Pythonintroduction

What Is Python?
Created in 1990 by Guido van Rossum
While at CWI, Amsterdam
Now hosted by centre for national research initiatives,
Reston, VA, USA
Free, open source
And with an amazing community
Object oriented language
“Everything is an object”

Why Python?Designed to be easy to learn and master
Clean, clear syntax
Very few keywords
Highly portable
Runs almost anywhere - high end servers and
workstations, down to windows CE
Uses machine independent byte-codes
Extensible
Designed to be extensible using C/C++, allowing
access to many external libraries

Python: a modern hybrid
A language for scripting and prototyping
Balance between extensibility and powerful built-in
data structures
genealogy:
Setl (NYU, J.Schwartz et al. 1969-1980)
ABC (Amsterdam, Meertens et al. 1980-)
Python (Van Rossum et all. 1996-)
Very active open-source community

Prototyping
Emphasis on experimental programming:
Interactive (like LISP, ML, etc).
Translation to bytecode (like Java)
Dynamic typing (like LISP, SETL, APL)
Higher-order function (LISP, ML)
Garbage-collected, no ptrs
(LISP, SNOBOL4)

Prototyping
Emphasis on experimental programming:
Uniform treatment of indexable structures (like
SETL)
Built-in associative structures (like SETL,
SNOBOL4, Postscript)
Light syntax, indentation is significant (from ABC)

Most obvious and notorious
features
Clean syntax plus high-level data types
Leads to fast coding
Uses white-space to delimit blocks
Humans generally do, so why not the language?
Try it, you will end up liking it
Variables do not need declaration
Although not a type-less language

A Digression on Block Structure
There are three ways of dealing with IF structures
Sequences of statements with explicit end (Algol-68,
Ada, COBOL)
Single statement
(Algol-60, Pascal, C)
Indentation (ABC, Python)

Sequence of Statements
IF condition THEN
stm;
stm;
..
ELSIF condition THEN
stm;
..
ELSE
stm;
..
END IF;
next statement;

Single Statement
IF condition THEN
BEGIN
stm;
stm;
END ..
ELSE IF condition THEN
BEGIN
stm;
..
END;
ELSE
BEGIN
stm;
..
END;
next-statement;

Indentation
IF condition:
stm;
stm;
..
ELSIF condition:
stm;
..
ELSE:
stm;
..
next-statement

Pythonwin
These examples use Pythonwin
Only available on Windows
GUI toolkit using Tkinter available for most platforms
Standard console Python available on all platforms
Has interactive mode for quick testing of code
Includes debugger and Python editor

Interactive Python
Starting Python.exe, or any of the GUI environments
present an interactive mode
>>>prompt indicates start of a statement or
expression
If incomplete, ...prompt indicates second and
subsequent lines
All expression results printed back to interactive
console

Variables and Types(1 of 3)
Variables need no declaration
>>> a=1
>>>
As a variable assignment is a statement, there is no
printed result
>>> a
1
Variable name alone is an expression, so the result is
printed

Variables and Types (2 of 3)
Variables must be created before they can be used
>>> b
Traceback (innermost last):
File "<interactive input>", line
1, in ?
NameError: b
>>>
Python uses exceptions - more detail later

Variables and Types (3 of 3)
Objects always have a type
>>> a = 1
>>> type(a)
<type 'int'>
>>> a = "Hello"
>>> type(a)
<type 'string'>
>>> type(1.0)
<type 'float'>

Assignment versus Equality Testing
Assignment performed with single =
Equality testing done with double = (==)
Sensible type promotions are defined
Identity tested with is operator.
>>> 1==1
1
>>> 1.0==1
1
>>> "1"==1
0

Simple Data Types
Strings
May hold any data, including embedded NULLs
Declared using either single, double, or triple quotes
>>> s = "Hi there"
>>> s
'Hi there'
>>> s = "Embedded 'quote'"
>>> s
"Embedded 'quote'"

Simple Data Types
Triple quotes useful for multi-line strings
>>> s = """ a long
... string with "quotes" or anything
else"""
>>> s
' a long012string with "quotes" or
anything else'
>>> len(s)
45

Simple Data TypesInteger objects implemented using C longs
Like C, integer division returns the floor
>>> 5/2
2
Float types implemented using C doubles
No point in having single precision since execution
overhead is large anyway

Simple Data Types
Long Integers have unlimited size
Limited only by available memory
>>> long = 1L << 64
>>> long ** 5
21359870359209100823950217061695521146027045223
56652769947041607822219725780640550022962086936
576L

High Level Data Types
Lists hold a sequence of items
May hold any object
Declared using square brackets
>>> l = []# An empty list
>>> l.append(1)
>>> l.append("Hi there")
>>> len(l)
2

>>> l
[1, 'Hi there']
>>>
>>> l = ["Hi there", 1, 2]
>>> l
['Hi there', 1, 2]
>>> l.sort()
>>> l
[1, 2, 'Hi there']

Tuples are similar to lists
Sequence of items
Key difference is they are immutable
Often used in place of simple structures
Automatic unpacking
>>> point = 2,3
>>> x, y = point
>>> x
2

Tuples are particularly useful to return multiple
values from a function
>>> x, y = GetPoint()
As Python has no concept of byref parameters, this
technique is used widely

