Python and VPython - Theoretical Chemistry Group … Teorico Modellistico 36 Functions Functions are defined with the def statements followed by the name of the function and the list

Python and VPythonSeminario Teorico Modellistico

Seminario Teorico Modellistico 2

What is Python?

It is a programming language created in 1990 by Guido Van Rossum

The Python Philosophy:


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

•clean •clear syntax•uses white space to delimit blocks

humans generally do, so why not the language?? ✦ Short

• it is typically one-third one-fifth the size of equivalent C++ or Java code (less to type=less to debug)

✦ Highly portable• run almost everywhere• porting a Python code from one platform to another it is just

a matter of copy-paste✦ Extensible

• large connection of prebuilt and portable functionality, standard libraries


Interpreted LanguagePython is an interpreted language or a ”dynamic language”

interpreted compiled

source code statements

Byte code

source code statements

binary machine code

Python interpreterexecutes the program step-by-step

platformindependent

executable file


What can i do with Python?

Not only a scripting language, but also an implementing standalone program

It is famous to be a scripting language. The scripting refers a “glue” layer to control and direct other applications

•Internet scripting•Good match for graphical user interface programs (GUIs)•Interface for all commonly used database (Oracle)

•Numerical and scientific programming (NumPy)•3D scientific programming (VPython)•Game programming (Pygame)•Image processing (PyOpenGL)


Who uses Python?

Some of the most famous users:

•Google:make extensive use of Python in its web search system and employs the Python’s creator

•YouTube:its video sharing service is largely written in Python

•BitTorrent:the popular peer-to-peer file sharing system is a Python program


Python Shell

• Interactive

✦ Modes:

• Executable scripts


Python Shell✦ Modes:

• FilePython programs and modules are written as text files with a .py extension

Programs and modules are differentiated only by the way they are called:

•.py files executed directly are programs•.py files referenced via the import statement are modules


Object typeVariables do not need declaration

✦ Numbers✦ Strings✦ Lists✦ Dictionaries✦ Tuples

Variables must be created before they can be used


Numbers

✦ Floating point

✦ Integers ✦ Complex

✦ Tests

•100/3•100.0/3•100.0//3•100%3•divmod(100,3)


Operators

x < y, x <= y, x > y, x >= y magnitude comparison

x == y, x != y values equality operators

x + y addition, concatenation

x - y subtraction

x * y multiplication, repetition

x % y remainder

x / y, x // y division: true and floor

x ** y power


Strings

define a string using single, double or triple quotes

gives you the length of the string

[i] gives the letter in the i-th position

[i:j] gives the letters from i to j

triple quotes are useful for multi-line strings

✦ strings are used to record textual information


Strings

✦ Operations

S + C concatenation

S * 3 repeat

len(S) length

String method calls

S.find(‘pa’) search

S.rstrip() remove whitespace

S.split(‘,’) split on delimiter

S.replace(‘pa’,’xx’) replacement

S.lowerS.upperS.title

case conversion


Strings✦ Operations

concatenation

case conversion

replacement


Strings✦ Curiously strings can not be modified, are immutable

you can only use .replace() or turn them into lists


Containers

✦ Tuples ()

✦ Lists []

Different types of containers are availablethey are efficient, comfortable to use and versatile

✦ Dictionaries {}


Lists []

✦ List is the most general sequence provided by the language

✦ Lists are positionally ordered collections of arbitrarily typed object

✦ no fixed size

✦ mutable

✦ unlike strings, lists can contain any sort of object


✦ lists indexes

Lists []


✦ Operations

L = [] empty list

L = [1, 2, 3, 4] four items: indexes 0,..,3

L = [‘abs’, [‘def’, ‘ghi’]] nested sublist

L[i] index

L[i][j] index of index

L[i : j] slice

len(L) length

L1+ L2 concatenation

L * 3 repetition

L.append() append in last position

L.extend([])in last position more

than one item

L.insert(I, X)insert in I position the

X item

L.index(X)search index of X

item

L.count(X) count the X items

L.sort() sort the list

L.reverse() reverse the list

del L[i : j] del some items

L.pop() del last item

L.remove() del a specific item

Lists []


✦ lists indexes

python - introduction - Simone Campagna 5212/20/2011

range

! Range è una funzione per generare liste di interi:>>> print range(3)

[0, 1, 2]

>>> print range(3, 7)

[3, 4, 5, 6]

>>> print range(3, 10, 2)

[3, 5, 7, 9]

✦ range function generates lists


extended slices/1La sintassi dello slicing accetta ora un parametro stride, sempre separato da :>>> l = range(20)

>>> l[1:18]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]

>>> l[1:18:3]

[1, 4, 7, 10, 13, 16]

>>> l[1::3]

[1, 4, 7, 10, 13, 16, 19]

>>> l[:5:3]

[0, 3]

>>> l[::3]

[0, 3, 6, 9, 12, 15, 18]

>>>

Lists []


✦ increase operators

✦ sort operators

Lists []


Dictionaries {}

✦ If lists are ordered collection of object, dictionaries are unordered collection

✦ Apart from lists, dictionaries are perhaps the most flexible data type in Python

✦ Dictionaries implement associative arrays, they associate an arbitrary value to an arbitrary key

✦ Dictionaries can contain all types of object, also heterogeneously


Dictionaries {}


dizionari/1

! I dizionari implementano array associativi; associano ad una chiave arbitraria un valore arbitrario:

