File Handling

Saving data

You will have to save data at some point in your program. 

There are various ways to do this:

• Files

• In the Registry

• In Databases

At Higher we will concentrate only on text/csv files. There are various methods to access files in Python but we will be using the methods in the File Object.

When a file is opened you have to specify how it should be opened. This is what type of access you will be using. When opening a file we can set the following modes:

• Reading (this is the default and opens the file in a read only mode)

• Writing (will usually overwrite the file)

• Appending (will add to the end of the existing contents)

Some other Python File Access Modes

• r - Open for reading (this is the default)

• w - Open for writing, truncating the file first

• a - Open for writing, appending to the end of the file if it exists

Assuming that the file exists in the same folder as the Python Program the pseudocode below shows opening a file for reading and reading all of the contents into a variable called filecontents.

OPEN FILE(SAMPLEFILE.txt) FOR READING

filecontents = FILE.READ

CLOSE FILE

There are different ways to read the contents of a file:

• ReadLine - Reads a single line of characters from the file and returns the data as a string.

• Read - Reads all characters from the current position to the end of the file.

What is a CSV File

A CSV file is simply a plain text file where each piece of data is separated by a comma. And each new record is on a new line e.g. below:

Mark,43

Joanna,54

It is important for you to analyse the file structure as this will dictate how the file is to be read and processed. The order of the pieces of data is important. Whether a file extension is CSV or plain text you should process the file in the same way.

In the most basic method of writing to a file you write a string at a time, and when needed will put a new line character at the end so that the next string will be on a new line.

When you need to create a CSV file from either variables, arrays or records you will need to create a single line of a file at one time. You will notice in the example below that we add the \n character at the end. This is Python's End of Line character so that the next writeline operation will take place on a new line.

The code below would take in an array of forenames, surnames and Scottish Candidate Numbers and will write them to a CSV file