The UOB Python Lectures:Part 1 - Introduction to

PythonHesham al-Ammal

University of Bahrain18/3/2013

Twitter: @heshaaamBlog and slides: heshaaam.wordpress.com

11Sunday, March 17, 13

Feel like flying?

Ref: xkcd.com2

2Sunday, March 17, 13

History: Name

Python is named based on the British comedy series Monty Python’s Flying Circus

Not the constricting snake

33Sunday, March 17, 13

History: BDFL

Guido van Rossom (Google 2012)

Principal author and Benevolent dictator (BDFL), central role in deciding direction of Python

Created in 1990 at CWI, Netherlands

Python’s Logo 1990-20054

4Sunday, March 17, 13

Interpreted vs CompiledProgramming Languages

Compiled Interpreted

C C++ Java Python Perl Lisp

Interactive vs. Batch

55Sunday, March 17, 13

Interactive Python$ pythonPython 2.7 (#1, Feb 28 2010, 00:02:06)Type "help", "copyright", "credits" or "license" for more information.>>>>>> the_world_is_flat = 1>>> if the_world_is_flat:... print "Be careful not to fall off!"...Be careful not to fall off!

66Sunday, March 17, 13

Batch mode

#! /usr/bin/env python the_world_is_flat=1if the_world_is_flat: print "Be careful not to fall off"

Edit

Save Debug/RunPython file.pyfile.py

77Sunday, March 17, 13

Python Versions

We will be using version 2.7

Versions 2.2-2.7 backward compatible

Version 3 is totally different fork of Python and thus not outward compatible

Version 2 will continue for a while

88Sunday, March 17, 13

Tutorial Outline

Data Types: Numbers, Strings, Lists

Control Flow: If, For, break, continue,...

Data structures

Modules

Input and output

Classes

The Standard Library

Ref: http://docs.python.org/2/tutorial/9

9Sunday, March 17, 13

Integers

Integer division: floor

Power

No limit to integer arithmetic except mempr (arbitrary long integers)

>>> 7/32>>> 2**521 -16864797660130609714981900799081393217269435300143305409394463459185543183397656052122559640661454554977296311391480858037121987999716643812574028291115057151Lhttp://en.wikipedia.org/wiki/Mersenne_prime10

10Sunday, March 17, 13

Floating Point / Complex

Full support for floating point numbers and mixed arithmetic

Complex numbers are also supported

>>> 3 * 3.75 / 1.57.5>>> 7.0 / 23.5

>>> 1j * 1J(-1+0j)>>> 1j * complex(0,1)(-1+0j)>>> 3+1j*3(3+3j)>>> (3+1j)*3(9+3j)>>> (1+2j)/(1+1j)(1.5+0.5j)

1111Sunday, March 17, 13

Strings

Strings can be enclosed in single quotes ‘’ or double quotes “”

Notice special characters

Multiple line strings are possible

>>> 'spam eggs''spam eggs'>>> 'doesn\'t'"doesn't">>> "doesn't""doesn't">>> '"Yes," he said.''"Yes," he said.'>>> "\"Yes,\" he said."'"Yes," he said.'>>> '"Isn\'t," she said.''"Isn\'t," she said.'

1212Sunday, March 17, 13

Strings (cont.)

Strings can be concatinated

Strings can be subscripted

>>> word = 'Help' + 'A'>>> word'HelpA'>>> '<' + word*5 + '>''<HelpAHelpAHelpAHelpAHelpA>'

>>> word[4]'A'>>> word[0:2]'He'>>> word[2:4]'lp'

1313Sunday, March 17, 13

Slicing strings

>>> word[:2] # The first two characters'He'>>> word[2:] # Everything except the first two characters'lpA'

1414Sunday, March 17, 13

Assigning to strings is an error

>>> word[0] = 'x'Traceback (most recent call last): File "<stdin>", line 1, in ?TypeError: object does not support item assignment>>> word[:1] = 'Splat'Traceback (most recent call last): File "<stdin>", line 1, in ?TypeError: object does not support slice assignment

>>> 'x' + word[1:]'xelpA'>>> 'Splat' + word[4]'SplatA'

However, this is OK: creating new strings

1515Sunday, March 17, 13

Negative string slicing

Indices may be negative numbers, to start counting from the right.

