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Reading and writing text files in Python

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  1. # The built-in function `open` opens a file and returns a file object.
  2.  
  3. # Read mode opens a file for reading only.
  4. try:
  5.     f = open("file.txt", "r")
  6.     try:
  7.         # Read the entire contents of a file at once.
  8.         string = f.read() 
  9.         # OR read one line at a time.
  10.         line = f.readline()
  11.         # OR read all the lines into a list.
  12.         lines = f.readlines()
  13.     finally:
  14.         f.close()
  15. except IOError:
  16.     pass
  17.  
  18.  
  19. # Write mode creates a new file or overwrites the existing content of the file. 
  20. # Write mode will _always_ destroy the existing contents of a file.
  21. try:
  22.     # This will create a new file or **overwrite an existing file**.
  23.     f = open("file.txt", "w")
  24.     try:
  25.         f.write('blah') # Write a string to a file
  26.         f.writelines(lines) # Write a sequence of strings to a file
  27.     finally:
  28.         f.close()
  29. except IOError:
  30.     pass
  31.  
  32. # Append mode adds to the existing content, e.g. for keeping a log file. Append
  33. # mode will _never_ harm the existing contents of a file.
  34. try:
  35.     # This tries to open an existing file but creates a new file if necessary.
  36.     logfile = open("log.txt", "a")
  37.     try:
  38.         logfile.write('log log log')
  39.     finally:
  40.         logfile.close()
  41. except IOError:
  42.     pass
  43.  
  44. # There is also r+ (read and write) mode.

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