Streams

• Data is often stored in files, so programs need to be able to read from and write to files.
• In either case, you first need to create a stream object. The built-in function open does this.
  • The first argument to open is a file name or a path to a file. If your're reading from the file, the file has to exist for this to work.

    >>> mydata = open('data.txt')
    Traceback (most recent call last):
      File "", line 1, in 
        mydata = open('data.txt')
    IOError: [Errno 2] No such file or directory: 'data.txt'

  • The second argument is a string telling whether you want to read from or write to the file. 'r' means read, 'w' means write, and 'a' means append, that is, write by adding to the file rather than writing over what is already there if the file already exists. 'r+' means that you can both read from and write to the file (by overwriting, not appending).
  • 'r' is the default for the second argument, and you can leave it out if you only want to read from the file.
  • If the file contains, or will contain, non-ASCII characters, you need to set its encoding when you open the stream; 'utf8' works in most cases.

    >>> español = open('es.txt', encoding='utf8')

• After you're done reading or writing from a file, you have to close the stream object. You do this with the file method close. The following opens a file to write to, does nothing with it, and then closes it.

  >>> data_file = open('data', 'w')
  >>> data_file.close()

Writing to files

• To write to a file, you can use the file method write, which takes a string as its argument. Include the newline escape character '\n' at the end if you want a newline.

  >>> data_file = open('data', 'w')
  >>> data_file.write("Here's a line of data.\n")
  >>> data_file.write("OK, that's all.\n")
  >>> data_file.close()

• You can also write using the print function, passing it file=stream, where stream is bound the stream object. The following does the same thing as in the previous bullet.

  >>> data_file = open('data', 'w')
  >>> print("Here's a line of data.", file=data_file)
  >>> print("OK, that's all.", file=data_file)
  >>> data_file.close()

• To avoid having to close the stream, you can use the special with ... as form. The following does the same thing as what's in the last two bullets.

  >>> with open('data', 'w') as data_file:
          print("Here's a line of data.", file=data_file)
      	print("OK, that's all.", file=data_file)

• Practice
  • Define a function that takes a filename and list of items. It writes each of the items on a separate line in the file and returns nothing.
  • Define a function that prints out an explanation, then asks a user for their name, SS#, and bank account PIN and appends these to a file containing all of the data you have gathered from other gullible users.

    >>> phish('phish_data')
    >>> phish('phish_data')
    You're bank accownt will soon gonna closed; please to cooparate that it continu.
    What is your name? William Jones
    What is your SS#? 383091055
    What is your bank account PIN? 3911
    Thank you; everything in order!

Reading from files

• One simple way to read lines from a file object is to use a for loop with the file object where a sequence would be expected. This binds the loop variable to each of the lines in the file, one at a time.

  >>> phish_file = open('phish_data')
  >>> for line in phish_file:
  	print(line, end='')
  
  William Jones
  383091055
  3911
  >>> phish_file.close()

  Note that end='' is required to prevent an extra newline character at the end of each printed line.
• The stream object method read returns the contents of the file as one long string. Also note that, as with write streams, you can use the special with...as form to avoid having to close the stream.

  >>> with open('data') as phish_file:
  	phish_file.read()
  
  'William Jones\n383091055\n3911\n'
  

• The stream object method readline returns a single line from the file. Note that a stream object maintains a pointer that keeps track of what point in the file reading has progressed to.

  >>> phish_file = open('phish_data')
  >>> phish_file.readline()
  'William Jones\n'
  >>> phish_file.readline()
  '383091055\n'
  >>> phish_file.readline()
  '3911\n'
  >>> phish_file.readline()
  ''
  >>> phish_file.close()
  >>> español = open('es.txt', encoding='utf8')
  >>> español.readline()
  'Buenos días.\n'
  >>> español.readline()
  '¿Cómo estás?\n'
  >>> español.readline()
  'Adiós.\n'
  >>> español.close()
  

• Practice: Define a function that looks for Social Security numbers in a file. It returns a list of all 9-digit numbers that are preceded by spaces or newlines and followed by spaces or periods. You may want to use the string methods split, replace, and isdigit (see the documentation on string methods).

  >>> get_SS('stuff.txt')
  [383091055, 983901100, 767389099]

Answers

Answers to problems in these notes