Python File Objects

Introduction 

------------ 

 

- reading and writing files is done via open() 

- open() returns a file object 

- syntax: 

 

    open('filename', 'mode') 

 

    where modes are: 

      r  - open file for reading (default) 

      w  - open file for writing (content will be overwritten) 

      a  - append 

      r+ - read and write 

 

- some basic operations on a file: `file.write()` and `file.read()` 

- when opening a binary file, append "b" in mode 

- when reading a file, line endings (\n on Unix, \n\r on Windows) are converted 

  to \n 

- when writing in text mode, \n is converted back to platform-specific line 

  endings which may corrupt a binary data when it is opened in text mode 

 

Daling with file objects 

------------------------ 

 

'r' = open for reading (default) 

'w' = open for writing, truncating the file first 

'x' = open for exclusive creation, failing if the file already exists 

'a' = open for writing, appending to the end of the file if it exists 

'b' = binary mode 

't' = text mode (default) 

'+' = open a disk file for updating (reading and writing) 

'U' = universal newlines mode (deprecated) 

 

https://docs.python.org/3/library/functions.html#open 

 

Reading	This opens the file for reading and closes it afterwards. It is advisable to close the file  after using it to free up resources; else, Python will destroy the object and close the file  for you but that may take some time.    This reads the entire content of a file. If the end of file has been reached, f.read() will  return an empty string ''  >>> f = open('/tmp/test.txt')  >>> f.read()  'hello python\n'  >>> f.read()  ''                >>> f.close()  >>> f.closed  True  >>>    To read a single line, use `file.readline()`  >>> f = open('test')  >>> f.readline()  '1st line\n'  >>> f.readline()  '2nd line\n'  >>> f.close()  >>>     `file.readlines()` will read all lines by storing each line to a list.  >>> f = open('test')  >>> print(f.readlines())  ['1st line\n', '2nd line\n']  >>>     This is a memory efficient way of reading a file.  >>> f = open('test')  >>> for line in f:  ...  print(line, end='')  ...   1st line  2nd line  >>>
Writing and appending	Writing to a file will overwrite its content (if there is any). It will also return the  number of characters written.  >>> f = open('/tmp/test.txt', 'w')  >>> f.write('This will overwrite the content')  31  >>> f.close()  >>> f = open('/tmp/test.txt')  >>> f.read()  'This will overwrite the content'  >>> f.close()  >>>      If the file doesn't exist, it will be created.  >>> newfile = open('/tmp/newfile', 'w')  >>> from subprocess import call  >>> call(["ls", "-l", "/tmp/newfile"])  -rw-r--r-- 1 merrell merrell 0 Aug 14 20:45 /tmp/newfile  0  >>>     Appending is as easy as `a`.  >>> f = open('/tmp/test.txt', 'a')  >>> f.write('This will be appended')  21  >>> f.close()  >>> f = open('/tmp/test.txt')  >>> f.read()  'This will overwrite the contentThis will be appended'  # notice line termination is incorrect  >>> f.close()  >>>
Positioning	`file.tell()` returns the file's current position in bytes (for files open in binary mode)  or in an opaque number (for files open in text mode).  >>> f.read()  '1st line\n2nd line\n'  >>> f.tell()  18  >>>     To change the file object's position, use `file.seek(offset, from_what)`. Position is computed  by adding an `offset` from the refernce point `from_what`. Reference points are "0" for  beginning of file, "1" for current position, and "2" for end of file.  >>> f = open('workfile', 'rb+')  >>> f.write(b'0123456789abcdef')  16  >>> f.seek(5)      # Go to the 6th byte in the file  5  >>> f.read(1)  b'5'  >>> f.seek(-3, 2)  # Go to the 3rd byte before the end  13  >>> f.read(1)  b'd'    So if you want to read again from the beginning, do this:  >>> f.read()  '1st line\n2nd line\n'  >>> f.seek(0,0)  0  >>> f.read()  '1st line\n2nd line\n'
Best practice in  dealing with file objects	Using `with` will automatically close the file for you which is the recommended way of  dealing with file objects.  >>> with open('/tmp/test.txt') as f:  ...   f.read()  ...   '1st line\n2nd line'  >>> f.closed  True
Replacing a line  persistently	for line in fileinput.input(['thefile.txt'], inplace=True):      print(line.replace('old stuff', 'shiny new stuff'), end='')
Checks if file is a  directory	os.path.isdir('/tmp/sample/dir1')