Introduction to Python

Michael Krisper Thomas Wurmitzer

October 21, 2014

Michael Krisper, Thomas Wurmitzer Introduction to Python October 21, 2014 1 / 26

Schedule

Tutorium IDates & DeadlinesSubmission SystemMap/Reduce ExampleIntroduction to Python (1)

Tutorium IIIntroduction to Python (2)Questions

DisclaimerEdited but mostly based on Michael Krisper’s Python Introduction (withpermission). Thank you!

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What is Python?

“Python is a dynamic, interpreted language. Source code does not

declare the types of variables or parameters or methods. This

makes the code short and flexible, and you lose the compile-time

type checking in the source code. Python tracks the types of all

values at runtime and flags code that does not make sense as it

runs.”

1

Huge standard library and community.Huge list of 3rd party libraries2.If you want to know more about Python’s history checkout Guido’sBlog3 on that topic.

1https://developers.google.com/edu/python/introduction

2https://github.com/vinta/awesome-python

3http://python-history.blogspot.co.at

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Installation

We use Python 2.7 - Python 3 is not supported and will most likely notwork.Linux: Most distributions already come with Python 2.7. If not installthem via your distributions packagemanager e.g. (pacman, apt, . . . )OSX: All recent versions ship with Python 2.7 out of the box.Windows: Windows Installer via python.org and check out theWindows FAQ4.

4https://docs.python.org/2/faq/windows.html

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Writing Python using REAL $EDITOR

Figure 1: https://xkcd.com/378

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Better than hoverboards!

Figure 2: https://xkcd.com/353/Michael Krisper, Thomas Wurmitzer Introduction to Python October 21, 2014 6 / 26

Writing Python: Hello World

% cat hello.pyprint �Hello World�% python hello.py # Run code inside fileHello World

% python -c "print �Hello World�" # Pass program as string.

% python # Interactive Mode% ipython # ... on steroids

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Writing Python: Hello World (Extended)

#!/usr/bin/env pythonimport sysimport math

def my_function(message):print messagereturn math.e # return constant from module

if __name__ == �__main__�:if len(sys.argv) < 2:

sys.exit(1)

result = my_function(sys.argv[1])print math.ceil(result) # function in module

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Writing Python: Calculations (interactive5)

% pythonPython 2.7.5 (default, Mar 9 2014, 22:15:05)[...]Type "help", "copyright", "credits" or "license" for more information.

>>> 3+4 # output: 7>>> 5*3.7+1 # output: 19.5>>> 2**3-1 # output: 7>>> 5**70 # output: 847032947254...13916015625L>>> 3/2 # output: 1>>> 3/2.0 # output: 1.5>>> 3.0//4.0 # output: 0.0>>> 5*"a" # output: "aaaaa">>> 3+4 == 2**3-1 # output: True

5https:

//docs.python.org/2/tutorial/interpreter.html#invoking-the-interpreter

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Writing Python: Variables & Assignments

>>> aTraceback (most recent call last):

File "<stdin>", line 1, in <module>NameError: name �a� is not defined>>> a = 42 # integer>>> a42>>> type(a) # output: <type �int�>>>> a += 1 # increase a by one

>>> b = 1.78 # float>>> c = "Hello" # string>>> d = [1,2,3,4] # list>>> e = (1,2,3) # tuple>>> f, g = True, None

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Writing Python: Bool67

bool(None) # Falsebool(0) # Falsebool({}) # Falsebool([]) # Falsebool("") # False

bool(1) # Truebool([1,2,3,4]) # Truebool("Hello") # True# ...

Python on Lovea

ahttps://twitter.com/johl/status/318494068365733889

>>> import this>>> love = this>>> this is love, love is not True or False, love is love>>> (True, True, True)

6https://docs.python.org/2/library/functions.html#bool

7https://docs.python.org/2/library/constants.html#False

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Writing Python: Strings8

>>> s = �Hello World�>>> s = "Hello World">>> s = """HelloWorld""" # Multiline

# Strings are Sequences>>> �lie� in �believe� # output: True>>> �execute�.find(�cute�) # output: 3>>> �foo� + �bar� # output: foobar>>> �\n\nValar Dohaeris �.strip() # output: Valar Dohaeris>>> �A;B;C\n;D�.split(�;�) # output: [�A�, �B�, �C\n�, �D�]

>>> help(str)

8http://docs.python.org/2/library/stdtypes.html#string-methods

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Writing Python: Conversion

>>> str(434) # �434�

>>> int(�956�) # 956>>> int(�\n\n210 \r\n�) # 210>>> int(�5a�)Traceback (most recent call last):

File "<stdin>", line 1, in <module>ValueError: invalid literal for int() with base 10: �5a�>>> int(�5a�, 16) # 90

>>> float(�3.14�) # 3.14

>>> type(�434�) # <type �str�>>>> type(434) # <type �int�>

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Writing Python: Lists9

>>> a = [4, 8, 15] # list definition>>> a[0] # get first element>>> len(a) # length of the list>>> a[1:3] # get a slice by range>>> a[-1] # get last element>>> a.append(16) # append element>>> a += [55, 23, 42] # concat lists>>> a.remove(55) # remove an element>>> del a[5] # delete element on index>>> sorted(a) # sort the list

9https://docs.python.org/2/tutorial/datastructures.html

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Writing Python: Dictionary10

