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Using lambda with filter()
The filter() function takes two arguments: function and a list
The function is called with all the items in the list and a new list is returned
which contains items for which the function evaluates to True.
Example:
Sample List: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Even List: [2, 4, 6, 8, 10]
Odd List: [1, 3, 5, 7, 9]](https://image.slidesharecdn.com/pythonunit2-231218222023-b89debaa/75/Python_Unit_2-pdf-47-2048.jpg)
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Using lambda with map()
The map() function takes two argument: function and a list
The function is called with all the items in the list and a new list is returned which
contains items returned by that function for each item.
Example:
OUTPUT:
List: [1, 2, 3, 4]
List: [1, 4, 9, 16]](https://image.slidesharecdn.com/pythonunit2-231218222023-b89debaa/75/Python_Unit_2-pdf-48-2048.jpg)
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The read() Method
The read() method reads a string from an open file. It is important to
note that Python strings can have binary data. apart from text data.
Syntax
fileObject.read([count]);
Here, passed parameter is the number of bytes to be read from the
opened file.
This method starts reading from the beginning of the file and if count is
missing, then it tries to read as much as possible, maybe until the end
of file.
110](https://image.slidesharecdn.com/pythonunit2-231218222023-b89debaa/75/Python_Unit_2-pdf-110-2048.jpg)
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Random Accessing of Binary files using mmap
print(mm.read()) #display entire file
print(mm.readline()) #display the first line of file
print(mm[5:]) #display from 5th byte till the end.
print(mm[5:10]) #display 5th byte to 9th byte.
mm[5:10] = str.encode() #store from 5th to 9th bytes.
n=mm.find(name) #return the position of name in file
mm.seek(10,0) #position the file pointer to 10th byte from](https://image.slidesharecdn.com/pythonunit2-231218222023-b89debaa/75/Python_Unit_2-pdf-141-2048.jpg)
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walk()
walk() generates the file names in a directory tree by walking the tree either top-
down or bottom-up.
Syntax
os.walk(top[, topdown=True[, onerror=None[, followlinks=False]]])
Parameters
top − Each directory rooted at directory, yields 3-tuples, i.e., (dirpath, dirnames,
filenames)
topdown − If optional argument topdown is True or not specified, directories are
scanned from top-down. If topdown is set to False, directories are scanned from
bottom-up.
onerror − This can show error to continue with the walk, or raise the exception to abort
the walk.
followlinks − This visits directories pointed to by symlinks, if set to true.](https://image.slidesharecdn.com/pythonunit2-231218222023-b89debaa/75/Python_Unit_2-pdf-158-2048.jpg)
Python_Unit_2.pdf
- 1.
- 2. BipinRupadiya.com GUJARAT TECHNOLOGICAL UNIVERSITY MASTEROF COMPUTER APPLICATIONS (MCA) SEMESTER: III Subject Name: Programming in Python Subject Code : 4639304 2
- 3. BipinRupadiya.com Unit-2 Functions, Exception, Modules and Files 3
- 4. BipinRupadiya.com Python Chapter-9 Functions Text Book: R NageswaraRao, Core Python Programming, 2nd Edition, Dreamtech Press
- 5. BipinRupadiya.com Introduction It issimilar to a program that consist of a group of statements that are intendant to perform a specific task. Two types of function: User Defined Built-In (abs(), len(), sort(), sqrt(), power())
- 6. BipinRupadiya.com Advantages of Functions: Used to process data, make calculations , perform task. Function as Reusable code Function provides Modularity Code maintenance is easy because of functions
- 7. BipinRupadiya.com Python Method: Methodis called by its name, but it is associated to an object (dependent). A method is implicitly passed the object on which it is invoked. It may or may not return any data. A method can operate on the data (instance variables) that is contained by the corresponding class Output: I am in method_abc of ABC class
- 8. BipinRupadiya.com Functions: Function isblock of code that is also called by its name. (independent) The function can have different parameters or may not have any at all. If any data (parameters) are passed, they are passed explicitly. It may or may not return any data. Function does not deal with Class and its instance concept. Output: -2 and 9
- 9. BipinRupadiya.com Difference between methodand function Simply, function and method both look similar as they perform in almost similar way, but the key difference is the concept of ‘Class and its Object‘. Functions can be called only by its name, as it is defined independently. But methods can’t be called by its name only, we need to invoke the class by a reference of that class in which it is defined, i.e. method is defined within a class and hence they are dependent on that class.
