Introduction to Python

Transcription

1 Introduction to Python

2 Development Environments what IDE to use? 1. PyDev with Eclipse 2. Sublime Text Editor 3. Emacs 4. Vim 5. Atom 6. Gedit 7. Idle 8. PIDA (Linux)(VIM Based) 9. NotePad++ (Windows) 10.BlueFish (Linux)

3 Python Version Python Version 2.7.x Download Link :

4 Python Interactive Shell % python Python (r266:84292, Nov , 10:50:32) [GCC (Red Hat )] on linux2 Type "help", "copyright", "credits" or "license" for more information. You can type things directly into a running Python session name Andrew name 'Andrew' >>> print "Hello", name Hello Andrew

5 Lists A compound data type: [5, "Hello", "there", 9.8] [5, "Hello", "there", [1, AI ] ] Use len() to get the length of a list >>> fruits = [ Apple", Banana", Olive"] >>> len(fruits) 3

6 Use [ ] to index items in the list fruits = [ Apple, Banana ] fruits Apple fruits Banana fruits Traceback (most recent call last): File "<stdin>", line 1, in <module> IndexError: list index out of range fruits [-1] Banana' fruits [-2] Apple [0] is the first item. [1] is the second item. Out of range values raise an exception. Negative values go backwards from the last element.

7 Strings share many features with lists smiles = "C(=N N)N.C(=O O O smiles 'C' smiles smiles[-1] O smiles Use slice notation to get a substring N smiles[10:-4] 'C(=O

8 String Methods: find, split smiles = "C(=N N)N.C(=O O O smiles.find O smiles.find Use find to find the start of a substring. smiles.find Start looking at position Find returns -1 if it couldn t find a match. smiles.split Split the string into parts with. as delimiter. 'C(=N N N', 'C(=O O O

9 String operators: in, not in if "Br" in Brother : print "contains! else: print doesn t contain _address = tberis not in _address: _address

10 Lists are mutable - some useful methods fruits = ["Apple", "Banana", "Olive"] fruits.append("strawberry") fruits [ Apple, Banana, Olive, Strawberry ] del fruits ['Banana', 'Olive', 'Strawberry'] #remove an element fruits.reverse() ['Strawberry', 'Olive', 'Banana'] #reverse the elements in a list fruits.sort() ['Banana', 'Olive', 'Strawberry'] #in place sort based of ASCII codes

11 Tuples: sort of an immutable list yellow = (255, 255, 0) # r, g, b yellow 255 yellow yellow Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'tuple' object does not support item assignment

12 Zipping lists together names [ Themis, Nikos, Despoina ] gender [ Male, Male, Female ] zip(names, gender) [( Themis, Male ), ( Nikos, Male ), ( Despoina, Female )]

13 Dictionaries Dictionaries are lookup tables. They map from a key to a value. symbol_to_name = { "H": "hydrogen", He helium "Li": "lithium", "C": "carbon O oxygen "N": "nitrogen" } Duplicate keys are not allowed Duplicate values are just fine

14 Dictionary Key access >>> symbol_to_name = {"H": "hydrogen, He helium "Li": "lithium,"c": "carbon O oxygen "N": "nitrogen } >>> symbol_to_name["c"] Get the value for a given key 'carbon' >>> Z" in symbol_to_name Test if the key exists( in only checks the keys, not the values.) False >>> symbol_to_name[ Z"] [] lookup failures raise an exception. Traceback (most recent call last): File "<stdin>", line 1, in <module> KeyError: Z' >>> symbol_to_name.get("p", "unknown") Use.get() if you want to return a default value. 'unknown' >>> symbol_to_name.get("c", "unknown") 'carbon >>> symbol_to_name["cu"] = copper Insert a new item >>> symbol_to_name["c"] = calcium Update single item

15 Keys can be any immutable value Numbers, strings, tuples are valid (immutable). Lists, dictionaries, sets are not valid (mutable). A set is an unordered collection with no duplicate elements. nobel_prize_winners = { (1979, "physics"): ["Glashow", "Salam", "Weinberg"], chemistry Hodgkin biology McClintock

16 Some useful dictionary methods >>> symbol_to_name = {"H": "hydrogen, He helium "Li": "lithium,"c": carbon O oxygen "N": "nitrogen } >>> symbol_to_name.keys() ['C', 'H O N Li He >>> symbol_to_name.values() ['carbon', 'hydrogen', 'oxygen', 'nitrogen', 'lithium', 'helium'] >>> symbol_to_name.update( {"P": "phosphorous", "S": "sulfur"} ) >>> symbol_to_name.items() [('C', 'carbon H hydrogen O oxygen N nitrogen P phosphorous S sulfur Li lithium He helium >>> del symbol_to_name['c'] >>> symbol_to_name {'H': 'hydrogen', 'O': 'oxygen', 'N': 'nitrogen', 'Li': 'lithium', 'He': 'helium'}

17 Control Flow Things that are False The boolean value False The numbers 0 (integer), 0.0 (float) The empty string "". The empty list [], empty dictionary {} and empty set set(). Things that are True The boolean value True All non-zero numbers. Any string containing at least one character. A non-empty data structure.

18 Control flow: if statement x = int(raw_input("please enter #:")) if x print 'Negative number' elif x print 'Zero' elif x print One' else: print 'More than one no switch case statements in python

19 Control flow: for a = ['cat', 'window', 'defenestrate'] for x in a: print x, len(x) no arithmetic progression, but range(10) # à [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] range(2,10,2) # à [2, 4, 6, 8] for i in range(len(a)): print i, a[i] break and continue like C and Java

20 User-defined functions def fib(n): """Print a Fibonacci series up to n.""" a b while b < n: print b a b b a+b fib(30)

21 File Manipulation

22 List Comprehension Construct lists in an alternative way

23 Classes class Person: def init (self, first, last): self.firstname = first self.lastname = last def get_fullname(self): return self.firstname + + self.lastname class Employee(Person): instances = 0 Class / static variables def init (self, first, last, afm): Person. init (self, first, last) self.afm = afm Employee.instances += 1 def get_employee(self): return self.get_fullname() + + self.afm employee1 = Employee( themis, beris, 123 ) Creates an instance of Employee employee2 = Employee( nikos, karalis, 456 ) Creates another instance of Employee print employee1.get_employee() print employee2.instances print employee1.instances == employee2.instances It will always be True