python tutorial #1

8/7/2019 Python Tutorial #1

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Beginning Python

Netsoc

Stephen Shaw

2010

Netsoc (Stephen Shaw) 2010 1 / 19


2/19

Getting started

SSH to one of our servers

$ ssh [email protected]

PuTTY: Enter login.netsoc.tcd.ie as the hostname

NX on cube if you want - all you need is a shell thoughNo netsoc account?

CS: macneill.scss.tcd.ie

Maths servers?Talk to an admin before you leave so you have an account for next time

http://macneill.scss.tcd.ie/


3/19

The plan

Schedule

This week - The basics

Week 2 - Modules, packages, the PYPI, Hacking up text with python -

screen scraping, regular expressions, unicodeWeek 3 - Miscellaneous fun (maybe a bit of PyGame?)

Most likely in the Arts Building Mac labs next week - details later in the

week

These slides: http://www.netsoc.tcd.ie/~stesh/python/1.pdf

http://www.netsoc.tcd.ie/~stesh/python/1.pdf


4/19

About python

Duck-typed, multiparadigm, general-purpose programming language

Born in the late '80s - older than Java

Awesome!

Python is an interpreted language (sort of)

Hacking up things

Google, YouTube, (netsoc!)

Libraries for almost any conceivable task



5/19

Diving into python

We'll use Python 3.1.3

At the prompt, type python3

stesh@spoon : [18:10]% python3

Python 3.1.3 (r313 :86834, Nov 28 2010, 10:01:07)

[GCC 4.4.5] on linux2

Type "help", "copyright", "credits" or "license" for more information .

>>>

You're now in the REPL - Read-Eval-Print-Loop

Python reads an expression from you, evaluates it, and prints it

Try it now!

If you don't know how something words, type help(thething)

Usual mathematical operators. Use ** for exponentiation



6/19

Functions

Functions look like this:

defarea(radius ):

i fradius < 0:

radius = - radius

pi = 3.14return pi * r**2

No need to say what types variables have

Variables can change type any time

Indentation - very important

Different indentation changes the meaning of your program!

Line-ending semicolons are optional



7/19

If, else

x, y = 4, 6

i fx != 5 and y < 100:something ()

e l i f x % 2 == 0 or 0 < x < y < 34:

something_else ()

e l i f not(True or False):

something_different() # Will never rune l s e :

stuff ()

Only need parentheses when things are ambiguous

and, or, not, ==, !=, ,

Multiple assignment and iterable unpacking

Indentation!

Comments begin with a #



8/19

Loops

ministers = ['Mary', 'Batt', 'Noel ', 'Dermott']f o r minister i n ministers:

p r i n t(minister + " has retired!")

f o r beatle i n {'Ringo', 'George', 'Paul', 'John'}:

p r i n t(beatle )

f o r c i n 'Stephen':

p r i n t(c)

f o r word i n 'This is a sentence '.split ():

p r i n t(word)



9/19

Lists

Python's equivalent to arrays

The `workhorse'LISP-like, I'm told

[1, 2, 3, 4, 5]

List of strings the lazy way:

mylist = 'A big long list of strings'.split()Lists are mutable (change their size whenever you want):

mylist.append(element)

Lists can hold anything, including objects of different types:

['a string', 1, True, ['2']]

Add lists to other lists: [1, 2, 3, 4] + [5, 6, 7, 8]

[1, 2, 3, 4, 5, 6, 7, 8]

Reversing: reversed(mylist)

Sorting: sorted(mylist)



10/19

Slicing

Access elements with [] notation: mylist[2] returns 'long'

Much richer than that:

mylist[1:] - Èverything except the first index'mylist[5:10] - Àll indices from 5 inclusive to 10'

mylist[5:10:2] - Àll indices from 5 inclusive to 10, counting up in twos'mylist[:-2] - Èverything except the last two indices'

mylist[1:-1] - Èverything except the first and last indices'

mylist[::] - Àll indices'mylist[::-1] - Àll indices, counting from the right (reverse the list)'

Using a list as a stack:

mylist.append(element)

mylist.pop()



11/19

More about strings

Replacing: 'DUCSS'.replace('U', 'I')

In most cases, you can treat a string as if it were a list:

'Netsoc'[2] will return t

reversed('apache') gives you a new string 'ehcapa''e' in 'Netsoc'

This is because both strings and lists are iterablesIterables are very widespread in Python

Let's look at some more iterables



12/19

Tuples

A bit like lists, except immutable: (1, 2, 3)

Once a tuple is created, it can't be changed, only accessed

mytuple + (5, 6, 7, 8) creates a whole new tupleYou can make a list from a tuple, and (sometimes) vice-versa:

tuple([1, 2, 3])

`A list is a pencil, a tuple is a pen'



13/19

Sets

Unordered collection of things with no repeated elements - noguarantee that elements will come out in the order you put them in

Often really fast

beatles = {'John', 'Paul', 'George', 'Ringo'}

OperationsA.add(element)

A.remove(element)

A.union(B)

A.intersection(B)

A - BA.difference(B)

A.issubset(B)



14/19

Comprehensions

Really powerful way to make iterable things

`Give me all the strings in mylist that don't contain a q':

[x for x in mylist if'q'not in x]

`Give me the set of all the even integers between 0 and 100':

{x for x in range(100)ifx % 2 == 0}

`Give me a copy of my list of integers as a list of strings':

[str(x) for x in mylist]



15/19

Dictionaries

Also very important

council = {

'secretary': 'br0kend'

'PRO': 'lux',

'treasurer': 'dalamar',

'amenities': 'don',

'auditor': 'mk429',

'admin': 'mu',

'webmaster': 'theorie'

}

Associate things with other things

council['auditor'] 'mk429'

'admin' in council returns True

Useful when you're counting occurrences of things



16/19

Anonymous functions

Cool

Functions are first-class objects in python

Can pass functions/methods as arguments to other functions/methods

Functions need not be bound to a name

# Sends x to itself raised to the power of 2

lambda x: x**2

# Binding to a name:

capitalize = lambda x: x.upper ()

# Passing a function to a function:

defapply(function , argument):

(function)(argument)

Used extensively in map, filter and reduce

A taste of functional programming


F l


17/19

Files

Easy!

with myfile as open('myfile.txt'):

while True:

line = mylist .readline ()

i fline == '':break

dosomething ()

use .read() to read it all at once

The with statement is relatively new

Does something vaguely similar to blocks in ruby


E i


18/19

Exceptions

No surprises hereExceptions have a big class hierarchy

raise to raise an exceptiontry to begin an excepting block

except to `catch'

finally cleaning up

t r y :

p r i n t(mydict [key ])

except KeyError:

p r i n t('Oh noes! ')

Can often be avoided:

i fkey i n mydict :

p r i n t(mydict [key ])

e l s e :

p r i n t('Oh noes! ')


I lli


19/19

Installing

Linux, OS X: you probably have python installed already

Python is an integral part of most *nixes.

Python 3 is in most repos (there is *usually* a non-broken package in

MacPorts)Python 3 default in Arch Linux (:-S)

There is a comprehensive python implementation for Windows

I've never tried it, so can't offer any advice there

Questions?


python tutorial #1

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