2.5. Files

This section fits here logically (as an important built-in type of object) but it is not needed for the next chapter, More On Flow of Control.

Thus far you have been able to save programs, but anything produced during the execution of a program has been lost when the program ends. Data has not persisted past the end of execution. Just as programs live on in files, you can generate and read data files in Python that persist after your program has finished running.

As far as Python is concerned, a file is just a string (often very large!) stored on your file system, that you can read or write, gradually or all together.

2.5.1. Writing Files

Open a directory window for your Python program directory. First note that there is no file named sample.txt.

Make sure you have started Idle so the current directory is your Python program directory (for instance in Windows with the downloaded shortcut to Idle).

Run the example program firstFile.py, shown below:

outFile = open('sample.txt', 'w')
outFile.write('My first output file!')
outFile.close()

The first line creates a file object, which links Python to your computer’s file system. The first parameter in the file constructor gives the file name, sample.txt. The second parameter indicates how you use the file. The 'w' is short for write, so you will be creating and writing to a file.

Warning

If the file already existed, the old contents are destroyed.

If you do not use any operating system directory separators in the name ('\' or '/' depending on your operating system), then the file will lie in the current directory. The assignment statement gives the Python file object the name outFile.

The second line writes the specified string to the file.

The last line is important to clean up. Until this line, this Python program controls the file, and nothing may be actually written to the file yet: Since initiating a file operation is thousands of times slower than memory operations, Python buffers data, saving small amounts and writing a larger chunk all at once.

Warning

The close line is essential for Python to make sure everything is really written, and to relinquish control of the file.

It is a common bug to write a program where you have the code to add all the data you want to a file, but the program does not end up creating a file. Usually this means you forgot to close the file.

Now switch focus and look at a file window for the current directory. You should now see a file sample.txt. You can open it in Idle (or your favorite word processor) and see its contents.

Run the example program nextFile.py, shown below, which has two calls to the write method:

outFile = open('sample2.txt', 'w')
outFile.write('My second output file!')
outFile.write('Write some more.')
outFile.close()

Now look at the file, sample2.txt. Open it in Idle. It may not be what you expect! The write method for the file is not quite like a print function. It does not add anything to the file except exactly the data you tell it to write. If you want a newline, you must indicate it explicitly. Recall the newline code '\n'. Run the example program revisedFile.py, shown below, which adds newline codes:

outFile = open('sample3.txt', 'w')
outFile.write('A revised output file!\n')
outFile.write('Write some more.\n')
outFile.close()

Check the contents of sample3.txt.

2.5.2. Reading Files

Run the example program printFile.py, shown below:

'''Quick illustration of reading a file.
(needs revisedFile.py run first to create sample3.txt)
'''

inFile = open('sample3.txt', 'r')
contents = inFile.read()
print(contents)

Now you have come full circle: what one Python program has written into the file sample3.txt, another has read and displayed.

In the first line an operating system file (sample3.txt) is associated again with a Python variable name (inFile). The second parameter again gives the mode of operation, but this time it is 'r', short for read. This file, sample3.txt, should already exist, and the intention is to read from it. This is the most common mode for a file, so the 'r' parameter is actually optional.

The read method returns all the file’s data as a single string, here assigned to contents. Using the close method is generally optional with files being read. There is nothing to lose if a program ends without closing a file that was being read. [1]

Note

There are three related but distinct concepts related to files. Beginners often try to merge several in their head or substitute one for another:

  1. The file name is a string that identifies the file in your computer’s file system.
  2. You need the file name to open a file creating a file object, but the file object (that I tend to call inFile or outFile) is not the same as the name of the file on your hard drive.
  3. There is still one more step to the most important part, the contents of the file. The read and write methods for a file object read existing content or write new content.

2.5.2.1. PrintUpper Exercise

Make the following programs in sequence. Be sure to save the programs in the same directory as where you start the idle shortcut and where you have all the sample text files:

  1. printUpper.py: read the contents of the sample2.txt file and print the contents out in upper case. (This should use file operations and should work no matter what the contents are in sample2.txt. Do not assume the particular string written by nextFile.py!)
  2. fileUpper.py: prompt the user for a file name, read and print the contents of the requested file in upper case.
  3. * copyFileUpper.py: modify fileUpper.py to write the upper case contents string to a new file rather than printing it. Have the name of the new file be dynamically derived from the old name by prepending ‘UPPER’ to the name. For example, if the user specified the file sample.txt (from above), the program would create a file UPPERsample.txt, containing ‘MY FIRST OUTPUT FILE!’. When the user specifies the file name stuff.txt, the resulting file would be named UPPERstuff.txt.

2.5.2.2. Mad Lib File Exercise

Write madlib3.py, a small modification of madlib2.py, requiring only a modification to the main function of madlib2.py. (Even better is to start from madlib2a.py if you did the exercise in Unique List Exercise). Also create a file myMadlib.py, as described below.

Your madlib3.py should

  • prompt the user for the name of a file that should contain a madlib format string as text (with no quotes around it).
  • Read in this file and use it as the format string in the tellStory function.

This is unlike in madlib2.py, where the story is a literal string coded directly into the program called originalStory. The tellstory function and particularly the getKeys function were developed and described in detail in this tutorial, but for this exercise there is no need to follow their inner workings - you are just a user of the tellstory function (and the functions that it calls). You do not need to mess with the code for the definition of tellStory or any of the earlier supporting functions. The original madlib string is already placed in a file jungle.txt as an example of the story file format expected. With the Idle editor, write another madlib format string into a file myMadlib.txt. If you earlier created a file myMadlib.py, then you can easily extract the story from there (without the quotes around it). Test your program both with myMadlib.py, and your new madlib story file.

[1]If, for some reason, you want to reread this same file while the same program is running, you need to close it and reopen it.

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