- 10. BipinRupadiya.com Defining a Function& Calling a function Output: -2 and 9 Output: -22 and 15
- 11. BipinRupadiya.com Returning a result/single valuefrom function def square(n): return n*n r= square(2) Print(r) # or print it as before print(square(2))
- 12. BipinRupadiya.com Returning multiple value fromfunction Python also has the ability to return multiple values from a function call, something missing from many other languages. In this case the return values should be a comma-separated list of values and Python then constructs a tuple and returns this to the caller. def square(x,y): return x*x, y*y t = square(2,3) print(t) # Produces (4,9)
- 13. BipinRupadiya.com Returning a multiplevalue from function An alternate syntax when dealing with multiple return values is to have Python "unwrap" the tuple into the variables directly by specifying the same number of variables on the left-hand side of the assignment as there are returned from the function. def square(x,y): return x*x, y*y xsq, ysq = square(2,3) print(xsq) # Prints 4 print(ysq) # Prints 9
- 14. BipinRupadiya.com Functions are firstclass objects Functions are first class objects, It means we can use function as perfect object. Since a functions are objects, we can pass functions to another functions just like an object. Also it is possible to return a function from another function. Following possibilities are important: 1. Possible to assign function to a variable 2. Possible to define a one function inside the another function. 3. Possible to pass a function as parameter to another function. 4. Possible that a function can return another function.
- 15. BipinRupadiya.com Possible to assignfunction to a variable:
- 16. BipinRupadiya.com Define a function insidethe another function
- 17. BipinRupadiya.com Pass a function asparameter to another function. Here new name of message() function is ‘fun’ (=reference name of message() function)
- 18. BipinRupadiya.com function can returnanother function
- 19. BipinRupadiya.com Pass by ObjectReference In function when we pass values, we can think of this two way. Pass by value or call by value Pass by reference or call by reference Pass by value represents that a copy of variable is passed to the function and modification to that value will not reflect outside of function. Pass by reference represents sending the reference or memory address of the variable to the function. Neither of this two concepts are applicable to Python.
- 20. BipinRupadiya.com Pass by ObjectReference In a python, values are sent to function by means of Object Reference. We know everything is considered as object in python. All numbers, strings, datatypes (like tuples, lists, dictionaries) are object in python. In python an object can be imagined as memory block where we can store some value. For example x=10 , here in python 10 is object and x is the name or tag given to it.
- 21. BipinRupadiya.com Pass by ObjectReference To know the location of the an object in heap, we can use id() function that gives an identity number of an object. X = 10 id(x) Output= 1154899390 This number may change computer to computer
- 22. BipinRupadiya.com Exampel-1: Pass byObject Reference A python program to pass an integer to a function and modify it. Here in function x= 15 but it will not be available outside of function when we modify its value by x=10
- 23. BipinRupadiya.com Exampel-2: Pass byObject Reference A program to modify the list to a function .
- 24. BipinRupadiya.com Exampel-3: Pass byObject Reference A python program to create a new object inside the function does not modify outside object.
- 25. BipinRupadiya.com Formal and actualargument Formal Arguments When you define function , it may have some parameters. These parameters are useful to receive values from outside of function. They are called ‘Formal Arguments’. Actual Arguments When we call the function we should pass the data or values to the function. These values are called ‘Actual Arguments’ Here, a and b formal arguments And x and y are actual arguments
- 26. BipinRupadiya.com Formal and actualargument Actual arguments used in a function are of 4 types. 1. Positional arguments 2. Keyword arguments 3. Default arguments 4. Variable length arguments
- 27. BipinRupadiya.com Positional Argument: Whenwe call a function with some values, these values get assigned to the arguments according to their position. These are the arguments passed to a function in correct positional order.
- 28. BipinRupadiya.com Keyword argument: Whenwe call a function with some values, these values get assigned to the arguments according to their position. Python allows functions to be called using keyword arguments. When we call functions in this way, the order (position) of the arguments can be changed. This kind of argument identify the parameter by their name
- 29. BipinRupadiya.com Default Argument: Functioncan have default argument using = operator It can assign from right to left only. Any number of arguments in a function can have a default value. But once we have a default argument, all the arguments to its right must also have default values. Default argument is optional during a call. If a value is provided, it will overwrite the default value. Example : def fun(a, b=8, c=90) Here b=8, c=90 is set default.
