Introduction to Python

Introduction to PythonIntroduction to PythonProgramming LanguagesProgramming Languages

Fall 2003Fall 2003

Adapted from Tutorial byAdapted from Tutorial by

Mark HammondMark Hammond

Skippi-Net, Melbourne, Skippi-Net, Melbourne, AustraliaAustralia

[email protected]@skippinet.com.auu

http://starship.python.net/crew/http://starship.python.net/crew/mhammondmhammond

What Is Python?What Is Python?

Created in 1990 by Guido van RossumCreated in 1990 by Guido van RossumWhile at CWI, AmsterdamWhile at CWI, AmsterdamNow hosted by centre for national Now hosted by centre for national

research initiatives, Reston, VA, USAresearch initiatives, Reston, VA, USAFree, open sourceFree, open source

And with an amazing communityAnd with an amazing communityObject oriented languageObject oriented language

““Everything is an object”Everything is an object”

Why Python?Why Python?Designed to be easy to learn and masterDesigned to be easy to learn and master

Clean, clear syntaxClean, clear syntaxVery few keywordsVery few keywords

Highly portableHighly portableRuns almost anywhere - high end servers and Runs almost anywhere - high end servers and

workstations, down to windows CEworkstations, down to windows CEUses machine independent byte-codesUses machine independent byte-codes

ExtensibleExtensibleDesigned to be extensible using C/C++, Designed to be extensible using C/C++,

allowing access to many external librariesallowing access to many external libraries

Python: a modern hybridPython: a modern hybrid

A language for scripting and prototypingA language for scripting and prototypingBalance between extensibility and Balance between extensibility and

powerful built-in data structurespowerful built-in data structuresgenealogy:genealogy:

Setl (NYU, J.Schwartz et al. 1969-1980)Setl (NYU, J.Schwartz et al. 1969-1980)ABC (Amsterdam, Meertens et al. ABC (Amsterdam, Meertens et al.

1980-)1980-)Python (Van Rossum et all. 1996-)Python (Van Rossum et all. 1996-)

Very active open-source communityVery active open-source community

PrototypingPrototyping

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

(LISP, SNOBOL4)(LISP, SNOBOL4)

PrototypingPrototyping

Emphasis on experimental Emphasis on experimental programming:programming:

Uniform treatment of indexable Uniform treatment of indexable structures structures (like SETL)(like SETL)

Built-in Built-in associative structuresassociative structures (like SETL, SNOBOL4, Postscript)(like SETL, SNOBOL4, Postscript)

Light syntax, indentation is Light syntax, indentation is significant significant (from ABC)(from ABC)

Most obvious and notorious Most obvious and notorious featuresfeatures

Clean syntax plus high-level data Clean syntax plus high-level data typestypesLeads to fast codingLeads to fast coding

Uses white-space to delimit blocksUses white-space to delimit blocksHumans generally do, so why not the Humans generally do, so why not the

language?language?Try it, you will end up liking itTry it, you will end up liking it

Variables do not need declarationVariables do not need declarationAlthough not a type-less languageAlthough not a type-less language

A Digression on Block A Digression on Block StructureStructure

There are three ways of dealing with There are three ways of dealing with IF structuresIF structuresSequences of statements with explicit Sequences of statements with explicit

end (Algol-68, Ada, COBOL)end (Algol-68, Ada, COBOL)Single statementSingle statement

(Algol-60, Pascal, C)(Algol-60, Pascal, C)Indentation (ABC, Python)Indentation (ABC, Python)

Sequence of StatementsSequence of Statements

IF condition THENIF condition THENstm;stm;stm;stm;....

ELSIF condition THENELSIF condition THENstm;stm;....

ELSEELSEstm;stm;....

END IF;END IF;next statement;next statement;

Single StatementSingle Statement IF condition THENIF condition THEN

BEGINBEGIN stm; stm;

stm; stm;ENDEND ....

