Python basics

10-Dec-22 Advanced Programming
Spring 2002
Python
Henning Schulzrinne
Department of Computer Science
Columbia University
(based on tutorial by Guido van Rossum)

Spring 2002
Introduction
 Most recent popular
(scripting/extension) language
 although origin ~1991
 heritage: teaching language (ABC)
 Tcl: shell
 perl: string (regex) processing
 object-oriented
 rather than add-on (OOTcl)

Spring 2002
Python philosophy
 Coherence
 not hard to read, write and maintain
 power
 scope
 rapid development + large systems
 objects
 integration
 hybrid systems

Spring 2002
Python features
no compiling or linking rapid development cycle
no type declarations simpler, shorter, more flexible
automatic memory management garbage collection
high-level data types and
operations
fast development
object-oriented programming code structuring and reuse, C++
embedding and extending in C mixed language systems
classes, modules, exceptions "programming-in-the-large"
support
dynamic loading of C modules simplified extensions, smaller
binaries
dynamic reloading of C modules programs can be modified without
stopping
Lutz, Programming Python

Spring 2002
Python features
universal "first-class" object model fewer restrictions and rules
run-time program construction handles unforeseen needs, end-
user coding
interactive, dynamic nature incremental development and
testing
access to interpreter information metaprogramming, introspective
objects
wide portability cross-platform programming
without ports
compilation to portable byte-code execution speed, protecting source
code
built-in interfaces to external
services
system tools, GUIs, persistence,
databases, etc.
Lutz, Programming Python

Spring 2002
Python
 elements from C++, Modula-3
(modules), ABC, Icon (slicing)
 same family as Perl, Tcl, Scheme, REXX,
BASIC dialects

Spring 2002
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

Spring 2002
What not to use Python (and
kin) for
 most scripting languages share these
 not as efficient as C
 but sometimes better built-in algorithms
(e.g., hashing and sorting)
 delayed error notification
 lack of profiling tools

Spring 2002
Using python
 /usr/local/bin/python
 #! /usr/bin/env python
 interactive use
Python 1.6 (#1, Sep 24 2000, 20:40:45) [GCC 2.95.1 19990816 (release)] on sunos5
Copyright (c) 1995-2000 Corporation for National Research Initiatives.
All Rights Reserved.
Copyright (c) 1991-1995 Stichting Mathematisch Centrum, Amsterdam.
All Rights Reserved.
>>>
 python –c command [arg] ...
 python –i script
 read script first, then interactive

Spring 2002
Python structure
 modules: Python source files or C extensions
 import, top-level via from, reload
 statements
 control flow
 create objects
 indentation matters – instead of {}
 objects
 everything is an object
 automatically reclaimed when no longer needed

Spring 2002
First example
#!/usr/local/bin/python
# import systems module
import sys
marker = '::::::'
for name in sys.argv[1:]:
input = open(name, 'r')
print marker + name
print input.read()

Spring 2002
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 """"

Spring 2002
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

Spring 2002
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

Spring 2002
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

Spring 2002
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

Spring 2002
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

Spring 2002
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'

Spring 2002
Do nothing
 pass does nothing
 syntactic filler
while 1:
pass

Spring 2002
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

Spring 2002
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?')

Spring 2002
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)

Spring 2002
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)

Spring 2002
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

Spring 2002
List methods
 count(x)
 how many times x appears in list
 sort()
 sort items in place
 reverse()
 reverse list

Spring 2002
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

Spring 2002
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}]

Spring 2002
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]

Spring 2002
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]
[]

Spring 2002
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]

Spring 2002
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')

Spring 2002
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

Spring 2002
Tuples
 Empty tuples: ()
>>> empty = ()
>>> len(empty)
0
 one item  trailing comma
>>> singleton = 'foo',

