File is a named location on disk to store related information. It is used to permanently store data in a non-volatile memory (e.g. hard disk).
Since, random access memory (RAM) is volatile which loses its data when computer is turned off, we use files for future use of the data.
When we want to read from or write to a file we need to open it first. When we are done, it needs to be closed, so that resources that are tied with the file are freed.
Hence, in Python, a file operation takes place in the following order.
- Open a file
- Read or write (perform operation)
- Close the file
Python has a built-in function
open() to open a file. This function returns a file object, also called a handle, as it is used to read or modify the file accordingly.
>>> f = open("test.txt") # open file in current directory >>> f = open("C:/Python33/README.txt") # specifying full path
We can specify the mode while opening a file. In mode, we specify whether we want to read
'w' or append
'a' to the file. We also specify if we want to open the file in text mode or binary mode.
The default is reading in text mode. In this mode, we get strings when reading from the file.
On the other hand, binary mode returns bytes and this is the mode to be used when dealing with non-text files like image or exe files.
|'r'||Open a file for reading. (default)|
|'w'||Open a file for writing. Creates a new file if it does not exist or truncates the file if it exists.|
|'x'||Open a file for exclusive creation. If the file already exists, the operation fails.|
|'a'||Open for appending at the end of the file without truncating it. Creates a new file if it does not exist.|
|'t'||Open in text mode. (default)|
|'b'||Open in binary mode.|
|'+'||Open a file for updating (reading and writing)|
f = open("test.txt") # equivalent to 'r' or 'rt' f = open("test.txt",'w') # write in text mode f = open("img.bmp",'r+b') # read and write in binary mode
Unlike other languages, the character
'a' does not imply the number 97 until it is encoded using
ASCII (or other equivalent encodings).
Moreover, the default encoding is platform dependent. In windows, it is
'utf-8' in Linux.
So, we must not also rely on the default encoding or else our code will behave differently in different platforms.
Hence, when working with files in text mode, it is highly recommended to specify the encoding type.
f = open("test.txt",mode = 'r',encoding = 'utf-8')
When we are done with operations to the file, we need to properly close the file.
Closing a file will free up the resources that were tied with the file and is done using Python
Python has a garbage collector to clean up unreferenced objects but, we must not rely on it to close the file.
f = open("test.txt",encoding = 'utf-8') # perform file operations f.close()
This method is not entirely safe. If an exception occurs when we are performing some operation with the file, the code exits without closing the file.
A safer way is to use a try...finally block.
try: f = open("test.txt",encoding = 'utf-8') # perform file operations finally: f.close()
This way, we are guaranteed that the file is properly closed even if an exception is raised, causing program flow to stop.
The best way to do this is using the
with statement. This ensures that the file is closed when the block inside
with is exited.
We don't need to explicitly call the
close() method. It is done internally.
with open("test.txt",encoding = 'utf-8') as f: # perform file operations
In order to write into a file in Python, we need to open it in write
'a' or exclusive creation
We need to be careful with the
'w' mode as it will overwrite into the file if it already exists. All previous data are erased.
Writing a string or sequence of bytes (for binary files) is done using
write() method. This method returns the number of characters written to the file.
with open("test.txt",'w',encoding = 'utf-8') as f: f.write("my first file\n") f.write("This file\n\n") f.write("contains three lines\n")
This program will create a new file named
'test.txt' if it does not exist. If it does exist, it is overwritten.
We must include the newline characters ourselves to distinguish different lines.
To read a file in Python, we must open the file in reading mode.
There are various methods available for this purpose. We can use the
read(size) method to read in size number of data. If size parameter is not specified, it reads and returns up to the end of the file.
>>> f = open("test.txt",'r',encoding = 'utf-8') >>> f.read(4) # read the first 4 data 'This' >>> f.read(4) # read the next 4 data ' is ' >>> f.read() # read in the rest till end of file 'my first file\nThis file\ncontains three lines\n' >>> f.read() # further reading returns empty sting ''
We can see that, the
read() method returns newline as
'\n'. Once the end of file is reached, we get empty string on further reading.
We can change our current file cursor (position) using the
seek() method. Similarly, the
tell() method returns our current position (in number of bytes).
>>> f.tell() # get the current file position 56 >>> f.seek(0) # bring file cursor to initial position 0 >>> print(f.read()) # read the entire file This is my first file This file contains three lines
We can read a file line-by-line using a for loop. This is both efficient and fast.
>>> for line in f: ... print(line, end = '') ... This is my first file This file contains three lines
The lines in file itself has a newline character
print() end parameter to avoid two newlines when printing.
Alternately, we can use
readline() method to read individual lines of a file. This method reads a file till the newline, including the newline character.
>>> f.readline() 'This is my first file\n' >>> f.readline() 'This file\n' >>> f.readline() 'contains three lines\n' >>> f.readline() ''
readlines() method returns a list of remaining lines of the entire file. All these reading method return empty values when end of file (EOF) is reached.
>>> f.readlines() ['This is my first file\n', 'This file\n', 'contains three lines\n']
There are various methods available with the file object. Some of them have been used in above examples.
Here is the complete list of methods in text mode with a brief description.
|close()||Close an open file. It has no effect if the file is already closed.|
|detach()||Separate the underlying binary buffer from the
|fileno()||Return an integer number (file descriptor) of the file.|
|flush()||Flush the write buffer of the file stream.|
|read(n)||Read atmost n characters form the file. Reads till end of file if it is negative or
|readline(n=-1)||Read and return one line from the file. Reads in at most n bytes if specified.|
|readlines(n=-1)||Read and return a list of lines from the file. Reads in at most n bytes/characters if specified.|
||Change the file position to offset bytes, in reference to from (start, current, end).|
|tell()||Returns the current file location.|
||Resize the file stream to size bytes. If size is not specified, resize to current location.|
|write(s)||Write string s to the file and return the number of characters written.|
|writelines(lines)||Write a list of lines to the file.|