Module 7
File Handling
7.2 File Handling Basic
Python provides basic functions and methods to manipulate files. Most of the manipulation is based on the file object.
Open Function
The open function is used to open a file. It returns a file object.
file_object = open(file_name, access_mode)
Here
- file_name is a string value that specifies the name of the file to be opened
-
access_mode determines the mode in which the file has to be opened - read, write, append
file = open(‘some_file.txt’, r)
You can also open a file using with keyword.
with open('some_file.txt') as f:
statement/s
It is good practice to use the with keyword when dealing with file objects. The advantage is that the file is properly closed after its suite finishes, even if an exception is raised at some point.
Access modes
| Access Mode | Description |
|---|---|
| r | Opens a file for reading only. The file pointer is placed at the beginning of the file. |
| rb | Opens a file for reading only in binary format. |
| r+ | Opens a file for both reading and writing. |
| rb+ | Opens a file for both reading and writing in binary format. |
| w | Opens a file for writing only. Overwrites the file if the file exists. If the file does not exist, creates a new file for writing. |
| wb | Opens a file for writing only in binary format. |
| w+ | Opens a file for both writing and reading. |
| wb+ | Opens a file for both writing and reading in binary format. |
| a | Opens a file for appending. The file pointer is at the end of the file if the file exists. That is, the file is in the append mode. If the file does not exist, it creates a new file for writing. |
| ab | Opens a file for appending in binary format. |
| a+ | Opens a file for both appending and reading. |
| ab+ | Opens a file for both appending and reading in binary format. |
Close method
The file object has a close method which flushes any unwritten information and closes the file object. After the file is closed, no more writing can be done.
Syntax
file.close()
Example
file = open('some_file.txt', r)
file.close()
Write method
The write() method writes some string to an open file. It doesn’t add a new line character to the end of the string.
Syntax
file.write(string)
Example
file = open('file.txt', w)
file.write('File handling is so easy in Python.')
file.close()
Read method
The read() method reads a string from an open file.
Syntax
file.read(count)
The read() method reads a string from an open file.
Here, count specifies the number of bytes to read starting from the beginning of the file.
Example
file = open('file.txt', r)
str = file.read(10)
print (str)
file.close()
If count is not specified, it reads until the end of file.
Example
with open('file.txt') as f:
str = f.read()
print(str)
Readline method
The readline() method reads a single line from the file and a newline character \n is left at the end of the string. It is only omitted on the last line of the file if the file doesn’t end in a newline.
You can also use the readlines() method which reads all the lines in the file and returns a list with the lines as members.
Syntax
file.readline()
Example
with open('file.txt') as f:
f.readline() # reads only the first line
f.readlines() # reads only the lines and returns a list
For reading lines from a file, you can loop over the file object. This is memory efficient, fast, and leads to simple code:
Example
for line in f:
print (line, end =' ')
File Positions
The tell() method returns an integer value giving the current position within the file.
Syntax
file.tell()
The seek() method changes the current file position.
Syntax
file.seek(offset, from)
Her
- offset indicates the number of bytes to be moved
- from specifies the reference position from where the bytes are to be moved
If from = 0 then it means the beginning of the file, if from = 1 then it means the current position and if from = 2 that means end of the file.
You can rename or delete file using Python’s os module.
Rename method
The rename() method renames a file.
Syntax
import os
os.rename('old_file_name.txt','new_file_name.txt')
Remove method
The remove() method deletes a file.
Syntax
import os
os.remove('some_file.txt')
Python Object Serialization
Serialization is the process whereby a Python object hierarchy is converted into a byte stream, and desialization is the inverse operation, whereby a byte stream (from a binary file or bytes-like object) is converted back into an object hierarchy.
Serialization is also known as pickling (and unpickling).
The pickle module implements binary protocols for serializing and de-serializing a Python object structure.
import pickle
The pickle module provides the following functions:
Dump Function
The dump() function writes a pickled representation of object to the open file object file.
Syntax
pickle.dump(obj, file)
Example
import pickle
list = ['foo','bar']
file = open('my_list.p', wb)
pickle.dump(list, file)
file.close()
Load Function
The load() function reads a pickled object representation from the open file object file and return the reconstituted object hierarchy specified therein.
Syntax
pickle.load(file)
Example
import pickle
file = open('my_list.p', wb)
list = pickle.load(file)
The following types can be pickled:
- None, True, and False
- integers, floating point numbers, complex numbers
- strings, bytes, bytearrays
- tuples, lists, sets, and dictionaries containing only picklable objects
- functions defined at the top level of a module
- built-in functions defined at the top level of a module
- classes that are defined at the top level of a module