>>> word[-1] # The last character'A'>>> word[-2] # The last-but-one character'p'>>> word[-2:] # The last two characters'pA'>>> word[:-2] # Everything except the last two characters'Hel'

0 1 2 3 4 5-5 -4 -3 -2 -1

H e l p A

Imagine numbers between characters

1616Sunday, March 17, 13

Lists

Many compound data types in Python

Most verstaile is a list

Comma-separated between square brackets

All slice operations return new lists

>>> a = ['spam', 'eggs', 100, 1234]>>> a['spam', 'eggs', 100, 1234]

>>> a[0]'spam'>>> a[3]1234>>> a[-2]100>>> a[1:-1]['eggs', 100]>>> a[:2] + ['bacon', 2*2]['spam', 'eggs', 'bacon', 4]>>> 3*a[:3] + ['Boo!']['spam', 'eggs', 100, 'spam', 'eggs', 100, 'spam', 'eggs', 100, 'Boo!']>>> a[:]['spam', 'eggs', 100, 1234]17

17Sunday, March 17, 13

More about lists>>> # Replace some items:... a[0:2] = [1, 12]>>> a[1, 12, 123, 1234]>>> # Remove some:... a[0:2] = []>>> a[123, 1234]>>> # Insert some:... a[1:1] = ['bletch', 'xyzzy']>>> a[123, 'bletch', 'xyzzy', 1234]>>> # Insert (a copy of) itself at the beginning>>> a[:0] = a>>> a[123, 'bletch', 'xyzzy', 1234, 123, 'bletch', 'xyzzy', 1234]>>> # Clear the list: replace all items with an empty list>>> a[:] = []>>> a[]

Lists are not immutable like strings

1818Sunday, March 17, 13

Even more about lists

You can use len()

Lists can be nested

>>> a = ['a', 'b', 'c', 'd']>>> len(a)4

>>> q = [2, 3]>>> p = [1, q, 4]>>> len(p)3>>> p[1][2, 3]>>> p[1][0]2>>> p[1].append('xtra') >>> p[1, [2, 3, 'xtra'], 4]>>> q[2, 3, 'xtra']

1919Sunday, March 17, 13

Our first useful program

>>> # Fibonacci series:... # the sum of two elements defines the next... a, b = 0, 1>>> while b < 10:... print b... a, b = b, a+b...112358

Multiple assignments

Indentation defines block

Interactive mode

In other languages:

temp = aa = bb = temp

In Python:

b, a = a, b

2020Sunday, March 17, 13

Our first useful program

#! /usr/bin/env python

# Fibonacci series:# the sum of two elements defines the nexta, b = 0, 1while b < 10: print b a, b = b, a+b

Batch mode

2121Sunday, March 17, 13

Interlude

And now for something completely different

2222Sunday, March 17, 13

The Zen of Python 1Beautiful is better than ugly.Explicit is better than implicit.Simple is better than complex.Complex is better than complicated.Flat is better than nested.Sparse is better than dense.Readability counts.Special cases aren't special enough to break the rules.Although practicality beats purity.Errors should never pass silently.Unless explicitly silenced.

UOB Students

2323Sunday, March 17, 13

The Zen of Python 2In the face of ambiguity, refuse the temptation to guess.There should be one—and preferably only one—obvious way to do it.Although that way may not be obvious at first unless you're Dutch.Now is better than never.Although never is often better than right now.If the implementation is hard to explain, it's a bad idea.If the implementation is easy to explain, it may be a good idea.Namespaces are one honking great idea—let's do more of those!

2424Sunday, March 17, 13

Coding style: Readability Counts

Programs must be written for people to read, and only incidentally for machines to execute.

