
modtime = os.stat(file).st_mtime
age = curtime - modtime
return age
def prune(file):
if fileage(file) > AGING:
return ("T")
else:
return ("File should not be deleted")
def purge(file):
if prune(file) == "T":
os.remove(file)
if __name__ == '__main__':
os.path.walk(sys.argv[1], lister, None)

Object Oriented Program
class Employee:
def __init__(self, name, salary = 0):
self.name = name
self.salary = salary
def giveRaise(self, percent):
self.salary += self.salary * percent
def work(self):
print self.name, "does stuff"
def __repr__(self):
return "<Employee: name=%s, salary=%s>" % (self.name, self.salary)
class Chef(Employee):
def __init__(self, name):
Employee.__init__(self, name, 50000)
def work(self):
print self.name, "makes food"
class Server(Employee):
Employee.__init__(self, name, 40000)
def work(self):
print self.name, "interfaces with customers"
class PizzaRobot(Chef):
Chef.__init__(self,name)
def work(self):
print self.name, "makes pizza"
if __name__ == "__main__":
bob = PizzaRobot('bob')
print bob
bob.giveRaise(0.2)
print bob; print
for klass in Employee, Chef, Server, PizzaRobot:
obj = klass(klass.__name__)
obj.work()

Functional Programming
• Functions are first class objects. Everything done with "data" can be done with
functions (e.g. passing a function to another function).
• Recursion is used as the primary control structure. This allows the compiler to
determine the layout of program statements rather than the programmer (e.g., loops
don’t have to be “unrolled” for efficiency)
• There is a focus on LIST Processing (for example, the name Lisp). Lists are often used
with recursion on sub-lists as a substitute for loops.
• "Pure" functional languages eschew side-effects. This excludes the almost ubiquitous
pattern in imperative languages of assigning first one, then another value to the same
variable to track the program state.
• FP either discourages or outright disallows statements, and instead works with the
evaluation of expressions (in other words, functions plus arguments). In the pure case,
one program is one expression (plus supporting definitions).
• FP worries about what is to be computed rather than how it is to be computed.
• FP uses "higher order" functions (functions that operate on functions that operate on
functions) [1]
• Note: “Currying” allows functions of more than one variable to be converted to
composite functions of one variable, e.g. F(x,y,z) = g(x) ◦ h(y) ◦ i(z)

Functional Code
Code Pattern Procedural Functional
If Statements if i=1:
namenum(i)='one'
elif i=2:
namenum(i)='two'
else:
namenum(i)='other'
>>> pr = lambda s:s
>>> namenum = lambda x: (x==1 and pr("one"))
.... or (x==2 and pr("two"))
.... or (pr("other"))
>>> namenum(1)
'one'
>>> namenum(2)
'two'
>>> namenum(3)
'other'
For Statements for e in list: func(e) map(func, list)
While Statements while <cond>:
<pre-suite>
if <break_condition>:
break
else:
<suite>
def while_block():
<pre-suite>
if <break_condition>:
return 1
else:
<suite>
return 0
while_FP = lambda: (<cond> and while_block()) or while_FP()
Example Program xs = (1,2,3,4)
ys = (10,15,3,22)
bigmuls = []
# ...more stuff...
for x in xs:
for y in ys:
# ...more stuff...
if x*y > 25:
bigmuls.append((x,y))
# ...more stuff...
# ...more stuff...
print bigmuls
# No side-effects possible
bigmuls = lambda xs,ys: filter(lambda (x,y):x*y > 25, combine(xs,ys))
combine = lambda xs,ys: map(None, xs*len(ys), dupelms(ys,len(xs))) dupelms
= lambda lst,n: reduce(lambda s,t:s+t, map(lambda l,n=n: [l]*n, lst))
print bigmuls((1,2,3,4),(10,15,3,22)) #replace with list comprehension
List
Comprehensions
None print [(x,y) for x in (1,2,3,4) for y in (10,15,3,22) if x*y > 25]

Functional Code part II
QuickSort void q_sort( input_type a[], int left, int right )
{
int i, j;
input_type pivot;
if (left + CUTOFF <= right) {
/* CUTOFF should be 20 but example is 2 */
pivot = median3( a, left, right );
i = left; j = right-1; /* why? */
for (;;) {
while (a[++i] < pivot);
while (a[--j] > pivot);
if (i < j)
swap( &a[i], &a[j] );
else
break;
}
swap( &a[i], &a[right-1] ); /* why? */
q_sort( a, left, i-1 );
q_sort( a, i+1, right);
}
}
void quick_sort( input_type a[], int n ) {
q_sort( a, 1, n);
insertion_sort( a, n );
/* n<=20: insertion is better, use CUTOFF */ }
def qsort(L):
if L == []: return []
return qsort([x for x in L[1:] if x< L[0] ]) + L[0:1] +
qsort([x for x in L[1:] if x>=L[0] ])

References
1. ActivePython from ActiveState
– http://www.activestate.com/Products/ActivePython/
1. Charming Python: Functional programming in Python, Part 1
– http://www-106.ibm.com/developerworks/library/l-prog.html
1. Currying: From Wikipedia
– http://en.wikipedia.org/wiki/Currying
1. Enhanced Python Distribution from EnThought
– http://www.enthought.com/python/
1. MatPlotLib: SourceForge
– http://matplotlib.sourceforge.net/
1. Numarray: Space Telescope Science Institute
– http://www.stsci.edu/resources/software_hardware/numarray
1. Python - Object/Type Structures
– http://wiki.cs.uiuc.edu/cs427/Python+-+Object%2FType+Structures
1. Python Software Foundation
1. http://www.python.org/
1. Python Programming Language: From Wikipedia
– http://en.wikipedia.org/wiki/Python_programming_language

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