The goal of this exercise is to learn the basics of reading and writing files in Python.

Getting set up

At this point, you should have (1) an account on Github and (2) been introduced to the very basics of Git.

Login to your Github account.
Fork this repository, by clicking the 'Fork' button on the upper right of the page.

After a few seconds, you should be looking at your copy of the repo in your own Github account.
Click the 'Clone or download' button, and copy the URL of the repo via the 'copy to clipboard' button.
In your terminal, navigate to where you want to keep this repo (you can always move it later, so just your home directory is fine). Then type:
```
$ git clone the-url-you-just-copied
```
and hit enter to clone the repository. Make sure you are cloning your fork of this repo.

Next, cd into the directory:

$ cd the-name-of-directory-you-just-cloned

At this point, you should be in your own local copy of the repository.
As you work on the exercise below, be sure to frequently add and commit your work and push changes to the remote copy of the repo hosted on GitHub. Don't enter these commands now; this is just to jog your memory:
```
$ # Do some work
$ git add file-you-worked-on.py
$ git commit
$ git push origin master
```

Learning objective

To become fluent in reading and writing files in Python.

Learning the basics

In Python, we use the open function to open a file. The basic syntax is:

file_object = open(path_to_file, access_mode)

The access_mode is usually 'r' for reading from the file or 'w' for writing to the file. There are other modes, but read and write are the most common and the only modes we will worry about here.

Let's work in the Python interpreter to learn some basics about how to work with files in Python:

$ python3

Let's try opening the dummy.txt file in this repo:

>>> file_object = open('dummy.txt', 'r')
>>> type(file_object)

Just like any object in Python, we can use help to find out more about it and see its methods:

>>> help(file_object)

We can use the read method to get a string of the entire file:

>>> s = file_object.read()
>>> print(s)

For larger files, getting a string of the entire file is not very efficient. Below, we will learn a more efficient approach.

When we are done with the file, whether reading from it or writing to it, make sure you close the file:

>>> file_object.close()

Below, we will see how we can use with statements so that files get closed automatically.

Now, let's create a new file and write a line in it:

>>> new_file = open('my-new-file.txt', 'w')
>>> new_file.write('Hello, this is my new file!')
>>> new_file.close()

Quit out of the Python interpreter:

>>> quit()

In your directory you should now see a new file called my-new-file.txt:

$ ls #In Windows: dir 
dummy.txt
my-new-file.txt
origin.txt
README.md

$ cat my-new-file.txt #In Windows: type  my-new-file.txt
Hello, this is my new file!

A simple example script

Now, let's write a short Python script to parse the dummy.txt file in this repo. Create a new file called dummy.py and open it with your preferred text editor. Add the following code to your file:

print('Opening dummy.txt')
with open('dummy.txt', 'r') as in_stream:
    print('Opening output.txt')
    with open('output.txt', 'w') as out_stream:
        for line in in_stream:
            line = line.strip()
            word_list = line.split()
            word_list.sort()
            for word in word_list:
                out_stream.write('{0}\n'.format(word))
print("Done!")
print('dummy.txt is closed?', in_stream.closed)
print('output.txt is closed?', out_stream.closed)

Run this script from the command line:

$ python3 dummy.py

Take a look at the output.txt file that was created by the script:

$ cat output.txt

Notice two things about this script:

We looped over the file object for dummy.txt line by line using a for loop (for line in in_stream). This is more efficient than reading the entire file as a string.
We used with open(...) as variable_name statements to ensure the files get closed. It is VERY easy to forget to close a file, so use with statements!

The Exercise

In this repo is a text version of On the Origin of Species by Charles Darwin. Check it out:

$ less origin.txt #In Windows, using Powershell: type file.txt | Out-Host -Paging OR type file.txt | oh -p

Your goal is to write a script that will read origin.txt and find all occurrences of certain words and write occurrences to a new file. Below, I outline which words to find and the format to use when writing them to the new file.

The words to find

Find all words related to heritability; your script should find heritable, inherit, inheritance, and other forms of these words. Use a regular expression pattern to find these.

The format of the output file

When you find such a word, write the line number it was found on and the word itself separated by a tab ('\t') character. For example, here is how the first 10 lines of the file should look:

471	Inheritance
660	inherited
799	inheritance
850	Inheritance
914	inheritance
991	INHERITANCE
993	inherited
1000	inherited
1046	inherited
1047	inheritable

If two words are found on the same line, both should be written to the file on separate lines (they will have the same line number).

Use best practices

Remember what you learned from the best practices exercise:

Put meaningful units of code into functions.
Make your script importable as a module; i.e., use the if __name__ == '__main__': flow control.
Write informative docstrings to document your script and functions.

Also, try to keep your code general/flexible. For example, write your functions so that they can be reused for other purposes; can you write them so they can be used with other files and other regular expression patterns?

Extra challenges (not required)

If you look at the contents of origin.txt, you will notice that the Gutenberg Project has added text to the beginning and end of the file. Can you you update your script to avoid searching this added text?

Try updating your script so that it can search for any regular expression pattern that is provided via the command line.

Try updating your script so that a path to any text file can be specified via the command line. Your script should then search this file rather than origin.txt.

Acknowledgments

Thanks to Project Gutenberg for providing a text version of On the Origin of Species. The origin.txt file was copied from http://www.gutenberg.org/cache/epub/2009/pg2009.txt.

Support

This work was made possible by funding provided to Jamie Oaks from the National Science Foundation (DEB 1656004).

License

The text version of On the Origin of Species in origin.txt was copied from Project Gutenberg under the Project Gutenberg License, which states:

This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.org

The full version of the Project Gutenberg License can be found at the end of the origin.txt file.

All content in this repository other than the origin.txt is licensed under a Creative Commons Attribution 4.0 International License.

charitety/python-file-io

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