Introduction to Python

Transcription

1 Introduction to Python MPBA Fondazione Bruno Kessler Trento Italy Roberto Visintainer 1

2 Intro Python is an open source scripting language. Developed by Guido van Rossum in the early 1990s Named after Monty Python Available for download from Why Python? Who? Rapid prototyping Reduced training time GUI application Portable (Fortran, C) Object oriented Mature language More and more companies and institutions are choosing Python 2

3 Compiled / Interpreted Language Executable Your code { Objects Your Code code Source code { Code Your Code code { } Code Code } Code Code Code } Different Files Line by Line Your code { Code Code } Code Code Compiler Executable Execution Result Interpreter Execution Result 3

4 Compiled / Interpreted Language Compiled: Interpreted: Pros Faster execution Can produce a distributable executable standalone files Pros Steep learning curve Takes automatically care of memory usage Allows fast prototyping Cons More complicated to build (many files) User has to administrate Memory usage Cons Usually slower Does not produce standalone programs 4

5 The Interpreter Interactive Interface to Python $ python Python (r266:84292, Sep , 16:22:56) [GCC 4.4.5] on linux2 Type "help", "copyright", "credits" or "license" for more information. Prompt Python Inputs are Interpreted ((2*6) 7)**2 25 To exit In Unix: Tipe Ctrl + D 5

6 Running programs You can create python programs with a text editor such emacs (using a python editing mode) It is possible to make your <program>.py program executable adding as first line of your code #!/usr/bin/python Execute your program with./<program>.py 6

7 Running programs Interpreter or $ emacs example.py $ chmod u+x example.py $./example.py Program #!/usr/bin/python for i in (1,2,3,4,5): print i 7

8 Main Instructions Allocation / Assignment a = 3 a 3 e = hi e 'hi' 'a' 5 b = 'a' b b = a + 2 b Basic Operations a a a a a a + b b * b / b % b ** b import math math.sqrt(b) math.log(a, base) math.log10(a) math.exp(b) math.degree(b) math.degrees(b) math.radiants(a) math.sin(a) math.cos(b) math.e math.pi help(math) help(math.ceil) 8

9 Control Instructions if Test if <Test> : Commands if <Test> : Commands elif <Test> : Commands If the test (boolean condition) is met Commands are executed Operators: == > < <= >=!= Conditions: and or not if <Test> : Commands else: Commands if <Test> : Commands elif <Test> : Commands else: Commands True / False Booleans: True False 0 # False '' # False [] {} () None # # # # False False False False 9

10 Control Instructions for Loops for i in <Sequence to iterate on> : Commands As far as there are elements in the sequence Commands are executed range(n) function range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] range( 10, 100, 30) [ 10, 40, 70] range(5,10) [5, 6, 7, 8, 9] range( 10, 100, 30) [] range(0,10,3) [0, 3, 6, 9] 10

11 Control Instructions while Loops Keeps running Commands as far as the condition is True (Condition = Boolean Function) while <Stop Condition> : Commands Increment / Decrement a Variable a, step = 1,.5 a += step a a = 2*step a i = 3 r = 0 while i!= 0: r = r + 2 i = 1 r 11

12 Data I/O From File f=open('<path/file>', '<mode>') mode (optional) can be: w write (OVERWRITES) r read (default) a append r+ read and write Read f.readline() txt line f.readlines() [ line, line,...] f.read(<b>) txt # Byte you want to read (default all the text) 12

13 Data I/O From File f.tell() # ask at what position of the file is the reader <position> # in bytes f.seek(<b>,<r>) # (B)yte you move to, (R)eference {0=beginning, # 1=current pos, 2=end} f.close() Write f = open('<path/file>', '<mode>') f.write('string') f.close() Output Formatting %s String print'i have %d %s and %d %s' % (2,'cats', 3, 'dogs') %d Int I have 2 cats and 3 dogs %f Float print 'dogs divided by cats %f' % (3/2.) Dogs divided by cats

14 Types of Variables type String 'A' + ' string' 'A string' 'A string'[3] # like a list 't' S = 'A string' S[3] = 't' # ERROR! # unlike list Numbers type type type type int : 0, 5, 9 # Up to 8 Bytes long : 0L, 5L, 9L float : 0.,.5, , 5.2e+9 complex : 5+6j (j= 1 ) NB.: Conversion string float is not automatic: b = '1.4' # b is a string 2.4 * b # ERROR! 2.4 * float(b) # WORKS 14

15 Types of Variables More on Numbers 1 b = '1.4' int(b) b = float(b) c = int(b) c a, b = 1, 2 a/2 0 a, b = 1, 2. a/b 0.5 # # # # b is a string ERROR Can't convert Conversion string float IT WORKS BY TRUNCATION # Initialize 2 int # Operations between int and int # GIVES ALWAYS AN INT # Initialize 1 int and 1 float # Operations between int and float # GIVES A FLOAT 15

