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9. More with Files

In chapter 6, we saw how to print to a file instead of to the screen. In this chapter, we will see how to read information from existing files. Then we will write programs to process data from files, and to edit files.

Reading from files

We shall consider the opening paragraph of Kafka’s “Metamorphosis”.


There are newline characters at the end of each line, save for the last. You can cut and paste or type this into a text file to try these examples out. Here, it is saved as gregor.txt. Now, we can read the whole contents of the file into a string using ’r’ for reading mode:


This single string contains the \n newline characters, of course. If we call again, the result is the empty string. This is because there is nothing else left to read – the contents of the file has already been read and we are at the end of the file.

Three ways to iterate over lines

Instead of reading the whole file as one big string, we may read the lines in turn, by repeated use of the readline method:


Notice that we omit the ’r’ argument to the open function – it is the default. Again, we know that there is no more to read when the result is the empty string. We can, alternatively, iterate directly over the contents of the file with a for loop:


Finally, we can use the list function to return a list of all the lines in the file in one go:


Example: reversing lines

We can write a program to read all the lines from a file, and write them in reverse order to another file:

>>> f = open('gregor.txt')
>>> f_out = open('output.txt', 'w')
>>> for x in reversed(list(f)):
...     print(x, end='', file=f_out)
>>> f.close()
>>> f_out.close()

Here is the contents of the output file:


We can use an extended version of the with … as structure we have already seen to prevent mistakes with matching up the opening and closing of files. Here is the same program in this simpler, safer, form:

>>> with open('gregor.txt') as f, open('output.txt', 'w') as f_out:
...     for x in reversed(list(f)):
...         print(x, end='', file=f_out)

Files and exceptions

Not only does the with … as construct prevent double-closing of a file, but also prevents any attempt to read from a file which has already been closed:

>>> f = open('gregor.txt')
>>> f.close()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: I/O operation on closed file.

There are still, however, some exceptions we may need to handle, even when using with … as – for example, a missing file:

>>> open('not_there.txt')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
FileNotFoundError: [Errno 2] No such file or directory: 'not_there.txt'

Example: text statistics

Consider this program to return the number of lines, characters (letters or other symbols), words, and sentences in a given file:


Notice we can use filter directly on line without turning it into a list. Here is the result:

>>> gregor_stats
(8, 472, 85, 4)

That is to say, 8 lines, 472 characters, 85 words, and 4 sentences. In the questions, you will be asked to extend this program to collect more statistics.

Common problems

In addition to situations which can lead to file-related exceptions, there are two more common issues which can occur when processing files. If we open a file which already exists, with the intention of writing to it, but we forget to open it in ’w’ mode, an exception occurs:

>>> with open('exists.txt') as f:
...     print('output', file=f)
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
io.UnsupportedOperation: not writable

When printing lines from a file which we have read using, for example, readlines, it is important to remember that they will always have a \n newline at the end already. If we print them just using plain print we will see double spacing:



We now know how to read from files as well as write to them. This means we can write file-processing programs, which read from one file, process the data in some way, and write to another. This is a significant class of useful programs. We made our programs cleaner and less error-prone by extending our use of the with … as construct. We learned how to iterate over the lines in the file, and so over the characters in each line, building our file statistics program. In some of the questions, you will be asked to extend this file statistics program in various ways.

In the next chapter, we fill in another gap in our knowledge: the real numbers – that is to say the ones which are not whole numbers.


  1. We wrote a program to print out the contents of a file line-by-line:

    >>> f = open('gregor.txt')
    >>> for line in f:
    ...     print(line, end='')

    Rewrite this program using the with … as construct.

  2. Give a function to write a dictionary with integer keys and string values to a given file. For example, the dictionary {1: ’oak’, 2: ’ash’, 3: ’lime’} should produce the file:

  3. Now write a function to read such a dictionary back from file. Make sure to handle exceptions arising from incorrect data. There is a built-in method strip which removes spaces and newlines from either end of a string which may prove useful.

  4. When we write to a file which already exists, its contents are overwritten. The file mode ’a’ allows information to be appended to a file instead. Use this to write a function which concatenates two files, writing the result to a third.

  5. Write a function which reads a file containing multiple numbers, separated by spaces, on multiple lines, and calculates their total.

  6. Write a function copy_file which, given two file names, reads the contents of the first, and writes it to the second.

  7. Extend our text statistics to print a histogram of the frequencies of each letter in the file. You might remember we wrote a similar histogram program in the questions to chapter 4.

  8. Extend it again to print a histogram of frequencies of words. How might punctuation and capital letters be dealt with? Hints:

  9. Write a function to search for a given word in a given file, listing the line numbers and lines at which it appears. Use our lessons from the previous question to deal with punctuation.

  10. Write a function top which prints the first five lines from a file, waiting for the user to press Enter for another five, and so on.