Python - Lecture 8

Python - Lecture 8

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  Python Lecture 8Lecture 8 -Ravi Kiran Khareedi
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  Files • Opening afile • Python has a built-in open() function, which takes a filename as an argument. • Filename argument can just denote a file• Filename argument can just denote a file name or the path of the file. • The path can be absolute or relative. • In python, / is used to specify the path • Ex: afile = open(‘test.txt’)
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  Absolute vs RelativePath • Absolute Path: – Always starts from the root path, in windows from the drive letter. • Relative Path:• Relative Path: – It’s the path with respect to the current dirctory. TASK: Try to specify the file path in windows style (using )
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  Stream Objects • Theopen() function returns a stream object, which has methods and attributes for getting information about and manipulating a stream of characters.of characters. afile = open(‘test.txt’) print afile.name print afile.mode • mode is ‘r’ by default
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  Files • Reading afile • File is read by calling a read() method of the stream object • The result is a string • TASK df = open(‘test.txt') print df.read() print "Read Again" print df.read()
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  Files - Reading •Reading a file twice don’t give any exception but simply returns a empty string. • So how to re-read a file? Let’s see: – afile.read() afile.seek(0) afile.read()afile.read() • seek() method moves the control to a specific byte position • read() method takes optional parameters to read the specific number of characters • tell() method of the stream object is used to know thw current position of the pointer(in simple words, the cursor)
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  Files Reading • seek(offset[,from]) • The offset argument indicates the number of bytes to be moved. • The from argument specifies the reference position from where the bytes are to be moved. – If from is set to 0, it means use the beginning of the file as the reference– If from is set to 0, it means use the beginning of the file as the reference position and 1 means use the current position as the reference position and if it is set to 2 then the end of the file would be taken as the reference position.
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  Files • Note: • Theseek() and tell() methods always count bytes, but since you opened this file as text, the read() method counts characters • (In case of English characters, both are same) • TASK df = open(‘test.txt') print df.read() Now try to move the file to some other folder. Lets see what happens
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  Files - Close •Closing a File • Open file consume some resources, depending on the file mode, Its important to close the file once its used. • afile.close() • Attribute closed will return if a file is closed or not.• Attribute closed will return if a file is closed or not. • afile.closed return true or false. • close() just closes a file but not destroy the afile object.
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  File - Close •You can’t read from a closed file; that raises an IOError exception. • You can’t seek in a closed file either. • There’s no current position in a closed file, so the tell() method also fails.method also fails. • Perhaps surprisingly, calling the close() method on a stream object whose file has been closed does not • raise an exception. It’s just a no-op. • Closed stream objects do have one useful attribute: the closed attribute will confirm that the file is closed.
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  File – AutomaticClose • To make sure the file is closed even if the program is crashed by using try(), finally block. • (Lets see what is it in later stages) • But python 2.6 has a better solution: with open('examples/chinese.txt', encoding='utf-8') as a_file: a_file.seek(17) a_character = a_file.read(1)a_character = a_file.read(1) print(a_character) • This code calls open(), but it never calls a file.close(). The with statement starts a code block, like an if statement or a for loop. • Inside this code block, you can use the variable a file as the stream object returned from the call to open(). All the regular stream object methods are available — seek(), read() etc • When the with block ends, Python calls a_file.close() automatically.
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  Reading a Lineat a time • for a_line in a_file: • Reads one line from a_file • TASK – GENERAL Write a program to generate first 10 numbers of the fibonacci• Write a program to generate first 10 numbers of the fibonacci series • Ex: 0,1,1,2,3,5 …..
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  File - Writing •For writing to a file, open the file in write mode. • There are 2 modes for writing: – Write mode - mode = ‘w’ as parameter in open() • Overwrite the data from the beginning of the file (Previous content is lost) – Append mode - mode = ‘a’ as parameter in open() • Adds data to the end of the file Both creates a file automatically if it doesn’t existBoth creates a file automatically if it doesn’t exist Ex: 1. df = open(‘test.txt’, mode = ‘w’) df.write(‘python’) df.close() 2. df = open(‘test.txt’, mode = ‘w’) df.write(‘python’) df.close() By default, the mode is ‘r’ (read) hence its not specific while reading.
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  Binary Files • Fewfiles has to be opened in binary mode. • Ex: Image Files • an_image = open('examples/beauregard.jpg', mode='rb’) • We just need to mention the mode as b along with w or r to• We just need to mention the mode as b along with w or r to specify it’s a binary file.
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  Renaming Files • Pythonos module provides methods that help you perform file-processing operations, such as renaming and deleting files. • The rename() Method:• The rename() Method: • os.rename(current_file_name, new_file_name) • Ex: import os # Rename a file from test1.txt to test2.txt os.rename( "test1.txt", "test2.txt" )
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  Deleting Files • Theremove() Method: • os.remove(file_name) • Syntax: import osimport os # Delete file test2.txt os.remove("text2.txt")
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  Python Directories • Pythoncan handle directory option using the os module • mkdir() • Creates directories in the current directory.• Creates directories in the current directory. • Syntax: • os.mkdir("newdir")
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  Python directories • Thegetcwd() Method: • getcwd() • The getcwd() method displays the current working directory. • os.getcwd() • The rmdir() Method: • os.rmdir('dirname') • The rmdir() method deletes the directory, which is passed as an argument in the method. • Before removing a directory, all the contents in it should be removed.
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  TASK - GENERAL •Write a program to check if a given string is palidrome • Ex: MADAM is a palindrome
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  References • Dive intoPython • http://www.tutorialspoint.com/python/pytho n_files_io.htm