- 30. BipinRupadiya.com Variable length argument: InPython, we can pass a variable number of arguments to a function using special symbols. There are two special symbols: *args (Non Keyword Arguments) **kwargs (Keyword Arguments) 30
- 31. BipinRupadiya.com Non Keyword Variablelength argument: When the programmer does not know how many argument a function may receive, at that time variable length concept is used. we should use an asterisk * before the parameter name to pass variable length arguments. The arguments are passed as a tuple and these passed arguments make tuple inside the function with same name as the parameter excluding asterisk *. Output <class 'tuple'> (1, 2, 3)
- 32. BipinRupadiya.com Keyword Variable lengthargument: Use double asterisk ** before the parameter name to denote this keyword variable length of argument. The arguments are passed as a dictionary and these arguments make a dictionary inside function with name same as the parameter excluding double asterisk **. Output <class 'dict'> {'a': 11, 'b': 33, 'c': 22}
- 33. BipinRupadiya.com Local Variables Alocal variable is a variable whose scope is limited only to that function where it is created.
- 34. BipinRupadiya.com Global Variables Whena variable is declared above a function, it becomes global variable.
- 35. BipinRupadiya.com The Global Keyword Sometimes global and local variable have same name. In that case, function by default refer to the local variable and ignores the global variable. So the global variable is not accessible inside the function but outside of it, it is accessible.
- 36. BipinRupadiya.com The Global Keyword if we wants to use global variable inside the function, Use the ‘global’ keyword before the variable in the beginning of function body as, global a;
- 37.
- 38. BipinRupadiya.com Passing a groupof Element to a function To pass a group of element like numbers or string, we can accept them into a list and then pass the list to the function . Example:
- 39. BipinRupadiya.com Passing a groupof Element to a function
- 40. BipinRupadiya.com Recursive Function Afunction that calls itself is known as ‘recursive function’. Eg:-factorial
- 41. BipinRupadiya.com Recursive Function: Towerof Hanoi Tower of Hanoi is a mathematical puzzle where we have three rods and n disks. The objective of the puzzle is to move the entire stack to another rod, obeying the following simple rules: 1) Only one disk can be moved at a time. 2) Each move consists of taking the upper disk from one of the stacks and placing it on top of another stack i.e. a disk can only be moved if it is the uppermost disk on a stack. 3) No disk may be placed on top of a smaller disk.
- 42.
- 43. BipinRupadiya.com Tower of HanoiExample OUTPUT: moving disk from A to C moving disk from A to B moving disk from C to B moving disk from A to C moving disk from B to A moving disk from B to C moving disk from A to C
- 44. BipinRupadiya.com Anonymous/Lambda Function Whatare lambda functions in Python? anonymous function is a function that is defined without a name. While normal functions are defined using the def keyword anonymous functions are defined using the lambda keyword. Hence, anonymous functions are also called lambda functions.
- 45. BipinRupadiya.com Anonymous/Lambda Function Howto use lambda Functions in Python? Syntax: lambda arguments: expression OUTPUT 30
- 46. BipinRupadiya.com Use of LambdaFunction We use lambda functions when we require a nameless function for a short period of time. we generally use it as an argument to a higher-order function Higher-order function is a function that takes in other functions as arguments Lambda functions are used along with built-in functions like filter(), map() etc.
- 47. BipinRupadiya.com Using lambda withfilter() The filter() function takes two arguments: function and a list The function is called with all the items in the list and a new list is returned which contains items for which the function evaluates to True. Example: Sample List: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] Even List: [2, 4, 6, 8, 10] Odd List: [1, 3, 5, 7, 9]
- 48. BipinRupadiya.com Using lambda withmap() The map() function takes two argument: function and a list The function is called with all the items in the list and a new list is returned which contains items returned by that function for each item. Example: OUTPUT: List: [1, 2, 3, 4] List: [1, 4, 9, 16]
- 49. BipinRupadiya.com lambda with reduce() The reduce() function takes two arguments: function and a list The function is called with a lambda function and a list and a new reduced result is returned. This performs a repetitive operation over the pairs of the list. This is a part of functools (function tools) module. Example: OUTPUT: sum : 45
- 50. BipinRupadiya.com Function Decorators Adecorator is a function that takes a function as its only parameter and returns a function. This is helpful to “wrap” functionality with the same code over and over again.
- 51. BipinRupadiya.com Function Decorators NestedFunction Working shown in example:
- 52.
- 53. BipinRupadiya.com Generators A generator-functionis defined like a normal function, but whenever it needs to generate a value, it does so with the yield keyword rather than return. If the body of a def contains yield, the function automatically becomes a generator function.