>>> numero_zampe = {'gatto': 4, 'oca': 2, 'formica': 6, 'ragno': 8}

>>> print numero_zampe['formica']

6

>>> print numero_zampe

{'oca': 2, 'ragno': 8, 'formica': 6, 'gatto': 4}

>>>


Dictionaries {}


dizionari/2! Non sono necessariamente omogenei:>>> d = {}

>>> d['alfa'] = 3

>>> d[2.5] = 'xyz'

>>> d[3+4j] = [3, 4, 5]

>>> d[(1,2,3)] = { 'x': 2, 'y': 3, 'z': 1 }

>>> print d

{2.5: 'xyz', (1, 2, 3): {'y': 3, 'x': 2, 'z': 1}, 'alfa': 3, (3+4j): [3, 4, 5]}

>>>

✦ Dictionaries can be heterogeneous


Dictionaries

✦ some operators


dizionari/3>>> print d.keys()

[2.5, (1, 2, 3), 'alfa', (3+4j)]

>>> print d.values()

['xyz', {'y': 3, 'x': 2, 'z': 1}, 3, [3, 4, 5]]

>>> print d.items()

[(2.5, 'xyz'), ((1, 2, 3), {'y': 3, 'x': 2, 'z': 1}), ('alfa', 3), ((3+4j), [3, 4, 5])]

>>> d.has_key('alfa')

True

>>> d.has_key('beta')

False

>>>


dizionari/2! Non sono necessariamente omogenei:>>> d = {}

>>> d['alfa'] = 3

>>> d[2.5] = 'xyz'

>>> d[3+4j] = [3, 4, 5]

>>> d[(1,2,3)] = { 'x': 2, 'y': 3, 'z': 1 }

>>> print d

{2.5: 'xyz', (1, 2, 3): {'y': 3, 'x': 2, 'z': 1}, 'alfa': 3, (3+4j): [3, 4, 5]}

>>>


Tuples ()

✦ Tuple constructs simple groups of object

✦ Tuple are lists that can not be modified

✦ Tuple do not support all the methods of the lists, but share most of their properties

✦ Tuple are particularly useful to return multiple values from a function


Tuples ()

✦ Unlike lists tuples uses ()


tuple/6Lo slicing consente di accedere a “porzioni” della tupla:>>> a = (0, 1, 2, 3, 4)

>>> print a[1:3] # dal secondo elemento (incluso)

# al quarto (escluso)

(1, 2)

>>> print a[:2] # dal primo (incluso) al terzo (escluso)

(0, 1)

>>> print a[2:] # dal terzo (incluso) all'ultimo (incluso)

(2, 3, 4)

>>> print a[2:] + a[:2]

(2, 3, 4, 0, 1)

>>>


Flow control statements

✦ If elif else✦ for✦ while

Blocks are delimited by indentation


if elif else

values equality operators


for for statements loops over sequences


for/1

! Il ciclo for consente di iterare su “oggetti iterabili”, come liste, tuple, set, dizionari, …

>>> for i in range(3):

... print i

...

0

1

2

>>>


for


for/2

>>> t = ('a', 'b', 10, 5.5)

>>> for i in t:

... print i

...

a

b

10

5.5

>>>


while

✦ while cycle is a generic cycle with a condition


while/1! Il ciclo while è il generico ciclo con una condizione:>>> i = 0

>>> while i < 4:

... print i

... i += 1

...

0

1

2

3

>>>

incremental operator


Break

✦ Break statements allows you to exit the loop


break

Break permette di uscire dal loop:>>> for i in range(10000):

... print i

... if i%3 == 2:

... break

...

0

1

2

>>>


Continue✦ continue statements allows you to go in the next

iteration


continue

Continue permette di passare all'iterazione successiva:>>> for i in range(4):

... if i == 1:

... continue

... print i

...

0

2

3

>>>


else clause

✦ else clause is available only when the loop does not end with a break statement


loops: else clause

! I loop (for e while) possono avere una clausola else, che viene eseguita solo se non si esce dal loop con un break; ovvero, se il loop completa naturalmente:

>>> for i in range(10):... if i > 3: break... print i... else:... print "finito!"...0123>>>

>>> for i in range(2):... if i > 3: break... print i... else:... print "finito!"...01finito!>>>


Functions

✦ Functions are defined with the def statements followed by the name of the function and the list of parameters


funzioni/2>>> def sum(a, b):

... return a+b

...

>>>

>>> print sum(3, 4)

7

>>> print sum("Ciao, ", "mondo!")

Ciao, mondo!

>>> print sum(3.2, 9.1)

12.3

>>> print sum(6, "ccc")

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

File "<stdin>", line 2, in sum

TypeError: unsupported operand type(s) for +: 'int' and 'str'

>>>


Functions

✦ return statement allows you to get back a variable, if you do not insert it you get back None


Debugging Python Code

✦ Do nothing

None of you ever have bugs in your code, but for less fortunate friends of yours who may, this is the strategy commonly used:

✦ Insert print statements

when you have a mistake in Python program, you get a useful and readable error message

read the error message and go fix tagged line and file


Documentation

✦ Python is very good documented http://docs.python.org/2.7/

✦ Tutorial: http://docs.python.org/2.7/tutorial/index.html

✦ Language reference: http://docs.python.org/2.7/reference/index.html

✦ Library reference: http://docs.python.org/2.7/library/index.html

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Python and VPython - Theoretical Chemistry Group … Teorico Modellistico 36 Functions Functions are defined with the def statements followed by the name of the function and the list