![ Iterators can be materialized as lists or tuples by
using the list() or tuple() constructor functions.
The following program shows how a list can be
converted to an iterator and then to materialize
to a tuple.
list=[1,2,3]
iterator=iter(list)
t=tuple(iterator)
print(t)
Output
(1,2,3)](https://image.slidesharecdn.com/9pythonprogramming-250503150622-936f39ff/85/9-Python-programming-notes-for-ktu-physics-and-computer-application-semester-4-14-320.jpg)
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9 Python programming notes for ktu physics and computer application semester 4
- 1. Priya B R Assistant Professor Christ Nagar College Maranalloor
- 2. Python allows us to connect to various databases through database interfaces. Python’s database interface is DB-API. You can choose the right database for your application. Python Database API supports a wide range of database servers such as − GadFly mSQL MySQL PostgreSQL Microsoft SQL Server 2000 Informix Interbase Oracle Sybase You must download a separate DB API module for each database you need to access. For example, if you need to access an Oracle database as well as a MySQL database, you must download both the Oracle and the MySQL database modules.
- 3. Create a database called SAMPLE in MySQL. The user id and password used to access SAMPLE database is “user” and “pass” respectively. Create a table STUDENT in SAMPLE. The table STUDENT has fields ROLLNO, NAME, AGE, COURSE, GRADE. The following code shows how to connect MySQL database with Python. import pymysql # Open database connection db = pymysql.connect("localhost","user",“pass",“SAMPLE" ) # prepare a cursor object using cursor() method cursor = db.cursor() # execute SQL query using execute() method. cursor.execute("SELECT VERSION()") # Fetch a single row using fetchone() method. data = cursor.fetchone() print "Database version : “,data # disconnect from server db.close()
- 4. Once a database connection is established, we are ready to create tables or records into the database tables using execute method of the created cursor. import pymysql # Open database connection db=MySQLdb.connect("localhost","testuser","test123","TESTDB" ) # prepare a cursor object using cursor() method cursor = db.cursor() # Drop table if it already exist using execute() method. cursor.execute("DROP TABLE IF EXISTS EMPLOYEE") # Create table as per requirement sql = """CREATE TABLE EMPLOYEE ( FIRST_NAME CHAR(20) NOT NULL, LAST_NAME CHAR(20), AGE INT, SEX CHAR(1), INCOME FLOAT )""" cursor.execute(sql) # disconnect from server db.close()I
- 5. It is required when you want to create your records into a database table. import MySQLdb # Open database connection db=MySQLdb.connect("localhost","testuser","test123","TESTDB" ) # prepare a cursor object using cursor() method cursor = db.cursor() # Prepare SQL query to INSERT a record into the database. sql = """INSERT INTO EMPLOYEE(FIRST_NAME, LAST_NAME, AGE, SEX, INCOME) VALUES ('Mac', 'Mohan', 20, 'M', 2000)""" # Execute the SQL command cursor.execute(sql) # Commit your changes in the database db.commit() except: # Rollback in case there is any error db.rollback() # disconnect from server db.close()
- 6. UPDATE Operation on any database means to update one or more records, which are already available in the database. import MySQLdb # Open database connection db=MySQLdb.connect("localhost","testuser","test123","TESTDB" ) # prepare a cursor object using cursor() method cursor = db.cursor() # Prepare SQL query to UPDATE required records sql = "UPDATE EMPLOYEE SET AGE = AGE + 1 WHERE SEX = '%c'" % ('M') # Execute the SQL command cursor.execute(sql) # Commit your changes in the database db.commit() except: # Rollback in case there is any error db.rollback() # disconnect from server db.close()
- 7. DELETE operation is required when you want to delete some records from your database. import MySQLdb # Open database connection db=MySQLdb.connect("localhost","testuser","test123","TESTDB" ) # prepare a cursor object using cursor() method cursor = db.cursor() # Prepare SQL query to DELETE required records sql = "DELETE FROM EMPLOYEE WHERE AGE > '%d'" % (20) # Execute the SQL command cursor.execute(sql) # Commit your changes in the database db.commit() except: # Rollback in case there is any error db.rollback() # disconnect from server db.close()
- 8. Transactions ensure the data consistency of the database. We have to make sure that more than one applications must not modify the records while performing the database operations. The transactions have the following properties. Atomicity Either the transaction completes, or nothing happens. If a transaction contains 4 queries then all these queries must be executed, or none of them must be executed. Consistency The database must be consistent before the transaction starts and the database must also be consistent after the transaction is completed. Isolation Intermediate results of a transaction are not visible outside the current transaction. Durability Once a transaction was committed, the effects are persistent, even after a system failure.
- 9. Commit is the operation, which gives a green signal to database to finalize the changes, and after this operation, no change can be reverted back. db.commit()
- 10. If you are not satisfied with one or more of the changes and you want to revert back those changes completely, then use rollback() method. db.rollback()
- 11. To disconnect Database connection, use close() method. db.close()
- 12. There are many sources of errors. A few examples are a syntax error in an executed SQL statement, a connection failure, or calling the fetch method for an already canceled or finished statement handle. The DB API defines a number of errors that must exist in each database module. The following table lists these exceptions. Warning Used for non-fatal issues. Must subclass StandardError. Error Base class for errors. Must subclass StandardError. InterfaceError Used for errors in the database module, not the database itself. Must subclass Error. DatabaseError Used for errors in the database. Must subclass Error. DataError Subclass of DatabaseError that refers to errors in the data.
- 13. There are many types of objects which can be used with a for loop. We can use a for loop for iterating through a list, string, dictionary or a file. These are called iterable objects. The Python iterators object is initialized using the iter() method. It uses the next() method for iteration. __iter__(): The iter() method is called for the initialization of an iterator. This returns an iterator object __next__(): The next method returns the next value for the iterable. When we use a for loop to traverse any iterable object, internally it uses the iter() method to get an iterator object, which further uses the next() method to iterate over. This method raises a StopIteration to signal the end of the iteration.