ELSE IF condition THENELSE IF condition THENBEGINBEGIN stm; stm; ....END;END;

ELSEELSEBEGINBEGINstm;stm;....END;END;

next-statement;next-statement;

IndentationIndentation

IF condition:IF condition: stm;stm;

stm;stm;....

ELSIF condition:ELSIF condition:stm;stm;....

ELSE:ELSE:stm;stm;....

next-statementnext-statement

PythonwinPythonwin

These examples use PythonwinThese examples use PythonwinOnly available on WindowsOnly available on WindowsGUI toolkit using Tkinter available for GUI toolkit using Tkinter available for

most platformsmost platformsStandard console Python available on all Standard console Python available on all

platformsplatformsHas interactive mode for quick Has interactive mode for quick

testing of codetesting of code Includes debugger and Python editorIncludes debugger and Python editor

Interactive PythonInteractive Python

Starting Python.exe, or any of the GUI Starting Python.exe, or any of the GUI environments present an interactive environments present an interactive modemode>>>>>> prompt indicates start of a prompt indicates start of a

statement or expressionstatement or expression

If incomplete, If incomplete, ...... prompt indicates prompt indicates second and subsequent linessecond and subsequent lines

All expression results printed back to All expression results printed back to interactive consoleinteractive console

Variables and TypesVariables and Types (1 of 3) (1 of 3)

Variables need no declarationVariables need no declaration>>> a=1>>> a=1>>>>>>

As a variable assignment is a As a variable assignment is a statement, there is no printed resultstatement, there is no printed result

>>> a>>> a11

Variable name alone is an expression, Variable name alone is an expression, so the result is printedso the result is printed

Variables and Types Variables and Types (2 of 3)(2 of 3)

Variables must be created before they Variables must be created before they can be usedcan be used

>>> b>>> bTraceback (innermost last):Traceback (innermost last): File "<interactive input>", line 1, File "<interactive input>", line 1, in ?in ?NameError: bNameError: b>>>>>>

Python uses exceptions - more detail Python uses exceptions - more detail laterlater

Variables and Types Variables and Types (3 of 3)(3 of 3)

Objects always have a typeObjects always have a type>>> a = 1>>> a = 1>>> type(a)>>> type(a)<type 'int'> <type 'int'> >>> a = "Hello">>> a = "Hello">>> type(a)>>> type(a)<type 'string'><type 'string'>>>> type(1.0)>>> type(1.0)<type 'float'><type 'float'>

Assignment versus Equality Assignment versus Equality TestingTesting

Assignment performed with single =Assignment performed with single =Equality testing done with double = (==)Equality testing done with double = (==)

Sensible type promotions are definedSensible type promotions are definedIdentity tested with Identity tested with isis operator. operator.

>>> 1==1>>> 1==111>>> 1.0==1>>> 1.0==111>>> "1"==1>>> "1"==100

Simple Data TypesSimple Data Types

StringsStringsMay hold any data, including embedded May hold any data, including embedded

NULLsNULLsDeclared using either single, double, or triple Declared using either single, double, or triple

quotesquotes>>> s = "Hi there">>> s = "Hi there">>> s>>> s'Hi there''Hi there'>>> s = "Embedded 'quote'">>> s = "Embedded 'quote'">>> s>>> s"Embedded 'quote'""Embedded 'quote'"


Triple quotes useful for multi-line stringsTriple quotes useful for multi-line strings>>> s = """ a long>>> s = """ a long... string with "quotes" or ... string with "quotes" or anything else"""anything else""">>> s>>> s' a long\012string with "quotes" ' a long\012string with "quotes" or anything else' or anything else' >>> len(s)>>> len(s)4545

Simple Data TypesSimple Data Types Integer objects implemented using C Integer objects implemented using C

longslongsLike C, integer division returns the floorLike C, integer division returns the floor>>> 5/2>>> 5/222

