Reading_and_writing_files

Reading And Writing Files
*************************
Filepath
--------
the file path is the path in which the file is present.
for ex: a file test.py present in the home directory's python folder has a path:
home/python/test.py 
for operations done on the os, we need to import the 'os module'
import os

The path join method of os module:
----------------------------------
the join method returns a string with correct file path including the separators.
eg:
import os
print(os.path.join('home','python','test.py'))
result: home/python/test.py

creating path for a set of files in a list
------------------------------------------

import os
files=['test.py','red.pdf','disc.mp3']
for filenames in files:
   print(os.path.join('home/desktop/users/python',filenames))

result: 
home/desktop/users/python/test.py
home/desktop/users/python/red.pdf
home/desktop/users/python/disc.mp3

Current Working Directory
-------------------------
All the directories other than the root directories can become current working directories.
syntax: os.getcwd()

eg: import os
print(os.getcwd())

result: home/killbox

Change Directory
----------------
the change directory method of os module is used to chage the current working directory.
eg:
import os
os.chdir('home/user/killbox')
print(os.getcwd())

Note: You cannot change to a directory which does not exist.


Absolute And Relative Paths
----------------------------
Absolute path: it starts with root directory
relative path: it is relative to the current working directory

here dot(.) means this directory
and dot dot(..) means parent folder.

Creating New Folders with OS.MAKEDIRS() method
----------------------------------------------
make directory method is used to create new directories

eg:
import os
os.makedirs('home/killbox/santhu')
print('new directory is created')

Handling Absolute and relative paths
------------------------------------
-> convert relative path to absolue path or generate absolute path

Syntax: os.path.abspath(path)
eg:
import os
print(os.path.abspath('d.py'))
result: home/killbox/d.py

-> Check if a path is absolute or relaive

Syntax: os.path.isabs(path)
eg:
import os
if os.path.isabs(home/killbox/santhu)==True
   print('it is absolute path')
else:
   print('it is relative path')

-> Print relative path from start path to a particular path

Syntax:os.path.relpath(path,start)
eg:
import os
print(os.path.relpath('home/killbox/python','home'))
print(os.path.relpath('home/killbox/python','home/killbox/testt'))

DIRECTORY AND BASE NAME
-----------------------

Directory name: first part of the path until the last backslash---os.path.dirname(path)
base name : the last part of path after the last backslash--------os.path.basename(path)
eg: path='home/killbox/desktop/python/d.py'
print(os.path.dirname(path)  # result is :'home/killbox/desktop/python'
print(os.path.basename(path) # result is :'d.py'

OS.PATH.SPLIT()
---------------
os.path.split() method of os module splits the path into dirname and base name in the form of tuples.
eg:
import os
path='home/killbox/desktop/python/d.py'
os.path.split(path)

Result: ('home/killbox/desktop/python','d.py')

the os.path.split() method never takes a path as string and returns a list of split path instead it generates tuples.
os.path.sep variable of os module stores the path without the backslash.Splitting the os.path.sep using python's split() method produces a list of the splitted elements of the path.
eg:
import os
path='home/killbox/desktop/python/d.py'
print(path.split(os.path.sep))

result: ['home', 'killbox', 'desktop', 'python', 'd.py']

Finding File Sizes and Folder Contents
--------------------------------------

Finding filesize
----------------
File sizes can be found out using 
os.path.getsize(path)
eg:
import os
path='/home/killbox/python/pgm1.py'
print(str(os.path.getsize(path))+' '+'byte(s)')

result: 71 byte(s)

Finding Folder Contents
-----------------------
File contents can be generated by using os module's listdir() method.
syntax: os.listdir(path)
eg:
import os
path='/home/killbox/python/'
print(os.listdir(path))

Result: ['pgm1.py', 'catnames.py', 'collatzui.py', 'collatz.py', 'collatz.pyc']


Finding size of a folder
------------------------
Finding size of a folder and listing its contents

import os
path='/home/killbox/python/'
total=0
# listing the contents
listt=os.listdir(path)
print(listt)
#finding total size of contents
for files in listt:
   total=total+os.path.getsize(os.path.join(path,files))
print(str(total)+' '+'bytes')

Checking Path Validity
----------------------
three types

->exists
  ------
Syntax:os.path.exists(path)
eg:
import os
path='/home/killbox/python/pgm1.py'
print(os.path.exists(path))

Result: 'True'

->is file
  -------
Syntax:os.path.isfile(path)
eg:
import os
path='/home/killbox/python/pgm1.py'
print(os.path.isfile(path))

Result: 'True'

->is directory
  ------------
Syntax:os.path.isdir(path)
eg:
import os
path='/home/killbox/python/pgm1.py'
print(os.path.isdir(path))

Result: 'True'

NOTE:We can check whether a flash drive is mounted or not using exists() method of os module.


FILE READING AND WRITING PROCESS
--------------------------------


--> Call the open() function to return a File object.

    myfile1=open('/home/killbox/python/pgm1.py')

--> Call the read() or write() method on the File object.

    files can be read using the file object's read() method.
    eg: content=myfile1.read()
        contentbylines=myfile.readlines()
    Note: by default the file will be opened in read mode. (myfile1=open('path','r') and myfile1=open(path) are  same)
    
    WRITE() and APPEND() Methods
    ----------------------------
    write() and append() are the 2 modes in writing to a file.
    Note:opening a file in write mode or append mode according to the need
    
    Write mode: myfile1=open('path','w')
    
    Append mode: myfile1=open('path','a')
    
    The difference is, in write mode if the file is not present, then the file is created and if present, it will be overwritten with he new data witten to the file.
    So inorder to retain the previous data and add/append new data to it, append() method of the file object is used.

--> Close the file by calling the close() method on the File object.
    close() method of file object is used to close the file. once the file is closed, it cannot be accessed until the file is opened again.
     
    myfile1.close()
    

SHELVE MODULE
-------------

The Shelve module can be used to store variable data to the computer harddisk for later use in the form of shelf(dbfile)

Syntax:  import shelve
         sfile=shelve.open('datashelf')
         var={'san','dsa','nsa'}
         sfile['variable']=var
         sfile.close()

result: the shelf is created.

==>KEYS AND VALUES: 

The shelf variable can be dictionaries and they have keys and values
--> keys: shelfobj.keys()
--> values: shelfobj.values()
NOTE: shelves return list like values but not actually lists so we have to convert them to lists

list(shelfobj.keys())
list(shelfobj.values())

eg:
import shelve
sfile=shelve.open('datashelf')
var={'san':'name','dsa':'com','nsa':'work'}
sfile['variable']=var
print(list(sfile.keys()))
print(list(sfile.values()))
sfile.close()


Saving Variables with the pprint.pformat() Function
---------------------------------------------------

pprint.pformat() method returns a string which is easily readable an syntactically correct python code which can be saved to a variable or as a python file for  later use by importing the module whenever needen.

import pprint
file=[{'san':'name','19':'age'},{'athul':'name','18':'age'}]
pprint.pformat(file)
fileobj=open('xs.py','w')
fileobj.write(pprint.pformat(file))
fileobj.close()

Result: file created and data is written to the file xs.py

import xs.py
print(xs.file)