16 Types of Variables Lists [ ] and Tuples ( ) L = ['1.4',1.4,'ciao',[1,2,3]] T = ('1.4',1.4,'ciao',[1,2,3]) L T L[2] T[2] len(t) len(l) L[1] = 4 L ['1.4', 4, 'ciao', [1, 2, 3]] T[1] = 4 # initialization of # initialization of # read list # read tuple # read third element # read third element # returns the length # returns the length # change element list tuple of T of L # ERROR! TUPLES ARE IMMUTABLE 16

17 Types of Variables more on Lists [ ] L = ['1.4',1.4,'ciao',[1,2,3]] L.append(<element>) L.remove(<element>) L.index(<element>) # Initialization of list # Adds <element> in the last # Place of the list # Removes the first <element> # from the list # Returns the position of the # first <element> of the list # # # L.insert(<position>, <element>) # # # L.sort() # L.reverse() # L.pop(<position>) Returns and eliminates the element in <position> (default <last position>). Inserts <element> in <position> Sorts the elements Reverses the order of the elements 17

18 Types of Variables more on Lists [ ] Slicing L = ['a','b','c','d','e','f','g','h'] L[4] 'e' L[ 1] L[:] 'h' ['a','b','c','d','e','f','g','h'] L[ 3] L[2:5] 'f' ['c', 'd', 'e'] L[ 3: 1] L[3:] ['f', 'g'] ['d', 'e', 'f', 'g', 'h'] L [1:4] = [] L[:3] L ['a', 'b', 'c'] ['a', 'e', 'f', 'g', 'h'] L[3] = L[3]+'igi' ['a', 'e', 'f', 'gigi', 'h'] 18

19 Types of Variables Iterating on List for item in L:... print 'the item is ',item for idx, it in enumerate(l):... print 'pos: ',idx,'val: ',it # At each iteration item takes # one of the values of L # At each iteration idx takes the value of # the position of L, it takes the value of # the item for idx in range(len(l)):... print 'L[%d] = %s' % (idx, L[idx]) 19

20 Types of Variables Dictionaries { } Is a kind of list where the index, referred to as key, can be an arbitrary text (any immutable object) # Initialization D = {} D['TEL NUM'] = ' ' D['HEIGHT'] = 1.90 D['WEIGHT'] = 100 D['GIRL'] = 'Brunilde' D['HOBY'] = ('python', 'guitar', 'beer') 20

21 Types of Variables Main Operations D['GIRL'] D.keys() # Extracts key corresponding to key GIRL # Returns a copy of list of keys D.has.key('COLOR') 'COLOR' in D # Does D have key davidecol? # Same test as above D.get('GIRL', 1.0) # As D['GIRL'] but returns 1.0 # in case the key does not exist D.items() # Returns a list of (key,value) # tuple del D['GIRL'] # Delete item len(d) # Returns the number of items 21

22 Functions What are they? A Function is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code. Can be called several times and/or from several places during a single execution including from other functions. What are they for? Judicious use of functions help a structured programming approach. Will heavily reduce the time for the development and maintenance of a large program, while increasing its quality and reliability. Functions, often collected into libraries, are an important mechanism for sharing and trading software. The discipline of object-oriented programming is based on objects and methods 22

23 Functions Function definition begins with def Function Name Function Arguments Colon def myfunction( <arg0>, a = <arg1>, b = <arg2> ): ''' < Documentation String > ''' <commandline 0> <commandline 1> return <something> indentation Key word return indicates the value to send back to the caller 23

24 Functions Function Variables are local: in general all variables allocated in a function are destroyed upon return. def myfun (a, b, c): d = a+b e = d*c return e RES = myfun(1,2,3) RES d # ERROR!! e # ERROR!! # definition # allocation of d # allocation of e NameError: name 'd' is not defined NameError: name 'e' is not defined 24

25 Functions Arguments Positional: they need to be passed to the function they need to be passed in the right order def power( a, b ): ''' returns a^b ''' c = a**b return c power (2,10)!= power (10,2) def sumsub( lnum, a ): ''' lnum + or a ''' for i in range(len(lnum)): if lnum[i] > 0: lnum[i] = lnum[i]+a elif lnum[i] < 0: lnum[i] = lnum[i] a return lnum sumsub(list, 10) OK sumsub(10, list) ERROR!! 25

26 Functions Arguments keyword arguments or named arguments: def wfile(filename, text, nlines = 10, mode = 'w' ): channel = open(filename, mode) for i in range(nlines): Default channel.write(text + '\n') channel.close() Call: wfile('intro','hi, my name is Mario',nlines =10,mode = 'w') Useless 26