Float types implemented using C Float types implemented using C doublesdoublesNo point in having single precision since No point in having single precision since

execution overhead is large anywayexecution overhead is large anyway


Long Integers have unlimited sizeLong Integers have unlimited sizeLimited only by available memoryLimited only by available memory>>> long = 1L << 64>>> long = 1L << 64>>> long ** 5>>> long ** 5213598703592091008239502170616955211460270452221359870359209100823950217061695521146027045223566527699470416078222197257806405500229620869356652769947041607822219725780640550022962086936576L36576L

High Level Data TypesHigh Level Data Types

Lists hold a sequence of itemsLists hold a sequence of itemsMay hold any objectMay hold any objectDeclared using square bracketsDeclared using square brackets

>>> l = []# An empty list>>> l = []# An empty list>>> l.append(1)>>> l.append(1)>>> l.append("Hi there")>>> l.append("Hi there")>>> len(l)>>> len(l)22


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


Tuples are similar to listsTuples are similar to listsSequence of itemsSequence of itemsKey difference is they are immutableKey difference is they are immutableOften used in place of simple structuresOften used in place of simple structures

Automatic unpackingAutomatic unpacking>>> point = 2,3>>> point = 2,3>>> x, y = point>>> x, y = point>>> x>>> x22


Tuples are particularly useful to Tuples are particularly useful to return multiple values from a return multiple values from a functionfunction

>>> x, y = GetPoint()>>> x, y = GetPoint()As Python has no concept of byref As Python has no concept of byref

parameters, this technique is used parameters, this technique is used widelywidely


Dictionaries hold key-value pairsDictionaries hold key-value pairsOften called maps or hashes. Often called maps or hashes.

Implemented using hash-tablesImplemented using hash-tablesKeys may be any immutable object, Keys may be any immutable object,

values may be any objectvalues may be any objectDeclared using bracesDeclared using braces

>>> d={}>>> d={}>>> d[0] = "Hi there">>> d[0] = "Hi there">>> d["foo"] = 1>>> d["foo"] = 1


Dictionaries (cont.)Dictionaries (cont.) >>> len(d)>>> len(d)22>>> d[0]>>> d[0]'Hi there''Hi there'>>> d = {0 : "Hi there", 1 : >>> d = {0 : "Hi there", 1 : "Hello"}"Hello"}>>> len(d)>>> len(d)22

BlocksBlocks

Blocks are delimited by indentationBlocks are delimited by indentationColon used to start a blockColon used to start a blockTabs or spaces may be usedTabs or spaces may be usedMixing tabs and spaces works, but is Mixing tabs and spaces works, but is

discourageddiscouraged>>> if 1:>>> if 1:... ... print "True"print "True"... ... TrueTrue>>>>>>

BlocksBlocks

Many hate this when they first see itMany hate this when they first see itMost Python programmers come to love itMost Python programmers come to love it

Humans use indentation when reading Humans use indentation when reading code to determine block structurecode to determine block structureEver been bitten by the C code?:Ever been bitten by the C code?:

if (1)if (1) printf("True"); printf("True"); CallSomething(); CallSomething();

LoopingLooping

The The forfor statement loops over sequences statement loops over sequences>>> for ch in "Hello":>>> for ch in "Hello":... ... print chprint ch... ... HHeelllloo>>> >>>

LoopingLooping

Built-in function Built-in function range()range() used to used to build sequences of integersbuild sequences of integers

>>> for i in range(3):>>> for i in range(3):... print i... print i... ... 001122>>> >>>

LoopingLooping

whilewhile statement for more traditional statement for more traditional loopsloops

>>> i = 0>>> i = 0>>> while i < 2:>>> while i < 2:... print i... print i... i = i + 1... i = i + 1... ... 0011>>> >>>

FunctionsFunctions

Functions are defined with the Functions are defined with the defdef statement:statement:

