STSCI Python Introduction

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1 STSCI Python Introduction Class 3 Jim Hare Today s Agenda Functions Passing Arguments to Modules File I/O User Defined Objects Variable Scopes System Commands and Controls 1

2 Function template Functions Consists of def statement, parameters and return def test1(x,y,j= testcase1 ) j= testcase1 is the default Example update_default_db.py Function Calls test1(2,3, testcase2 ) # ordered call test1(j= testcase2, y= 3, x=2) # no order test1(2,3) # this time j is the default testcase1 2

3 Passing Arguments main is the name of the originating or first module run in the current execution of python def run(database, account='sogsspss'): #database #account os.system('printf "sp_defaultdb %s,%s\\ngo" isql' % (account,database)) os.system('printf "select db_name()\\ngo" isql') return None Passing Arguments if name == main : if name == ' main ': if len(sys.argv)!= 3: print Error in arguments try again. sys.exit() database = string.lower(sys.argv[1]) account = sys.argv[2] run(database,account) 3

4 Functions Returning Values def splitonperiod(instring): # is a string import string stringlist = string.split(instring,. ) test = 1 return test,stringlist astring = md.spring.2003.stsci.edu status,varlist = splitonperiod(astring) print astring [ md, spring, 2003, STSCI, EDU ] # file : div.py def divide(a,b): q=a/b r = a q*b return (q,r) Namespace 4

5 import div Namespace cont. a,b = div.divide(2305,29) import div as foo a,b = foo.divide(2305,29) from div import divide a,b = divide(2305,29) from div import * # all in current namespace Modules dir() import string dir(string) # all modules have dir function [ builtins, doc, file, name, _idmap, _idmapl, _lower, _swapcase, _upper, atof, atof_error, atoi, atoi_error, atol, atol_error, capitalize, capwords, center, count, digits, expandtabs, find, 5

6 Raw Input Command first = raw_input( Type your first name: ) mid = raw_input( Type your middle name: ) last = raw_input( Type your last name: ) print Your full name is %s %s %s \ % (first,mid,last) Example: myname.py Files myfile = open( build_id.data, r ) File modes are: r, w, or a. b following means binary ie. rb Updates r+ or w+, can perform both input and output, as long as output operations flush their data before subsequent input operations ie. f.flush() 6

7 File Methods - Read F.read - Reads at most n bytes F.readline() - Reads a single line of input F.readlines() - Reads all the lines and returns a list. mylist = myfile.readlines() File Methods - Write F.write(S) F.writelines(L) - Writes string S - Writes all strings in List L 7

8 F.close() F.tell() File Methods - closes the file F - returns the file pointer F.seek(offset [,where]) seek to new file position F.isatty() F.flush() F.truncate([size]) F.fileno() - 1 if F is interactive terminal - Flushes output buffers - truncates to size bytes - integer descriptor File Attributes F.closed - Boolean 0 = open, 1=closed F.mode - I/O mode of file F.name - name of file or source F.softspace - indicator of whether a space character needs to be printed before another value when using the print statement. 8

9 Standard Input, Output, and Error import sys - to get access to Standard I/O files sys.stdin sys.stdout sys.stderr - Standard Input - Standard Output - Standard Error Object Oriented Programming Concepts Class an object/statement defining inherited members and methods Instance object created from a class. Member an attribute of an object, the data bound to the object Method function attribute of an object Self the name given to the implied instance object in methods. 9

10 Class Example Class PhoneDir Name AccountID Balance Insert Add Balance Object MyAccount Paul Lee $10.00 withdraw deposit balance Instances of Class Account Object YourAccount Jane Doe $ withdraw deposit balance Object Example Class Account Name AccountID Balance Withdraw Deposit Balance Object MyAccount Paul Lee $10.00 withdraw deposit balance Instances of Class Account Object YourAccount Jane Doe $ withdraw deposit balance 10

11 Class statement class <name> (<superclass>,<superclass>, ): Example: class vector3d: def init (self,x,y,z): self.x = x self.y = y self.z = z myvector = vector3d(4,7,2) # myvector is instance Operator Overloading Operator Overloading occurs when an operator s meaning is changed. The new method of the class is said to overload the operator. Example of overloading + operator: def add (self,other): return vector3d(self.x + other.x, self.y + other.y, self.z + other.z) 11

12 Special Class Methods init (self,args ) - Instance constructor del (self) Instance destructor repr (self) string representation for print, also str (self) works the same cmp (self,other) function for comparisons of instances of objects There are many others p Chapter 3 Types and Objects review. Object Oriented Programming Concepts continued Inheritance when an object is given access to the attributes of another class Superclass the class or classes another class inherits attributes from, (base class) Subclass A class that inherits attributes from another class Read Chapter 7 of Python Essential Reference 12

13 Inheritance class A: vara = 42 def method1(self) print This is method 1 class B(A): # class B inherits from A def method2(self) print This is method 2 IofB = B() # create instance of B IofB.method2 # invokes B.method2 IofB.method1 # invokes A.method1 Name Resolution: The LGB Rule First local function level Names assigned inside a function def Second global module Names assigned at top-level of a module Names declared global in function Third built-in - Python Predefined names open, len, print, etc. 13

14 LGB Scope Example # global scope X = 99 # X and addy: global def addy(y): # Y and Z : locals # local scope Z = X + Y # X is global return Z # return and def built-in addy(2) # addy in module: 101 LGB Scope Example 2 Y, Z = 1, 2 # global names in module def sum(): global X X = Y + Z # return None sum() print X # declare globals assigned # None is a built-in meaning Null # execute the function # I can print X outside the local function 14

15 System Command Controls commands module p. 155 Python Ess. Ref. getoutput(command) getstatusoutput(command) os module: popen(command [, mode [,bufsize]]) p. 185 see call functions in spss.py,misc import commands commands module x = commands.getoutput('spss_time') print x # prints the output from the command Import string spss_time xlist = string.split(x, \n ) # to create a list by line of output 15

16 commands.getstatusoutput import commands status,commandoutput = commands.getstatusoutput( cclist get testccl ) print status print commandoutput popen >>> commandoutput = \ os.popen( cclist get testccl 2>&1","r") >>> out_string = commandoutput.read() >>> print out_string >>> stat = commandoutput.close() >>> print stat 16

STSCI Python Introduction. Class URL

STSCI Python Introduction. Class URL STSCI Python Introduction Class 2 Jim Hare Class URL www.pst.stsci.edu/~hare Each Class Presentation Homework suggestions Example files to download Links to sites by each class and in general I will try