Step 5 File
Table of contents
- What is a File?
- Task 5.1 Save Remaining Cups
- Task 5.2 Retrieve Remaining Cups
- Task 5.3 Deal with Missing Information
In Step 3, we played with different operators. For instance, we used assignments and conditions to decrease the remaining cups. You might have noticed that each time you execute the code, the number of cups is always 10 (or the number you set), and then it decreases by one. It is because the computer ‘forget’ between each execution of the code. The variables that keep the information when the code is running are only available during the execution. As soon as the program stop, all this information disappear.
In this step, we explore the use of files to store and retrieve information. We do so at the beginning and the end of the program to ‘remember’ the number of remaining cups.
What is a File?
A computer has two types of memory: the short-term that we often call RAM (for Random Access Memory) and the long-term that we often call drive (e.g. HDD or SSD). On the former, the computer handles the variables of all programs currently running while. The latter is for everything to keep for later use.
Files is a common way to structure long-term memory. You certainly use files to store pictures and other documents. A file is characterised by a path (where to find it), a type of content (e.g. text, picture, video) and a size.
Task 5.1 Save Remaining Cups
We want to save the number of remaining cups in a file. Thus, we will focus on text files. To write in a file, we need the following elements:
pathwhere to find the filefile variabletextwhat we want to save in the file
Write in file Algorithm
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Open the file located at [path] in write mode and keep it in the [file variable]
Write [text] into the file
Close the file
We recognise the path that we mentioned and the text that we want to save into the file. We keep our access to the file in a variable to write text in it and close it at the end. The Python syntax would look as follows:
Write in File Python syntax
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file_variable = open("path", "w")
file_variable.write(text)
file_variable.close()
In this syntax, we can recognise that the first line ‘opens’ a file, using the path. w stands for writing mode. Like a string, the file_variable is an object. This data structure combines several data types, enabling us to manipulate a file easily. write and close are actions that we can perform on this file object.
Save Remaining Cups Algorithm
With this new knowledge, we can save the number of remaining cups before the end of our program, for example:
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Open the file cups.txt in 'write' mode and keep it in the 'cup_file' variable
Convert number_remaining_cups into string
Write number_remaining_cups in cup_file
Close the file cup.txt
Note the conversion of the number into a string: we can only store a piece of text.
In your Replit project, you can add this algorithm and translate each line into Python syntax at the end of the code.
Execute the code to check that it behaves the way you expect.
When your program works correctly, you should see a new file, ‘cups.txt’, in the left panel of Replit. You can open it and check if it contains the correct number of remaining cups.
Task 5.2 Retrieve Remaining Cups
Once we save information, we need to retrieve it at the beginning of our program to use it. First, we want to read from the file.
Read from file Algorithm
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Open the file located at [path] in read mode and keep it in the [file variable]
Read from the file and keep it in [text variable]
Close the file
This algorithm is similar to the one writing into files. However, we note the read mode instead of write. This is because we now read information from the file into a text variable.
Read from File Python syntax
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file_variable = open("path", "r")
text = file_variable.read()
file_variable.close()
We can use the read from file algorithm to retrieve the number of remaining cups that we saved in the file ‘cups.txt’.
Retrieve Remaining Cups
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Open the file cups.txt in 'read' mode and keep it in the 'cup_file' variable
Read the number of remaining cup from cup_file as text
Convert number_remaining_cups into a number
Close the file cup.txt
Note the conversion again from string to integer. With this conversion, we can use the text from the file as a number.
In your Replit project, you can add this algorithm and translate each line into Python syntax at the beginning of the code.
Execute the code to check that it behaves the way you expect.
Task 5.3 Deal with Missing Information
This code does not need to initialise number_remaining_cups because it comes from the file. However, there is still one situation when we need to do so: when there is no existing file to tell us the number of remaining cups.
Let’s test! Go ahead, right-click on the file ‘cups.txt’ and delete it. Then, execute the code again. An error fails the program because the file does not exist.
We can reuse the Exception mechanism we introduced in the previous step. We want to try to open and read the file. If this action fails, we need to initialise the number of remaining cups. It would look like this:
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Try to do something
Open the file cups.txt in 'read' mode and keep it in the 'cup_file' variable
Read the number of remaining cup from cup_file as text
Convert number_remaining_cups into a number
Close the file cup.txt
If it fails
Create a variable called number_remaining_cups of type int with the initial value 10
In your Replit project, you can transform your algorithm to add the exception mechanism and translate each line into Python syntax.
Execute the code to check that it behaves the way you expect.