>>> d = { "key": "value", "key2": "value2" }>>> d["key"]>>> d.keys()>>> d.values()>>> d["key3"] = 45>>> "key" in d>>> len(d)>>> d.get("nokey", "default") # = "default">>> d.setdefault ("nokey", "default")

10https://docs.python.org/2/tutorial/datastructures.html

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Writing Python: Built-in Datatypes (Excerpt)

int Integer 42

Bool Boolean: True, False True

Long Long Integer 10000000000000000L

Float Double 3.85

Complex Complex Number (3.1+0.9j)

Str String “Jaqen H’ghar”List List / Array [1, 2, 5.5, "asdf", 0]

Dict Dictionary {"a":"foo", "b":"bar"}

Set Set Set([1, 2, 1, 3, True])

Tuple Tuple (1, 2, None, True, 4)

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Writing Python: Built-in Functions11 (Excerpt)len(...) Get length of a sequencemax(...)/min(...) Get max / min element of a sequenceopen(...) Open a file for read/writeprint Output to consoleinput(...) Read from consolerange(...)/xrange(...) Create a counter sequencesorted(...) Sort a sequencesum(...) Calculate the sum of a sequencetype(...) Get the type of a variable

Others: abs, dir, eval, format, hash, help, next, enumerate,ord, map, reduce, slice, unicode, zip, apply, ...

11https://docs.python.org/2/library/functions.html#built-in-funcs

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Writing Python: Built-in Functions12 (Excerpt)

print "Valar morghulis" # with newline

print "Valar morghulis", # without newline

print "a = ", 1, " b = ", 2

print "a = %d b = %d" % (1,2)

print "{} of {}".format(�mother�, �dragons�)

import syssys.stdout.write("Who wrote the pink letter?")

12https://docs.python.org/2/library/functions.html#built-in-funcs

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Control Flow13

if points < 10 or �bird� is not word:print "Erm, seriously?"

elif points < 40:print "Seriously?"

else:print "Good job!"

for word in [�ham�, �sausage�, �spam�]:print word

while answer < 42:answer +=1

Additionally statements like pass, break or continue can be used tofurther control the above.

13https://docs.python.org/2/tutorial/controlflow.html

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Functions14

def say(string):print string

>>> say(�Hello�) # Output: Hello>>> say # Output: <function say at 0x102697938>>>> s = say>>> s(�Hodor!�) # Output: Hodor!

14https:

//docs.python.org/2/tutorial/controlflow.html#defining-functions

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Classes15

class Human(object): # Inherits from �object�def __init__(self, name): # Constructor

self.name = name

def speak(self):print self.name, ": Valar Morghulis."

jh = Human("Jaqen H�ghar");jh.speak()

15https://docs.python.org/2/tutorial/classes.html

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File I/O 16

fh = open(�filename.txt�, �r�)lines = fh.readlines()fh.close()

with open(�filename.txt�, �w�) as f:f.write("\n" % )

with open(�filename.txt�, �w�) as f:f.write("%d + %d = %d\n" % (2,3,2+3))f.write("Another line\n")f.write("Another line\n")

16https:

//docs.python.org/2/tutorial/inputoutput.html#reading-and-writing-files

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DateTime17

>>> import datetime>>> a = datetime.datetime(2014, 10, 26, 9, 15, 10)>>> b = datetime.datetime(2024, 10, 26, 9, 15, 10)>>> type(a) # Output: <type �datetime.datetime�>>>> a # 26.10.2014 09:15:10>>> a < b # Output: True

>>> st = "2013-11-06 09:57:52">>> ft = "%Y-%m-%d %H:%M:%S">>> c = datetime.datetime.strptime(st, ft)>>> type(c) # Output: <type �datetime.datetime�>>>> c # 06.11.2013 09:57:52

>>> �2013-11-06 09:57:52� < �2013-11-06 10:57:52� # ???>>> �2013-11-06 09:57:52� > �2013-11-06 10:57:52� # ???

17http://docs.python.org/2/library/datetime.html

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(Common) Exceptions18

>>> 2 + �Hodor!�Traceback (most recent call last):

File "<stdin>", line 1, in <module>TypeError: unsupported operand type(s) for +: �int� and �str�

>>> 2 + aTraceback (most recent call last):

File "<stdin>", line 1, in <module>NameError: name �a� is not defined

>>> while �Hodor� print "Hodor!"File "<stdin>", line 1

while �Hodor� print "Hodor!"^

SyntaxError: invalid syntax

18https://docs.python.org/2/tutorial/errors.html

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Help & ResourcesUse help and dir in interactive mode.Python Language Reference

I https://docs.python.org/2/reference/index.html

Google’s Python ClassI https://developers.google.com/edu/python/

Think Python: How to Think Like a Computer ScientistI http://www.greenteapress.com/thinkpython/

Code Academy’s Python TrackI http://www.codecademy.com/en/tracks/python

StackOverflowI http://stackoverflow.com/questions/tagged/python

Reddithttp://reddit.com/r/pythonhttp://reddit.com/r/learnpython

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Questions?

eMail: thomas.wurmitzer@tugraz.atI Please use [DB2] or the course number 707.030 as a subject.

Newsgroup: tu-graz.lv.dbase2Question Hours

I 14.12.2015, 13:00 (HS i9 PZ2EG048)I 11.01.2016, 13:00 (HS i9 PZ2EG048)

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