- 54. BipinRupadiya.com Generators Generators areused to create iterators, but with a different approach. Generators are simple functions which return an iterable set of items, one at a time, in a special way. When an iteration over a set of item starts using the for statement, the generator is run. Once the generator's function code reaches a "yield" statement, the generator yields its execution back to the for loop, returning a new value from the set. The generator function can generate as many values (possibly infinite) as it wants, yielding each one in its turn.
- 55.
- 56. BipinRupadiya.com Structured Programming Thepurpose of programming is to solve problems related to various areas. Starting problems like adding two numbers to complex problem like designing engine of air craft we can solve this problem through programming. Solving a complex problem , structured programming is the strategy used by the programmers. In structured programming, the main task is divided into several parts called Sub tasks and each this task is represented by one or more functions.
- 57.
- 58. BipinRupadiya.com Structured Programming Letstake an example of salary of an employee.(diagram)
- 59. BipinRupadiya.com Creating our ownmodule in python A module represents a group of classes, methods , functions and variables. While we are developing software, there may be several classes, methods and functions. We should first group them on depending on their relationship into various modules and later use these module in the other programs. It means, when a module is developed, it can be reused in any program that needs the module. In python , there is several built-in modules like sys, io , time etc. Just like this , we can create our own module too, and use them whenever we need them.
- 60. BipinRupadiya.com Creating our ownmodule in python
- 61. BipinRupadiya.com Creating our ownmodule in python
- 62. BipinRupadiya.com The Special Variable_name_ When a program is executed in python, there is a special variable internally created by the name ‘_name_’ This variable stores information regarding weather the program is executed as an individual program or as a module. When the program is executed directly, the python interpreter stores the value ‘_main_’ into this variable. When the program is imported as another program, then python interpreter stores the module name into this variable. So by observing _name_ we can understand how program is executed.
- 63. BipinRupadiya.com The Special Variable_name_ Lets assume that we have written a program. When this program is run, python interpreter stores the value ‘__main__’ into the special “__name__” Hence we can check if this program run directly as a program or not as: If __name__ == ‘__main__’:
- 64.
- 65. BipinRupadiya.com Python Chapter-16 Exception Text Book: R NageswaraRao, Core Python Programming, 2nd Edition, Dreamtech Press
- 66. BipinRupadiya.com Introduction The errorsin software are called bugs. And process of removing them is called debugging.
- 67. BipinRupadiya.com Errors in Python There are three types of error. 1. Compile time errors 2. Run time errors 3. Logical errors
- 68. BipinRupadiya.com Compile time errors: These are syntactical errors found in the code, due to which program fails to compile. For example like …….forgetting a colon in statement like if, while, for , def….etc will result in compile time error. This will raise Syntax Error.
- 69.
- 70. BipinRupadiya.com Run Time Error: when PVM cant execute byte code, it flags runtime error. Runtime errors are not detected by python its detected by PVM at runtime.
- 71. BipinRupadiya.com Solution… If correctargument are passes then it work properly
- 72. BipinRupadiya.com Logical error: Theseerrors are either because of wrong formula or wrong design of program itself. Logical errors are not detected by python compiler or PVM
- 73. BipinRupadiya.com Exception It isa run time error which can be handled by the programmer. That means if a programmer can guess an error in the program and he can do something to eliminate the harm caused by that error then it is called an exception. If the programmer can’t do anything in case of an error, then it is called an error not an exception. All the exception are represented as classes in python.
- 74. BipinRupadiya.com Exception Exceptions whichare already available in python are called as ‘built in’ exception. Exceptions which are user defined are developed by Programmer. Base class for built in exception is ‘BaseException’ class From BaseException class Subclass Exception has been derived. And from Exception Subclass ‘StandardError’ and ‘Warning’ are derived. Here StandardError Must be handled through exception but warning is fine in program.
- 75.
- 76. BipinRupadiya.com Exception Handling Thepurpose of handling error is to make the program Robust. To handle the exception, programmer should perform the following three steps: 1. try 2. except 3. finally
- 77. BipinRupadiya.com try block 1. Observethe statements in program and find out the possibilities where exception is needed. Such a statements must be write inside a try block. The greatness of try block is that even if exception arise inside the block ,then also program will not be terminated. try: Statements.
- 78. BipinRupadiya.com except block 2. Write‘except’ block where he should display the exception detail to the user. This helps to understand the user that there is an error. except exceptionname: Statements The statements inside an except block are called handlers. Since they handle the situations when the exception error occurs.