Spring 2002
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

Spring 2002
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

Spring 2002
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

Spring 2002
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

Spring 2002
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

Spring 2002
Comparing sequences
 unlike C, can compare sequences (lists,
tuples, ...)
 lexicographical comparison:
 compare first; if different  outcome
 continue recursively
 subsequences are smaller
 strings use ASCII comparison
 can compare objects of different type, but
by type name (list < string < tuple)

Spring 2002
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)

Spring 2002
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

Spring 2002
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)

Spring 2002
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 *

Spring 2002
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
FilesPython2.2DLLs', 'C:Program
FilesPython2.2lib', 'C:Program
FilesPython2.2liblib-tk', 'C:Program
FilesPython2.2', 'C:Program FilesPython2.2libsite-
packages']

Spring 2002
Compiled Python files
 include byte-compiled version of module if
there exists fibo.pyc in same directory as
fibo.py
 only if creation time of fibo.pyc matches
fibo.py
 automatically write compiled file, if possible
 platform independent
 doesn't run any faster, but loads faster
 can have only .pyc file  hide source

Spring 2002
Standard modules
 system-dependent list
 always sys module
>>> import sys
>>> sys.p1
'>>> '
>>> sys.p2
'... '
>>> sys.path.append('/some/directory')

Spring 2002
Module listing
 use dir() for each module
>>> dir(fibo)
['___name___', 'fib', 'fib2']
>>> dir(sys)
['__displayhook__', '__doc__', '__excepthook__', '__name__', '__stderr__', '__st
din__', '__stdout__', '_getframe', 'argv', 'builtin_module_names', 'byteorder',
'copyright', 'displayhook', 'dllhandle', 'exc_info', 'exc_type', 'excepthook', '
exec_prefix', 'executable', 'exit', 'getdefaultencoding', 'getrecursionlimit', '
getrefcount', 'hexversion', 'last_type', 'last_value', 'maxint', 'maxunicode', '
modules', 'path', 'platform', 'prefix', 'ps1', 'ps2', 'setcheckinterval', 'setpr
ofile', 'setrecursionlimit', 'settrace', 'stderr', 'stdin', 'stdout', 'version',
'version_info', 'warnoptions', 'winver']

Spring 2002
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)

Spring 2002
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)

Spring 2002
Class definitions
Class ClassName:
<statement-1>
...
<statement-N>
 must be executed
 can be executed conditionally (see Tcl)
 creates new namespace

Spring 2002
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

Spring 2002
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

Spring 2002
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

Spring 2002
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

Spring 2002
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

Spring 2002
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)

Spring 2002
Notes on classes
 Data attributes override method
attributes with the same name
 no real hiding  not usable to
implement pure abstract data types
 clients (users) of an object can add
data attributes
 first argument of method usually called
self
 'self' has no special meaning (cf. Java)

Spring 2002
Another example
 bag.py
class Bag:
def __init__(self):
self.data = []
def add(self, x):
self.data.append(x)
def addtwice(self,x):
self.add(x)
self.add(x)

Spring 2002
Another example, cont'd.
 invoke:
>>> from bag import *
>>> l = Bag()
>>> l.add('first')
>>> l.add('second')
>>> l.data
['first', 'second']

Spring 2002
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

Spring 2002
Multiple inheritance
class DerivedClass(Base1,Base2,Base3):
<statement>
 depth-first, left-to-right
 problem: class derived from two classes
with a common base class

Spring 2002
Private variables
 No real support, but textual
replacement (name mangling)
 __var is replaced by
_classname_var
 prevents only accidental modification,
not true protection

Spring 2002
~ C structs
 Empty class definition:
class Employee:
pass
john = Employee()
john.name = 'John Doe'
john.dept = 'CS'
john.salary = 1000

Spring 2002
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

Spring 2002
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

Spring 2002
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

Spring 2002
Language comparison
Tcl Perl Python JavaScript Visual
Basic
Speed development     
regexp   
breadth extensible   
embeddable  
easy GUI   (Tk) 
net/web     
enterprise cross-platform    
I18N    
thread-safe   
database access     

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