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Safari | Python Developer's Handbook -> File Handling





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File Handling

Python's core language supports all the basic functions that are necessary to manipulate files. It isn't necessary to import any modules to use them. Whenever you use the open function to get access to a file, Python creates a file object that supports all the built-in methods that apply to this new object.

Opening a File

basic syntax: file = open ( filename[, mode[, buffersize]] )

The mode can be r, w, or a (read, write, and append, respectively). If none of them are mentioned, read mode is assumed.

If you are working with a binary file, add the letter b to the mode indicator (for example, rb or wb). The b stands for binary mode ( text translation mode).

You can also place a + sign to the mode letter to indicate a read/write open (for example, r+ or w+)it is useful when you need to perform both operations (read and write) in the file. Remember that if you use w+, it will first truncate the file length to zero.

The last argument in the open syntax is the buffersize clause, which means

  • 0 = unbuffered

  • 1 = line buffered

  • If buffersize is greater than 1, its value is equal to the buffer size, in bytes.

  • If negative, the buffer size is the system default(default behavior).

Here's an example:

						
file = open("foo.txt", "r")
line = file.readline()
line = line[:-1]        #chop off the newline character
while line:
    print line
    line = file.readline()
    line = line[:-1]
file.close()
				
					

Supported Methods

The following methods are supported by all file objects.

read()

It reads up to n bytes. But, if you don't provide any argument, read() reads all available data from the file.

basic syntax: file.read( [nbytes] )

							
>>> file = open("foo.txt").read()

						

If you say file = open("foo.txt").read(100), Python will read the file up to its first 100 bytes.

readline()

It reads only one line at a time (until, and including, the newline character).

basic syntax: file.readline()

							
>>> file=open("test.txt","r")
>>> while 1:
       line = file.readline()
       if not line:
           break


						

Both read() and readline() functions return an empty string for EOF.

readlines()

It reads the entire file into a list of strings.

basic syntax: file.readlines()

							
>>> file=open("test.txt","r")
>>> for line in file.readlines():
       print line


						
write()

It writes a string to a file.

basic syntax: file.write(string)

							
>>> file.write('Spam')

						
writelines()

It writes a list of strings to a file.

basic syntax: file.writelines(list)

							
>>> file.writelines(["We are the knights who say ","ni!"])

						
seek()

It goes to a new file position. If how=0, it starts from the beginning of the file; if how=1, the position is relative to the current position; if how=2, the position is relative to the end of the file. The default value for how is 0.

basic syntax: file.seek( position[, how] )

tell()

It returns the current file pointer.

basic syntax: file.tell()

Fileno()

It returns an integer file descriptor.

basic syntax: file.fileno()

flush()

It flushes the internal buffer.

basic syntax: file.flush()

close()

It closes the file.

basic syntax: file.close()

truncate()

It truncates the file.

basic syntax: file.truncate( [size] ... Expand> <Close

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