- 79. BipinRupadiya.com Finally block 3. Theprogrammer should perform clean up action like closing the files and terminating any other process which are running. And the program should write a finally block. finally: statements
- 80.
- 81. BipinRupadiya.com Exception Handling trycontains the statement where there may be one or more exceptions. except block execute when an exception occur in try block else block execute when there is no exception will execute finally block will always is executed. try: statements except Exception1: statements except Exception2: statements except Exception3: statements else: statements finally: statements
- 82. BipinRupadiya.com Exception Handling KeyPoints A single try can be followed by several except blocks. Multiple except blocks can be used to handle multiple exception We cant write except block without try block We can write try blocks without any except block Else block and Finally block are not compulsory. When there is no exception, else block is executed after try block. Finally block is always executed.
- 83. BipinRupadiya.com Types of Exceptions Exception Baseclass for all exceptions StopIteration Raised when the next() method of an iterator does not point to any object. SystemExit Raised by the sys.exit() function. StandardError Base class for all built-in exceptions except StopIteration and SystemExit. ArithmeticError Base class for all errors that occur for numeric calculation. OverflowError Raised when a calculation exceeds maximum limit for a numeric type. FloatingPointError Raised when a floating point calculation fails. ZeroDivisonError Raised when division or modulo by zero takes place for all numeric types. AssertionError Raised in case of failure of the Assert statement. AttributeError Raised in case of failure of attribute reference or assignment. EOFError Raised when there is no input from either the raw_input() or input() function and the end of file is reached. ImportError Raised when an import statement fails. 83
- 84. BipinRupadiya.com KeyboardInterrupt Raised when theuser interrupts program execution, usually by pressing Ctrl+c. LookupError Base class for all lookup errors. IndexError Raised when an index is not found in a sequence. KeyError Raised when the specified key is not found in the dictionary. NameError Raised when an identifier is not found in the local or global namespace. UnboundLocalError Raised when trying to access a local variable in a function or method but no value has been assigned to it. EnvironmentError Base class for all exceptions that occur outside the Python environment. IOError Raised when an input/ output operation fails, such as the print statement or the open() function when trying to open a file that does not exist. OSError Raised for operating system-related errors. 84 Types of Exceptions
- 85. BipinRupadiya.com SyntaxError Raised when thereis an error in Python syntax. IndentationError Raised when indentation is not specified properly. SystemError Raised when the interpreter finds an internal problem, but when this error is encountered the Python interpreter does not exit. SystemExit Raised when Python interpreter is quit by using the sys.exit() function. If not handled in the code, causes the interpreter to exit. TypeError Raised when an operation or function is attempted that is invalid for the specified data type. ValueError Raised when the built-in function for a data type has the valid type of arguments, but the arguments have invalid values specified. RuntimeError Raised when a generated error does not fall into any category. NotImplementedError Raised when an abstract method that needs to be implemented in an inherited class is not actually implemented. 85 Types of Exceptions
- 86. BipinRupadiya.com Types of Exceptions OUTPUT: EnterData : 1w invalid Data
- 87. BipinRupadiya.com Handle IOError Apython program to handle IOError produced by open() function.
- 88.
- 89. BipinRupadiya.com Except Block exceptblock is used to catch exception that is raised in try block. It is written in various format 1. we can write except block with the exception class name as: except Exceptionclass: 2. We can catch the exception as an object that contain some information about the catch exception. except Exceptionclass as obj; 3. To catch a multiple exception, we can write multiple catch block. you can also write all the exception as tuple inside parentheses as : except (Exceptionclass1,Exceptionclass2,….): 4. When we are not bother about which type of exception it is , to catch this type of exception we need below syntax: except:
- 90. BipinRupadiya.com Use of finallyand try blocks
- 91. BipinRupadiya.com The Assert Statement It is useful to ensure that a given condition is True. If it is not true, it raise AssertionError. Syntax assert condition, message If the condition is false, then AssertionError except is raised along with the message. You can raise AssertionError exception without message too.
- 92.
- 93.
- 94. BipinRupadiya.com User Defined Exception Python also allows you to create your own exceptions by deriving classes from the standard built-in exceptions. It is also called custom exception To create the user defined exception , user needs to follow below steps: 1. All exceptions are class, so programmer needs to create his own exception as class. Also he has to make his class as subclass to built-in class Exception. class MyException(Exception): def __init__(self,arg): self.msg = arg Here this class MyException has a constructor where a variable ‘msg’ is defined. The ‘msg receives a message passed from outside through ‘arg’.
