Section 3: Writing Data to Files

 Table of Contents > Chapter 6 > Section 3 

We have seen how data stored in files can be read and processed by a Python program.  Similarly, data that is produced by a Python program can be written to file where it will persist after the program has been terminated.

Before we can write data to a file, we must first open it for writing.  We use the same function that we used to open a file for reading but specify the 'w' or write mode:

with open('myFile.txt', 'w') as f:

     ...f
When we open a file for writing, it is created for us if it does not already exist.  Be careful when opening an existing file for writing as anything previously stored in the file will be erased when the file is opened.  If you wish to append new data to the end of the file, use the 'a' or append mode.   

The file class has a write method that writes a string to a file.  We have seen that it is convenient to read data from a file a line at a time, although this is not the only way that it can be done.  When it comes to writing data to a file, we are free to choose the level of granularity - from single character, to word, to line, to entire contents of the file - it depends what is stored in the string that we pass to the write method. 

As a simple example, Code Explorer 7.2 shows a Python function that makes a copy of a file.

Some comments are in order:

1. When designing the tests, note that we have used a StringIO object to represent the input and output file.  When constructing the StringIO object that represents the output file, we passed an empty string as a parameter, thereby simulating an empty file.
   
   
2. The StringIO class has a method getvalue() that returns the entire contents of the "file".  We use this in the tests to ensure that the output file contains an exact copy of the input file.
   
   
3. We would not normally copy a file this way.  Operating systems provide methods for copying files that are far more efficient. 

Assuming that we want to copy the file data.txt to a file named dataCopy.txt, we call our copy_file function from the Python shell as follows:

>>> with open('data.txt', 'U') as inf:

>>>     with open('dataCopy.txt', 'w') as outf:

>>>         copy_file(inf, outf)
Again, the with statement ensures that the files data.txt and dataCopy.txt are closed after the call to copy_file terminates.