>>> def foo(bar):>>> def foo(bar):... return bar... return bar>>> >>>

This defines a trivial function named This defines a trivial function named foofoo that takes a single parameter that takes a single parameter barbar

FunctionsFunctions

A function definition simply places a A function definition simply places a function object in the namespacefunction object in the namespace

>>> foo>>> foo<function foo at fac680><function foo at fac680>

>>>>>> And the function object can obviously And the function object can obviously

be called:be called: >>> foo(3)>>> foo(3)33>>>>>>

ClassesClasses

Classes are defined using the Classes are defined using the classclass statementstatement

>>> class Foo:>>> class Foo:... def __init__(self):... def __init__(self):... self.member = 1... self.member = 1... def GetMember(self):... def GetMember(self):... return self.member... return self.member... ... >>> >>>

ClassesClasses

A few things are worth pointing out in the A few things are worth pointing out in the previous example:previous example:The constructor has a special name The constructor has a special name __init____init__, while a destructor (not shown) , while a destructor (not shown) uses uses __del____del__

The The selfself parameter is the instance (ie, the parameter is the instance (ie, the thisthis in C++). In Python, the self parameter in C++). In Python, the self parameter is explicit (c.f. C++, where it is implicit) is explicit (c.f. C++, where it is implicit)

The name The name selfself is not required - simply a is not required - simply a conventionconvention

ClassesClasses

Like functions, a class statement simply Like functions, a class statement simply adds a class object to the namespaceadds a class object to the namespace

>>> Foo>>> Foo<class __main__.Foo at 1000960><class __main__.Foo at 1000960>>>>>>>

Classes are instantiated using call syntaxClasses are instantiated using call syntax >>> f=Foo()>>> f=Foo()>>> f.GetMember()>>> f.GetMember()11

ModulesModules

Most of Python’s power comes from Most of Python’s power comes from modulesmodules

Modules can be implemented either in Modules can be implemented either in Python, or in C/C++Python, or in C/C++

importimport statement makes a module statement makes a module availableavailable

>>> import string>>> import string>>> string.join( ["Hi", "there"] )>>> string.join( ["Hi", "there"] )'Hi there''Hi there'>>>>>>

ExceptionsExceptions

Python uses exceptions for errorsPython uses exceptions for errorstrytry / / exceptexcept block can handle block can handle

exceptionsexceptions >>> try:>>> try:... 1/0... 1/0... except ZeroDivisionError:... except ZeroDivisionError:... print "Eeek"... print "Eeek"... ... EeekEeek>>> >>>

ExceptionsExceptions

trytry / / finallyfinally block can guarantee block can guarantee execute of code even in the face of execute of code even in the face of exceptionsexceptions

>>> try:>>> try:... 1/0... 1/0... finally:... finally:... print "Doing this anyway"... print "Doing this anyway"... ... Doing this anywayDoing this anywayTraceback (innermost last): File "<interactive Traceback (innermost last): File "<interactive input>", line 2, in ?input>", line 2, in ?ZeroDivisionError: integer division or moduloZeroDivisionError: integer division or modulo>>>>>>

ThreadsThreadsNumber of ways to implement threadsNumber of ways to implement threadsHighest level interface modelled after Highest level interface modelled after

JavaJava >>> class DemoThread(threading.Thread):>>> class DemoThread(threading.Thread):

... def run(self):... def run(self):

... for i in range(3):... for i in range(3):

... time.sleep(3)... time.sleep(3)

... print i... print i

... ... >>> t = DemoThread()>>> t = DemoThread()>>> t.start()>>> t.start()>>> t.join()>>> t.join()001 1 <etc><etc>

Standard LibraryStandard Library

Python comes standard with a set of Python comes standard with a set of modules, known as the “standard library”modules, known as the “standard library”

Incredibly rich and diverse functionality Incredibly rich and diverse functionality available from the standard libraryavailable from the standard libraryAll common internet protocols, sockets, CGI, All common internet protocols, sockets, CGI,