- 95. BipinRupadiya.com User Defined Exception 2.The programmer can write his codes, when a programmer suspects the possibilities of exception at that time , he should raise an exception. raise MyException(‘message’) 3. Try and catch block is shown as below: try: code except MyException as me: print(me)
- 96. BipinRupadiya.com User Defined Exception OUTPUT: ANew Exception occurred: 6
- 97. BipinRupadiya.com Logging and Exceptions It is a good idea to store all the error message to in some file. So a log file is used to store error and exception messages. When we store the messages into log file ,we can print or read later that error. This helps to understand programmer , that how many errors are there. Python provides logging module to create a log file, that can store all the error messages. There may be different levels of error messages. For example, warning errors have less priority than error which crashes system So depending on seriousness, they are classified into 6 levels in logging module.
- 98. BipinRupadiya.com Logging Error andtheir Numeric value Level Numeric constant When it’s used NOTSET 0 Represent that level is not set DEBUG 10 Detailed information, typically of interest only when diagnosing problems. INFO 20 Confirmation that things are working as expected. WARNING 30 An indication that something unexpected happened, or indicative of some problem in the near future (e.g. ‘disk space low’). The software is still working as expected. ERROR 40 Due to a more serious problem, the software has not been able to perform some function. CRITICAL 50 A serious error, indicating that the program itself may be unable to continue running. 98
- 99. BipinRupadiya.com Logging and Exceptions To understand the levels of logging messages, see the code below:
- 100. BipinRupadiya.com Logging Exceptions Programto store the message released by an exception into the file
- 101.
- 102.
- 103. BipinRupadiya.com Python Chapter-17 Modules and Files TextBook: R Nageswara Rao, Core Python Programming, 2nd Edition, Dreamtech Press
- 104. BipinRupadiya.com Introduction Data isvery important in our day to day life. Every organization is depend on its data for continuing its business. If the data is lost , the organization has to be closed. Computers are primarily created for data handling, for storing and retrieving a data. To store a data in computer, we need file. a file is nothing but the collection of data that is available to a computer. For example we can store employee data like name, number , salary etc in file.
- 105. BipinRupadiya.com Types of filesin python In python there are two types of files. 1. Text files 2. Binary files Text file stores data in the form of characters... Example: If we store 'Ganesh' then will be stored as 6 characters. If we store Employee salary 8900.75 will be store as 7 characters Binary file stores entire data in the form of bytes Example: character is stored as a byte and integer is stored in the form of 8 bytes. Binary file can be used to store text, images, audio and video.
- 106. BipinRupadiya.com Open a file Syntax: file handler =open ("file_name", "open_mode", "buffering") Example: f =open("myfile.txt","r") Parameter: File_name= name of file Open_mode= mode of open Buffering= temporary block of memory, It is an optional integer used to set the size of buffer. In binary mode, we can pass 0 to indicate no use of buffer In text mode , use 1 for buffering to retrieve the data from file one line at a time.
- 107. BipinRupadiya.com access modes foropen() READ r: Opens the file in read-only mode. Starts reading from the beginning of the file and is the default mode for the open() function. rb: Opens the file as read-only in binary format and starts reading from the beginning of the file. While binary format can be used for different purposes, it is usually used when dealing with things like images, videos, etc. r+: Opens a file for reading and writing, placing the pointer at the beginning of the file. WRITE w: Opens in write-only mode. The pointer is placed at the beginning of the file and this will overwrite any existing file with the same name. It will create a new file if one with the same name doesn't exist. wb: Opens a write-only file in binary mode. w+: Opens a file for writing and reading. wb+: Opens a file for writing and reading in binary mode. APPEND a: Opens a file for appending new information to it. The pointer is placed at the end of the file. A new file is created if one with the same name doesn't exist. ab: Opens a file for appending in binary mode. a+: Opens a file for both appending and reading. ab+: Opens a file for both appending and reading in binary mode. 107
- 108. BipinRupadiya.com Closing a File A file which is opened should be closed using close() method. The data file may be corrupted or deleted if you don't close file. If you don't close the file, memory utilized by the file is not freed, leading to the problem like insufficient memory. So it is mandatory to close the file. Example f.close() Here file object f is closed, means f is deleted/ freed from the memory.
- 109.
- 110. BipinRupadiya.com The read() Method The read() method reads a string from an open file. It is important to note that Python strings can have binary data. apart from text data. Syntax fileObject.read([count]); Here, passed parameter is the number of bytes to be read from the opened file. This method starts reading from the beginning of the file and if count is missing, then it tries to read as much as possible, maybe until the end of file. 110
- 111.