—Abelson & Sussman, Structure and Interpretation of Computer Programs

2525Sunday, March 17, 13

Back to our regularly scheduled programming

2626Sunday, March 17, 13

More control statements: if

>>> x = int(raw_input("Please enter an integer: "))Please enter an integer: 42>>> if x < 0:... x = 0... print 'Negative changed to zero'... elif x == 0:... print 'Zero'... elif x == 1:... print 'Single'... else:... print 'More'...More

2727Sunday, March 17, 13

for loops

>>> # Measure some strings:... words = ['cat', 'window', 'defenestrate']>>> for w in words:... print w, len(w)...cat 3window 6defenestrate 12

Unlike for loops in C++, it iterates on elements of a sequence like a list or a string. For exmaple (no pun intended):

2828Sunday, March 17, 13

for loops and slicing

If you need to edit the sequencemake a copy of the list using slicing

>>> for w in words[:]: # Loop over a slice copy of list. ... if len(w) > 6:... words.insert(0, w)...>>> words['defenestrate', 'cat', 'window', 'defenestrate']

2929Sunday, March 17, 13

The range() functionFor iterating over a range of numbers>>> range(10)[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> range(5, 10)[5, 6, 7, 8, 9]>>> range(0, 10, 3)[0, 3, 6, 9]>>> range(-10, -100, -30)[-10, -40, -70]

>>> a = ['Mary', 'had', 'a', 'little', 'lamb']>>> for i in range(len(a)):... print i, a[i]...0 Mary1 had2 a3 little4 lamb

3030Sunday, March 17, 13

Enough with control structures

You just should know that there are more control structures when you need them:

break

continue

pass

3131Sunday, March 17, 13

Defining Functions

>>> def fib(n): # write Fibonacci series up to n... """Print a Fibonacci series up to n."""... a, b = 0, 1... while a < n:... print a,... a, b = b, a+b...>>> # Now call the function we just defined:... fib(2000)0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597

Documentation string literal

3232Sunday, March 17, 13

Return values

33

>>> def fib2(n): # return Fibonacci series up to n... """Return a list containing the Fibonacci series up to n."""... result = []... a, b = 0, 1... while a < n:... result.append(a) ... a, b = b, a+b... return result...>>> f100 = fib2(100) # call it>>> f100 # write the result[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

Functions without return just return

None

Method append belongs to the object result

33Sunday, March 17, 13

More on functions: Default Arguments

34

def ask_ok(prompt, retries=4, complaint='Yes or no, please!'): while True: ok = raw_input(prompt) if ok in ('y', 'ye', 'yes'): return True if ok in ('n', 'no', 'nop', 'nope'): return False retries = retries - 1 if retries < 0: raise IOError('refusing user') print complaint

Options• only the mandatory argument: ask_ok('Do you really want to quit?')• one of the optional arguments: ask_ok('OK to overwrite the file?', 2)• all arguments: ask_ok('OK to overwrite the file?', 2, 'Come on, only yes or no!')

Notice the in keyword

34Sunday, March 17, 13

Things I will not mention in functions

Arbitrary argument lists

Lambda forms

Documentation strings

3535Sunday, March 17, 13

Coding StyleUse 4-space indentation, and no tabs.

Wrap lines so that they don’t exceed 79 characters.

Use blank lines to separate functions and classes, and larger blocks of code inside functions.

When possible, put comments on a line of their own.

Use docstrings.

Use spaces around operators and after commas, but not directly inside bracketing constructs: a = f(1, 2) + g(3, 4).

3636Sunday, March 17, 13

Data Structures

3737Sunday, March 17, 13

Lists and their methods 1

38

list.append(x)

Add an item to the end of the list; equivalent to a[len(a):] = [x].

list.extend(L)

Extend the list by appending all the items in the given list; equivalent to a[len(a):] = L.

list.insert(i, x)

Insert an item at a given position.

38Sunday, March 17, 13

Lists and their methods 2

39

list.remove(x)

Remove the first item from the list whose value is x. It is an error if there is no such item.

list.pop([i])

Remove the item at the given position in the list, and return it.

list.index(x)

Return the index in the list of the first item whose value is x.

