Python Lists

I am in the process of learning the Python language and have been thinking of ways to help reinforce what I have learned. One thing that helps me in the learning process is to write about the subject I am learning. So I thought I would start a Python category on my personal blog and start documenting the journey I have been on. In my first post I focused on my interest in programming and using the main() function in Python programs (a tip my youngest enlightened me on). In my second post I cover how to open text files, read them and then close them. This post will be a multi-parter on lists.

From the very first program I worked on (a notes processing application) it became very apparent that I was going to be using a lot of lists. A list in Python is a group of things (strings, integers, etc...). Lists are quite malleable. You can have duplicate items in them and you can move, add or remove items on a whim. Lists are ordered so items can be identified or manipulated by their index number. The items of a list are encapsulated within brackets ([]).

fruit = ['apple','banana','orange']

In the example above I created a list of fruit. Fruit is the variable I have assigned the list using the equals sign. Inside the brackets are my list items: apple, banana and orange. The single quotes around each item in the list indicates that the items are strings, but the list could consist of integers as well. If you want to see the contents of a list you can use the print() method:

print(fruit)
['apple','banana','orange']

Once you have a list defined you can do interesting things like loop through them and compare items in the list to other things like strings. In future posts I will cover ways to update lists, retrieve values, loop through them, and run comparisons on list items with strings.

Opening and reading text files in Python

So far a lot of projects that I have worked on in Python have utilized external files for some kind of processing. Early on I had to become familiar with the open() function and some supporting functions for working with external files. Initially I worked with plain text files, but have moved on to more complex file types and have learned how to use a few new libraries to help process those. For this post I will stick with plain text.

The open() function does just what it describes and opens the specified file. This function takes two parameters: filename and mode.

open('file.txt','r')

The filename parameter is a text string for the file one wants to open and will need to include the full path if necessary. The mode parameter has the following options:

• Read "r" - The read parameter opens the file for reading. This is the default value if this parameter is left blank. If the file does not exist an error message will be returned.

• Append "a" - The append parameter opens the file for appending additional data. If the file does not exist the function will create a new file.

• Write "w" - The write parameter opens the file for writing data. This parameter will overwrite and existing file or create a new file.

• Create "x" - The create parameter creates the specified file. An error will be returned if it already exists. Additionally, one can specify if the file should be handled as binary "b" or text "t" (text is the default mode).

Once the text file has been opened it is time to read the contents for processing. I have used both the read() and readlines() functions in the projects I have worked on. The read() function will read the entire document as one block of text. The readlines() function will read each line of text as a list. Once the text is in a readable format for Python one can start manipulating or processing it.

file = open('file.txt','r')

readTheFile = file.read()
print(readTheFile)
This is some text.
Here is some additional text.

readlinesTheFile = file.readlines()
print(realinesTheFile)
['This is some text.\n','Here is some additional text.']

Of course, once you are finished with the file it is good practice to close it and that is where the close() function comes into play.

file.close()

You can get a bit fancier and have Python close the file automatically once one has finished processing it, but I will save that for another post.

So there you have it, the basics of opening, reading and closing files in Python. I am sure there are many ways people have discovered with file management in Python so if you have a tip please leave it in the comments!