Learning Python

Chapter 5

1. Basics, import. This one is simpler than you may think. When you’re done, your file and interaction should look close to the following code; remember that Python can read a whole file into a string or lines list, and the len built-in returns the length of strings and lists:

   % cat mymod.py
   
   def countLines(name):
       file = open(name, 'r')
       return len(file.readlines())
   
   def countChars(name):
       return len(open(name, 'r').read())
   
   def test(name):                                  # or pass file object
       return countLines(name), countChars(name)    # or return a dictionary
   
   % python
   >>> import mymod
   >>> mymod.test('mymod.py')
   (10, 291)

   On Unix, you can verify your output with a wc command. Incidentally, to do the “ambitious” part (passing in a file object, so you only open the file once), you’ll probably need to use the seek method of the built-in file object. We didn’t cover it in the text, but it works just like C’s fseek call (and calls it behind the scenes); seek resets the current position in the file to an offset passed in. To rewind to the start of a file without closing and reopening, call file.seek (0 ); the file read methods all pick up at the current position in the file, so you need to rewind to reread. Here’s what this tweak would look like:

   % cat mymod2.py def countLines(file): file.seek(0) # rewind to start of file return len(file.readlines()) def countChars(file): file.seek(0) # ditto (rewind if needed) return len(file.read()) def test(name): file = open(name, 'r') # pass file object return countLines(file), ...