39Sunday, March 17, 13

Lists and their methods 3

40

list.count(x)

Return the number of times x appears in the list.

list.sort()

Sort the items of the list, in place.

list.reverse()

Reverse the elements of the list, in place.

40Sunday, March 17, 13

Lists as stacks

41

>>> stack = [3, 4, 5]>>> stack.append(6)>>> stack.append(7)>>> stack[3, 4, 5, 6, 7]>>> stack.pop()7>>> stack[3, 4, 5, 6]>>> stack.pop()6>>> stack.pop()5>>> stack[3, 4]

41Sunday, March 17, 13

Lists as queues

42

>>> from collections import deque>>> queue = deque(["Eric", "John", "Michael"])>>> queue.append("Terry") # Terry arrives>>> queue.append("Graham") # Graham arrives>>> queue.popleft() # The first to arrive now leaves'Eric'>>> queue.popleft() # The second to arrive now leaves'John'>>> queue # Remaining queue in order of arrivaldeque(['Michael', 'Terry', 'Graham'])

use collections.deque which was designed to have fast appends and pops from both ends

42Sunday, March 17, 13

Functional Programming Tools

4343Sunday, March 17, 13

filter(function,sequence)

44

>>> def f(x): return x % 2 != 0 and x % 3 != 0...>>> filter(f, range(2, 25))[5, 7, 11, 13, 17, 19, 23]

44Sunday, March 17, 13

map(function, sequence)

45

>>> def cube(x): return x*x*x...>>> map(cube, range(1, 11))[1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]

45Sunday, March 17, 13

reduce(function, sequence)

46

>>> def add(x,y): return x+y...>>> reduce(add, range(1, 11))55

returns a single value constructed by calling the binary function function on the first two items of the sequence, then on the result and the next item, and so on.

46Sunday, March 17, 13

List comprehension

47

>>> squares = []>>> for x in range(10):... squares.append(x**2)...>>> squares[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

squares = [x**2 for x in range(10)]

You can do this

or get the same result with this

47Sunday, March 17, 13

More list comprehension

48

>>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y][(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]

>>> combs = []>>> for x in [1,2,3]:... for y in [3,1,4]:... if x != y:... combs.append((x, y))...>>> combs[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]

is equivalent to

48Sunday, March 17, 13

Dictionaries

49

>>> tel = {'jack': 4098, 'sape': 4139}>>> tel['guido'] = 4127>>> tel{'sape': 4139, 'guido': 4127, 'jack': 4098}>>> tel['jack']4098>>> del tel['sape']>>> tel['irv'] = 4127>>> tel{'guido': 4127, 'irv': 4127, 'jack': 4098}>>> tel.keys()['guido', 'irv', 'jack']>>> 'guido' in telTrue

Associative memory

49Sunday, March 17, 13

What have we learned?

Hopefully some

basic syntax Python

Phylosophy of Python

All of the tools that you need, elegantly

5050Sunday, March 17, 13

What we haven’t learnt

The Python Standard Library

Other useful libraries

5151Sunday, March 17, 13

Why Python?

Python as glue: hundreds of libraries in C++, Fortran, etc.

Almost no need for a second language.

Quick prototyping, scripting, etc.

Strong support for scientific computing.

Support for web dev, mobile dev

5252Sunday, March 17, 13

Why Python?

Open source, join the revolution

Runs on all platforms

Cython for high performance

iPython (replace Mathematica)

MatPlotLib (replace Matlab)

5353Sunday, March 17, 13

More Python?

python.org

The tutorial

http://docs.python.org/2/tutorial/

Many video lectures on many topics

http://pyvideo.org/

5454Sunday, March 17, 13

iPython

Powerful interactive shells (terminal and Qt-based).

A browser-based notebook with support for code, text, mathematical expressions, inline plots and other rich media.

Support for interactive data visualization and use of GUI toolkits.

Flexible, embeddable interpreters to load into your own projects.

Easy to use, high performance tools for parallel computing.

5555Sunday, March 17, 13

5656Sunday, March 17, 13

57

Thank you for listening

57Sunday, March 17, 13