Introduction to Python

Transcription

1 Introduction to Python Michael Krisper Thomas Wurmitzer March 22, 2014 Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

2 Schedule Tutorium Dates & Deadlines Submission System Assignment I: Example (Short) Python Introduction networkx, numpy & matplotlib Disclaimer Edited but mostly based on Michael Krisper s Python Introduction (with permission). Thank you! Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

3 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 libraries 2. If you want to know more about Python s history checkout Guido s Blog 3 on that topic Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

4 Installation We use Python Python 3 is not supported (but may work)! Linux: Most distributions already come with Python 2.7. If not install them 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 the Windows FAQ Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

5 Writing Python using REAL $EDITOR Figure 1: Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

6 Better than hoverboards! Figure 2: Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

7 Writing Python: Hello World % cat hello.py print 'Hello World' % python hello.py # Run code inside file Hello World % python -c "print 'Hello World'" # Pass program as string. % python # Interactive Mode % ipython #... on steroids Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

8 Writing Python: Hello World (Extended) #!/usr/bin/env python import sys import math def my_function(message): print message return 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 Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

9 Writing Python: Calculations (interactive 5 ) % python Python (default, Mar , 22:15:05) >>> 3+4 # output: 7 >>> 5*3.7+1 # output: 19.5 >>> 2**3-1 # output: 7 >>> 5**70 # output: L >>> 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 Hint: from future import division Before: 3/2 -> 1, After: 3/2 -> https: //docs.python.org/2/tutorial/interpreter.html#invoking-the-interpreter Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

10 Writing Python: Variables & Assignments >>> a Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'a' is not defined >>> a = 42 # integer >>> a 42 >>> 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 Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

11 Writing Python: Bool 67 bool(none) # False bool(0) # False bool({}) # False bool([]) # False bool("") # False bool(1) # True bool([1,2,3,4]) # True bool("hello") # True # Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

12 Writing Python: Strings 8 >>> s = 'Hello World' >>> s = "Hello World" >>> s = """ Hello World """ # 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) 8 Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

13 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'> Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

14 Writing Python: Lists 9 >>> 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 9 Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

15 Writing Python: Dictionary 10 >>> 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") 10 Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

16 Writing Python: Built-in Datatypes (Excerpt) int Integer 42 Bool Boolean: True, False True Long Long Integer L Float Double 3.85 Complex Complex Number ( j) 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) Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

17 Writing Python: Built-in Functions 11 (Excerpt) len(...) max(...)/min(...) open(...) print input(...) range(...)/xrange(...) sorted(...) sum(...) type(...) Get length of a sequence Get max / min element of a sequence Open a file for read/write Output to console Read from console Create a counter sequence Sort a sequence Calculate the sum of a sequence Get the type of a variable Others: abs, dir, eval, format, hash, help, next, enumerate, ord, map, reduce, slice, unicode, zip, apply, Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

18 Writing Python: Built-in Functions 12 (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 sys sys.stdout.write("who wrote the pink letter?") 12 Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

19 Control Flow 13 if points < 3 or 'bird' is not word: print "Erm, seriously?" elif points < 10: print "Seriously?" else: print "Good job!" for word in ['ham', 'sausage', 'spam']: print word for i, word in enumerate(['ham', 'sausage', 'spam']): print i, word while answer < 42: answer += Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

20 Functions 14 def say(string): print string >>> say('hello') # Output: Hello >>> say # Output: <function say at 0x > >>> s = say >>> s('hodor!') # Output: Hodor! 14 https: //docs.python.org/2/tutorial/controlflow.html#defining-functions Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

21 Classes 15 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() 15 Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

22 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") 16 https: //docs.python.org/2/tutorial/inputoutput.html#reading-and-writing-files Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

23 (Common) Exceptions 17 >>> 2 + 'Hodor!' Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unsupported operand type(s) for +: 'int' and 'str' >>> 2 + a Traceback (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 17 Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

24 JSON 18 The json module provides methods for easily reading and writing data from and to files. import json json_string = '{'list': [1,2,3,4], 'key': 'value', 'truth': 42 }' content = json.loads(json_string) print contents['truth'] # output: 42 print contents['key'] # output: value print contents['list'] # output: [1,2,3,4] 18 Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

25 networkx 19 The networkx module/library provides functionality to create, analyze and draw complex networks. import networkx as nx G = nx.graph() G.add_node(1) G.add_edge(1, 2) G.add_edge(2, 3) import matplotlib.pylab as plot plot.figure() nx.draw(g) plot.savefig("graph.png") 19 http: //networkx.github.io/documentation/latest/tutorial/tutorial.html Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

26 Help & Resources Use help and dir in interactive mode. Library Documentation: networkx, numpy, matplotlib Python Language Reference Google s Python Class Think Python: How to Think Like a Computer Scientist Code Academy s Python Track The Hitchhiker s Guide to Python! StackOverflow Reddit Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27

27 Questions? 1 Ask now. 2 Ask in the newsgroup tu-graz.lv.web-science. 3 Send an Dominik Mösslang, dominik.moesslang@student.tugraz.at Ingo Holzmann, ingo.holzmann@student.tugraz.at Emanuel Lacić, emanuel.lacic@tugraz.at Thomas Wurmitzer, t.wurmitzer@student.tugraz.at Please use [WSWT] or the course number as a subject. Please post general questions regarding the assignment tasks to the tu-graz.lv.web-science. If you have a particular, specific problem, you can contact us per as well. Answering delays might be longer, though. Michael Krisper, Thomas Wurmitzer Introduction to Python March 22, / 27