- 112. BipinRupadiya.com Store a groupof strings into a text file we have to use the write() method inside a loop.
- 113. BipinRupadiya.com Read all stringsfrom a text file
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- 115. BipinRupadiya.com Knowing weather afile exists or not The operating system (os) module has a sub module by the name 'path' that contains a method is file() This method is used to know , weather the file we are opening is really exists or not.
- 116. BipinRupadiya.com Knowing weather afile exists or not
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- 118. BipinRupadiya.com Working with thebinary files Binary files handle data in the form of bytes. They can be used to read or write text, images, audio and video files. To open a binary file for reading purpose, we can use 'rb' mode. And for writing purpose use 'wb' mode b is attached to 'r ' to show that is binary file. Same for 'wb'.
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- 120. BipinRupadiya.com The With statement The with statement can be used while opening a file. The advantage of with statement is that it will take care of closing a file which is opened by it. So we don't need to close the file explicitly. In case of an exception also, 'with' statement will close the file before the exception is handled. Syntax with open("filename","openmode") as fileobject:
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- 123. BipinRupadiya.com Pickel in python So far we have work with simple file, but what happens if we want to store some structured data in the file? For example, we want to store some employee detail like employee identification (int), name(string), salary(float) in file. This data is called structured and got different type. To store such a data we need to create class Employee with the instance variable I'd, name, sal,..
- 124. BipinRupadiya.com Pickel in python Then we create an object to this class and store actual data into that object. Later this object should be stored into a binary file in the form of bytes. This is called pickle or serialization. So Pickle is the process of converting a class objects into a byte stream so that it can be stored into a file.----this is called object serialization.
- 125. BipinRupadiya.com Pickel in python Pickling is done using the dump() method of ‘pickle’ module: pickle.dump(object,file) This statement will store the object into the binary file. Once the object are stored into a file, we can read from the file at any time.
- 126. BipinRupadiya.com Pickel in python Unpickle is the process whereby a byte stream is converted back into a class object. It means unpickle represents reading the class objects from the file. Unpickling is also called de-serialization. Unpickling is done using the load() method of ‘pickle’ module: object= pickle.load(file)
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- 131. BipinRupadiya.com The seek() andthe tell() method In the binary files data is stored in the binary form, so when we conduct the reading or writing operation of binary file, a file pointer moves inside the file depending on how many bytes are written or read from the file.
- 132. BipinRupadiya.com tell() method Toknow the position of the file pointer, we can use the tell() method. It returns the current position of the file pointer from the beginning of the file. Syntax: n= f.tell() where f=handler/file object n=integer which represents byte.
- 133. BipinRupadiya.com seek() Now ifwe want to move a file pointer to another position, we can use the seek() method Syntax: f.seek(offset, fromwhere) offset 0 means beginning of the file 1 means current position of the file pointer 2 means end of the file f.seek(10) same as f.seek(10,0) here pointer will go from 0 to 11th (10+1) byte position form the beginning of the file. f.seek(-10,2) here pointer will go from ending of the file to 9th byte(- 10+1). Here 2 represents the ending of the file.
- 134. BipinRupadiya.com Example: seek() TheString’s characters and their bytes positions in file f.seek(3) : print(f.read(2)) : ans=zi it will read to byte from the current pointer position which is ‘z’ so ans will be ‘zi’ print(f.tell()) : ans=5 print(f.seek(4,1)) : ans= ‘y’ it will move pointer from current position, so it will point to ‘y’ character. print(f.read(1)): ans= y print(f.tell()) : ans= 10
- 135. BipinRupadiya.com Random Accessing ofBinary files Data in binary files is stored in the form of continues bytes. Lets take binary file having 1000 bytes of data. If we want to access the last 10 bytes of data, it is not needed to search the file bytes by byte from the beginning. It is possible to go to the 991st byte using seek() method only. f.seek(900) Then read the last 10 bytes using read() method, read(10) In this way, we can directly go to any byte in the binary file is called random accessing.
- 136. BipinRupadiya.com Strings in Binaryfiles A problem with binary file is that , they accept data in the form of bytes or binary format. For example, if we store a string into a binary file, as we shown in the following statement, we will end up in the error.