Dictionaries hold key-value pairs
Often called maps or hashes. Implemented using hash-
tables
Keys may be any immutable object, values may be any
object
Declared using braces
>>> d={}
>>> d[0] = "Hi there"
>>> d["foo"] = 1

Dictionaries (cont.)
>>> len(d)
2
>>> d[0]
'Hi there'
>>> d = {0 : "Hi there", 1 :
"Hello"}
>>> len(d)
2

Blocks
Blocks are delimited by indentation
Colon used to start a block
Tabs or spaces may be used
Mixing tabs and spaces works, but is discouraged
>>> if 1:
... print "True"
...
True
>>>

Blocks
Many hate this when they first see it
Most Python programmers come to love it
Humans use indentation when reading code to
determine block structure
Ever been bitten by the C code?:
if (1)
printf("True");
CallSomething();

Looping
The for statement loops over sequences
>>> for ch in "Hello":
... print ch
...
H
e
l
l
o
>>>

Looping
Built-in function range() used to build sequences of
integers
>>> for i in range(3):
... print i
...
0
1
2
>>>

Looping
while statement for more traditional loops
>>> i = 0
>>> while i < 2:
... print i
... i = i + 1
...
0
1
>>>

Functions
Functions are defined with the def statement:
>>> def foo(bar):
... return bar
>>>
This defines a trivial function named foo that takes a
single parameter bar

Functions
A function definition simply places a function object
in the namespace
>>> foo
<function foo at fac680>
>>>
And the function object can obviously be called:
>>> foo(3)
3
>>>

Classes
Classes are defined using the class statement
>>> class Foo:
... def __init__(self):
... self.member = 1
... def GetMember(self):
... return self.member
...
>>>

Classes
A few things are worth pointing out in the previous
example:
The constructor has a special name __init__, while a
destructor (not shown) uses __del__
The self parameter is the instance (ie, the this in C+
+). In Python, the self parameter is explicit (c.f. C++,
where it is implicit)
The name self is not required - simply a convention

Classes
Like functions, a class statement simply adds a class
object to the namespace
>>> Foo
<class __main__.Foo at 1000960>
>>>
Classes are instantiated using call syntax
>>> f=Foo()
>>> f.GetMember()
1

Modules
Most of Python’s power comes from modules
Modules can be implemented either in Python, or in
C/C++
import statement makes a module available
>>> import string
>>> string.join( ["Hi", "there"] )
'Hi there'
>>>

Exceptions
Python uses exceptions for errors
try / except block can handle exceptions
>>> try:
... 1/0
... except ZeroDivisionError:
... print "Eeek"
...
Eeek
>>>

Exceptions
try / finally block can guarantee execute of code
even in the face of exceptions
>>> try:
... 1/0
... finally:
... print "Doing this anyway"
...
Doing this anyway
Traceback (innermost last): File "<interactive
input>", line 2, in ?
ZeroDivisionError: integer division or modulo
>>>

Threads
Number of ways to implement threads
Highest level interface modelled after Java
>>> class DemoThread(threading.Thread):
... def run(self):
... for i in range(3):
... time.sleep(3)
... print i
...
>>> t = DemoThread()
>>> t.start()
>>> t.join()
0
1 <etc>

Standard Library
Python comes standard with a set of modules, known
as the “standard library”
Incredibly rich and diverse functionality available
from the standard library
All common internet protocols, sockets, CGI, OS
services, GUI services (via Tcl/Tk), database, Berkeley
style databases, calendar, Python parser, file
globbing/searching, debugger, profiler, threading and
synchronisation, persistency, etc

External library
Many modules are available externally covering
almost every piece of functionality you could ever
desire
Imaging, numerical analysis, OS specific functionality,
SQL databases, Fortran interfaces, XML, Corba, COM,
Win32 API, etc
Way too many to give the list any justice

Python ProgramsPython programs and modules are written as text files
with traditionally a .py extension
Each Python module has its own discrete namespace
Name space within a Python module is a global one.

Python ProgramsPython modules and programs are differentiated only
by the way they are called
.py files executed directly are programs (often referred
to as scripts)
.py files referenced via the import statement are
modules

Python Programs
Thus, the same .py file can be a program/script, or a
module
This feature is often used to provide regression tests
for modules
When module is executed as a program, the regression
test is executed
When module is imported, test functionality is not
executed

More Information on Python
Can’t do Python justice in this short time frame
But hopefully have given you a taste of the language
Comes with extensive documentation, including
tutorials and library reference
Also a number of Python books available
Visit www.python.org for more details
Can find python tutorial and reference manual

Scripting Languages
What are they?
Beats me 
Apparently they are programming languages used for
building the equivalent of shell scripts, i.e. doing the
sort of things that shell scripts have traditionally been
used for.
But any language can be used this way
So it is a matter of convenience

Characteristics of Scripting Languages
Typically interpretive
But that’s an implementation detail
Typically have high level data structures
But rich libraries can substitute for this
For example, look at GNAT.Spitbol
Powerful flexible string handling
Typically have rich libraries
But any language can meet this requirement

Is Python A Scripting Language?
Usually thought of as one
But this is mainly a marketing issue
People think of scripting languages as being easy to
learn, and useful.
But Python is a well worked out coherent dynamic
programming language
And there is no reason not to use it for a wide range of
applications.

Pythonintroduction

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