OS services, GUI services (via Tcl/Tk), OS services, GUI services (via Tcl/Tk), database, Berkeley style databases, calendar, database, Berkeley style databases, calendar, Python parser, file globbing/searching, Python parser, file globbing/searching, debugger, profiler, threading and debugger, profiler, threading and synchronisation, persistency, etcsynchronisation, persistency, etc

External libraryExternal library

Many modules are available externally Many modules are available externally covering almost every piece of covering almost every piece of functionality you could ever desirefunctionality you could ever desireImaging, numerical analysis, OS specific Imaging, numerical analysis, OS specific

functionality, SQL databases, Fortran functionality, SQL databases, Fortran interfaces, XML, Corba, COM, Win32 API, interfaces, XML, Corba, COM, Win32 API, etc etc

Way too many to give the list any Way too many to give the list any justicejustice

Python ProgramsPython ProgramsPython programs and modules are Python programs and modules are

written as text files with traditionally written as text files with traditionally a a .py.py extension extension

Each Python module has its own Each Python module has its own discrete namespacediscrete namespace

Name space within a Python module Name space within a Python module is a global one.is a global one.

Python ProgramsPython ProgramsPython modules and programs are Python modules and programs are

differentiated only by the way they differentiated only by the way they are calledare called.py files executed directly are programs .py files executed directly are programs

(often referred to as scripts)(often referred to as scripts).py files referenced via the .py files referenced via the importimport

statement are modulesstatement are modules

Python ProgramsPython Programs

Thus, the same .py file can be a Thus, the same .py file can be a program/script, or a moduleprogram/script, or a module

This feature is often used to provide This feature is often used to provide regression tests for modulesregression tests for modulesWhen module is executed as a program, When module is executed as a program,

the regression test is executedthe regression test is executedWhen module is imported, test When module is imported, test

functionality is not executedfunctionality is not executed

More Information on PythonMore Information on Python

Can’t do Python justice in this short time Can’t do Python justice in this short time frameframeBut hopefully have given you a taste of the But hopefully have given you a taste of the

languagelanguageComes with extensive documentation, Comes with extensive documentation,

including tutorials and library referenceincluding tutorials and library referenceAlso a number of Python books availableAlso a number of Python books available

Visit Visit www.python.orgwww.python.org for more details for more detailsCan find python tutorial and reference Can find python tutorial and reference

manualmanual

Scripting LanguagesScripting Languages

What are they?What are they?Beats me Beats me Apparently they are programming Apparently they are programming

languages used for building the languages used for building the equivalent of shell scripts, i.e. doing the equivalent of shell scripts, i.e. doing the sort of things that shell scripts have sort of things that shell scripts have traditionally been used for.traditionally been used for.

But any language can be used this wayBut any language can be used this waySo it is a matter of convenienceSo it is a matter of convenience

Characteristics of Scripting Characteristics of Scripting LanguagesLanguages

Typically interpretiveTypically interpretiveBut that’s an implementation detailBut that’s an implementation detail

Typically have high level data structuresTypically have high level data structuresBut rich libraries can substitute for thisBut rich libraries can substitute for thisFor example, look at GNAT.SpitbolFor example, look at GNAT.Spitbol

Powerful flexible string handlingPowerful flexible string handlingTypically have rich librariesTypically have rich libraries

But any language can meet this requirementBut any language can meet this requirement

Is Python A Scripting Is Python A Scripting Language?Language?

Usually thought of as oneUsually thought of as oneBut this is mainly a marketing issueBut this is mainly a marketing issue

People think of scripting languages as People think of scripting languages as being easy to learn, and useful.being easy to learn, and useful.

But Python is a well worked out But Python is a well worked out coherent dynamic programming coherent dynamic programming languagelanguageAnd there is no reason not to use it for a And there is no reason not to use it for a

wide range of applications.wide range of applications.

Documents

Introduction to Python