- 137. BipinRupadiya.com Strings in Binaryfiles Converting a string literal into the binary format can be done by prefixing the characteristic ‘b’ before the string, as: b’Hello’ as: b’Dear’ On the other hand, to convert a string variable into binary format, we have to use encode() method as: str.encode() similar, to read a string from binary format to readable text, we have to use decode() method as: str.decode()
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- 140. BipinRupadiya.com Random Accessing ofBinary files using mmap mmap-memory mapped file mmap is a module in python It is useful to map or link to a binary file and manipulate the data of the file as well just like string mm=mmap.mmap (f.fileno(),0) This will map the currently opened file (f) with the object ‘mm’ f.fileno() indicates that fileno() is a handle to the file object ‘f’. This ‘f’ represents the actual binary file that is being mapped. The second argument is 0 which represents the total size of file should be considered for mapping. So entire file represented by ‘f’ is mapped in memory to object ‘mm’. So now mm will now onwards behave like file
- 141. BipinRupadiya.com Random Accessing ofBinary files using mmap print(mm.read()) #display entire file print(mm.readline()) #display the first line of file print(mm[5:]) #display from 5th byte till the end. print(mm[5:10]) #display 5th byte to 9th byte. mm[5:10] = str.encode() #store from 5th to 9th bytes. n=mm.find(name) #return the position of name in file mm.seek(10,0) #position the file pointer to 10th byte from
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- 143. BipinRupadiya.com mmap example Python codeto do random access In binary file using mmap() We will do Display Search and Update
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- 145. BipinRupadiya.com What is azip file? ZIP is an archive file format that supports lossless data compression. By lossless compression, we mean that the compression algorithm allows the original data to be perfectly reconstructed from the compressed data. So, a ZIP file is a single file containing one or more compressed files, offering an ideal way to make large files smaller and keep related files together. Why do we need zip files? To reduce storage requirements. To improve transfer speed over standard connections. 145
- 146. BipinRupadiya.com Zipping and unzippingfiles In zipping the file content, following two things could happen: The file contents are compressed and hence the size will be reduce. The format of data will be changed making it unreadable. While zipping a file content , a zipping algorithm(logic) is used in such a way that the algorithm first find out which bit pattern is most often repeated in the original file and replaces that bit pattern with a 0. Then the algorithm searches for next bit pattern which is most often repeated in input file. In this place 1 is substituted. Third repeated bit pattern will be replaced by 10, the fourth by 11 , fifth by 100 and so on. In this way, the original bit patterns are replaced by lesser numbers of bits. The file with lesser number of bits is called ‘zipped file’ or ‘compressed file’.
- 147. BipinRupadiya.com Zipping and unzippingfiles To get back the original data from the zipped file, we can follow a reverse algorithm, which substitute the original bit pattern wherever particular bits are found. Fig.17.4 1010 1010 1111 0010 0101 1110 1111 0001 0 1 10 11 zipping unzipping Original file compressed file
- 148. BipinRupadiya.com Zipping and unzippingfiles In a python, zipfile contains ZipFile class that help us to zip or unzip a file content. For example to zip the file we should first pass the zip file name in write mode with an attribute ZIP_DEFLATED to the zipfile calss object as: f = zipfile(‘test.zip’, ‘w’, ZIP_DEFLATED) Here ‘f’ is the zipfile class object to which test.zip file name is passed. Next step is to add the filenames that are to be zipped, using write() method as: f.write(‘file1.txt’) f.write(‘file2.txt’)
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- 151. BipinRupadiya.com Working with theDirectories The os module represents operating system dependent functionality. The module is useful to perform some operation on directories.
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- 158. BipinRupadiya.com walk() walk() generates thefile names in a directory tree by walking the tree either top- down or bottom-up. Syntax os.walk(top[, topdown=True[, onerror=None[, followlinks=False]]]) Parameters top − Each directory rooted at directory, yields 3-tuples, i.e., (dirpath, dirnames, filenames) topdown − If optional argument topdown is True or not specified, directories are scanned from top-down. If topdown is set to False, directories are scanned from bottom-up. onerror − This can show error to continue with the walk, or raise the exception to abort the walk. followlinks − This visits directories pointed to by symlinks, if set to true.
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- 160. BipinRupadiya.com Running other programsfrom python program The ‘os’ module has the system() method that is useful to run an executable program from our python program. This is similar to system() function of C language. Note: * wild card character means ‘all’
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- 162. BipinRupadiya.com Bipin S. Rupadiya (MCA,PGDCA, BCA) Assistant Professor, JVIMS-MCA (537), Jamnagar www.BipinRupadiya.com 162