27 Functions Variable number of Arguments def fwrite(file, L): channel = open(file, 'w') for i in L: channel.write(i + '\n') channel.close() Call: fwrite('intro',['hi,','my name is Mario','and I am','a farmer']) 27

28 Modules A module is a file containing Python definitions and statements. The file name is the module name with the suffix.py appended. # Fibonacci numbers module def fib(n): # write Fibonacci series up to n a, b = 0, 1 while b < n: print b, a, b = b, a+b def fib2(n): # return Fibonacci series up to n result = [] a, b = 0, 1 while b < n: result.append(b) a, b = b, a+b return result create a file called fibo.py and write this code in it 28

29 Modules How to use a module import fibo # Using the module name you can access the functions fibo.fib(1000) fibo.fib2(1000) # This does not enter the names of the functions defined # in fibo directly in the current symbol table import fibo as fi fi.fib(1000) fi.fib2(1000) # As before, but very useful for functions used very often # or modules with long names NB.: Modules can import other modules. Put the import statements at the beginning of a module or script. ref.: 29

30 Modules More on How to use a module from fibo import fib, fib2 fib(1000) fib2(1000) # It is like cutting and pasting the definition of the functions in your code To import all the functions of a library: from fibo import * fib(1000) fib2(1000) These last two are deprecated because the reduce the readability of the code and could cover other functions without knowing it. import fibo as fi Normally If you change your module, you have to restart the interpreter to use the modified functions, or you can use: fi=reload(fi) Remember the function help(<modulename>) 30

31 Classes What Are They? A class defines the structure and the behavior of an object are defined by, which is a definition, or model, of all objects of a specific type. Why Should we use them? Example: Library We could define a Class book, each object of this class will have a 31

32 Classes What Are They? A class defines the structure and the behavior of an object are defined by, which is a definition, or model, of all objects of a specific type. Why Should we use them? Example: Library We could define a Class book, each object of this class will have a Attributes State (structure): Title Author Number of Pages List People who borrowed Presence/Absence Modules (functions): Borrow Return Is the Book Available? 32

33 Classes What Are They? A class defines the structure and the behavior of an object are defined by, which is a definition, or model, of all objects of a specific type. Why Should we use them? Example: Library We could define a Class book, each object of this class will have a Attributes State (structure): Title Author Never Change Number of Pages List People who borrowed Presence/Absence Modules (functions): Borrow Return Is the Book Available? Returns Yes/No 33

34 Classes Class Definition class MyClass: "A simple example class" def init (self): DEFINE STRUCTURE def f(self): DEFINE METHODS 34

35 Classes Class Definition Class Instantiation class MyClass: "A simple example class" def init (self): self.i = def f(self): return 'hello world' x = MyClass() Creates a new instance of the class and assigns this object to the local variable x. Use the structure of x x.i x.f() hello world 35

36 Classes Class Definition e = 123 class Test: def init (self): self.e = 5 def go(self): print e class Test2: def init (self): self.e = 5 def go(self): print self.e Class Instantiation x = Test() x2 = Test2() x.go() 123 x2.go() 5 self indicates the Object itself. self.e indicates the Attribute of the Object (reachable because self is in passed to the function) 36

37 Classes Class Definition class Bag: def init (self): self.data = [] def add(self, x): self.data.append(x) def addtwice(self, x): self.add(x) self.add(x) Recursions are ALLOWED Class Instantiation MULTIUPLE Instantiations Handag = Bag() Backpack = Bag() Suitcase = Bag() 37

38 Classes Class Definition class MyClass2: ''' A simple class to increase and subtract ''' def init (self, orig): self.i = orig def tellme(self): return self.i def increase(self, p): self.i = self.i + p def decrease (self, m): self.i = self.i m Class Instantiation goofy = MyClass2(10) Use the Object goofy: goofy.tellme() 10 goofy.increase(15) goofy.tellme() 25 goofy.decrease(50) goofy.tellme() 25 38

39 Classes Class Definition class MyClass2: A simple class to increase and subtract def init (self, orig): self.i = orig def tellme(self): "A comment on tellme()" return self.i def increase(self, m): A comment on increase() self.i = self.i + m def decrease (self, m): A comment on decrease() self.i = self.i m # Comments on DOCUMENTATION!!! Writing undocumented code is a waste of time because nobody else will be able to use it. goofy = MyClass2() Help! goofy. doc help(paperino) line 39

40 Classes class arithmetic: def init (self, first, second): self.a = first self.b = second def times(self): c = self.a * self.b return c def division(self): c = self.a/self.b return c QUESTIONS: goofy = arithmetic(3,6) goofy.division() >>?? goofy = arithmetic('ciccio', 6) goofy.times() >>?? goofy.division() >>?? >> ERROR?? 40

41 References Official Website: Official Tutorial: Official Documentation: Cookbook: Google 41