- 14. Iterators can be materialized as lists or tuples by using the list() or tuple() constructor functions. The following program shows how a list can be converted to an iterator and then to materialize to a tuple. list=[1,2,3] iterator=iter(list) t=tuple(iterator) print(t) Output (1,2,3)
- 15. Often, when dealing with iterators, we also need to keep a count of iterations. Python eases the programmers’ task by providing a built-in function enumerate() for this task. The enumerate () method adds a counter to an iterable and returns it in the form of an enumerating object. This enumerated object can then be used directly for loops or converted into a list of tuples using the list() function. Syntax: enumerate(iterable, start=0) Parameters: Iterable: any object that supports iteration Start: the index value from which the counter is to be started, by default it is 0
- 16. Generators are a special class of functions that simplify the task of writing iterators. Regular functions compute a value and return it, but generators return an iterator that returns a stream of values. A generator is a special routine that can be used to control the iteration behaviour of a loop. A generator is similar to a function returning an array. A generator has parameters, it can be called and it generates a sequence of numbers. Function returns a whole array, a generator yields one value at a time. Generators require less memory Generators in Python are defined with the def keyword use the yield keyword may use several yield keywords returns an iterator When a function is called, a private namespace is allocated for the local variables and when a function reaches a return statement, the local variables are destroyed and the value is returned to the caller. But in generators local variables aren’t thrown away on exiting a function
- 17. Decorators are used to add functionality to an existing code. This is also called metaprogramming as a part of the program tries to modify another part of the program at compile time There are two types of decorators in Python Function decorators Class decorators A decorator in Python is any callable Python object that is used to modify a function or a class.
- 18. The Common Gateway Interface(CGI) is a standard way for a web server to pass a web user’s request to an application program and to receive data back to forward to the user. The Common Gateway Interface, or CGI, is a set of standards that define how information is exchanged between the web server and a custom script. To understand the concept of CGI, let us see what happens when we click a hyper link to browse a particular web page or URL. Your browser contacts the HTTP web server and demands for the URL, i.e., filename. Web Server parses the URL and looks for the filename. If it finds that file then sends it back to the browser, otherwise sends an error message indicating that you requested a wrong file. Web browser takes response from web server and displays either the received file or error message.
- 19. All HTTP headers will be in the following form: HTTP Field Name: Field Content Example Content-type: text/html
- 20. Environment variables are a series of hidden values that the web server sends to every CGI program we run. Our program can parse them and use the data they send. Environment variables are stored in a hash named %ENV:. Some of the environment variables are DOCUMENT_ROOT : The root directory of our server. HTTP_COOKIE : The visitor’s cookie, if one is set. HTTP_HOST : The hostname of the page being attempted. REMOTE_ADDR : The IP address of the visitor
- 21. A form is an area that can contain form elements. Form elements allow the user to enter information in a form Get method The GET method is the default method to pass information from browser to web server and it produces a long string that appears in your browser's Location:box. Never use GET method if you have password or other sensitive information to pass to the server. Post method A generally more reliable method of passing information to a CGI program is the POST method. This packages the information in exactly the same way as GET methods, but instead of sending it as a text string after a ? in the URL it sends it as a separate message.
- 22. Radio Buttons are used when only one option is required to be selected. Here is example HTML code for a form with two radio buttons − <form action = "/cgi-bin/radiobutton.py" method = "post" target = "_blank"> <input type = "radio" name = "subject" value = "maths" /> Maths <input type = "radio" name = "subject" value = "physics" /> Physics <input type = "submit" value = "Select Subject" /> </form>
- 23. Drop Down Box is used when we have many options available but only one or two will be selected. Here is example HTML code for a form with one drop down box − <form action = "/cgi-bin/dropdown.py" method = "post" target = "_blank"> <select name = "dropdown"> <option value = "Maths" selected>Maths</option> <option value = "Physics">Physics</option> </select> <input type = "submit" value = "Submit"/> </form>
- 24. Checkboxes are used when more than one option is required to be selected. Here is example HTML code for a form with two checkboxes − <form action = "/cgi-bin/checkbox.cgi" method = "POST" target = "_blank"> <input type = "checkbox" name = "maths" value = "on" /> Maths <input type = "checkbox" name = "physics" value = "on" /> Physics <input type = "submit" value = "Select Subject" /> </form>
- 25. TEXTAREA element is used when multiline text has to be passed to the CGI Program. Here is example HTML code for a form with a TEXTAREA box − <form action = "/cgi-bin/textarea.py" method = "post" target = "_blank"> <textarea name = "textcontent" cols = "40" rows = "4"> Type your text here... </textarea> <input type = "submit" value = "Submit" /> </form>
- 26. An HTTP cookie is a small piece of data sent from a website and stored on the user’s computer by the user’s web browser while the user is browsing. Cookies are designed to be a reliable mechanism for web sites to remember information or to record the user’s browsing activity. Cookies are a plain text data record of 5 variable-length fields − Expires − The date the cookie will expire. If this is blank, the cookie will expire when the visitor quits the browser. Domain − The domain name of your site. Path − The path to the directory or web page that sets the cookie. This may be blank if you want to retrieve the cookie from any directory or page. Secure − If this field contains the word "secure", then the cookie may only be retrieved with a secure server. If this field is blank, no such restriction exists. Name=Value − Cookies are set and retrieved in the form of key and value pairs.
- 27. To upload a file, the HTML form must have the enctype attribute set to multipart/form- data. The input tag with the file type creates a "Browse" button.