Welcome back for another instalment in our series of programming with Python. In our last article we talked about variables and numeric data types and expanded on the work we had been doing to create a very small program that extracted data from a text file.
We are going to keep going with our example, but don't worry if you didn't go through the last article as the code we will be using will all be presented fresh again in this article.
If you open up the interactive shell and run the sample we have provided below, you will see that we added "What is your name?" in the brackets of the input() function. This is presented to the user, and when a response is provided, the answer is then provided back to the program. In the example below, we aren't doing anything with the output, but we can assign it to a variable or use the answer for something.
>>> input("What is your name? ")
What is your name? Vince
'Vince'
As you can see we are provided a string returned to us, so this will be perfect to use as part of our application where we are adding details of the money we have saved. As you can see in the example below, we are asking the question, "How much did you save today?" with the answer being stored in a variable named savings. This variable can then be used to write to our savings.txt file. Make note that we also add a newline(\n) before adding the savings variable into our file.
>>> savings = input("How much did you save today?")
How much did you save today?200
>>> print(savings)
200
>>> f = open("savings.txt","a")
>>> f.write("\n{}".format(savings))
3
>>> f.close()
>>> exit()
Instead of having to add all these details into the interactive shell every time we wanted to add some details to the savings.txt file, we can create a simple program that can do this for us. So every time we want to add in a new details of money we have saved, we simply run the program below instead. Create a file called my_savings.py as we have below before adding in the code:
touch my_savings.py
Make sure you don't add in the line numbers on the left as they are simply there for reference. It shouldn't be anything too new, we are using comments in this program for the first time. As you can see line 3 and 6 start with the hash(#) character and anything to the right of the hash symbol is not read by Python, but is used to provide details to anyone reading the code. Live four asks the user question and stores the answer into the variable named savings. We then open up our text file, and write the details of the variable into file before closing it.
1 #!/usr/bin/env python
2
3 # Prompt the user for an input
4 savings = input("How much money did you save today? ")
5
6 # Use the details provided and add the new value to the file
7 f = open("savings.txt","a")
8 f.write("\n{}".format(savings))
9 f.close()
Run the program as we have below and we can add a value to the amount of money we have saved today:
python3 my_savings.py
How much money did you save today? 200
This is great progress, but if you have created the check_savings.py program from our last article, you would have guessed by now, this program won't really work very well now.
It's ok if you haven't created this program yet, as we will be recreating it in a moment.
If you run our check_savings.py program and if we have used it more than once as we have in the following example, this is what happens:
python3 check_savings.py
I have saved $500
200 so far this year
Not the best output as every entry we make in the savings.txt file will now put a separate entry on our output. Open your interactive shell again, and we will show you how to work with this kind of problem.
>>> f = open("savings.txt","r")
>>> total_savings = f.readlines()
f.close()
The difference here is that the variable total_savings will not be a string, instead by using the readlines() function, we have created a List datatype, which we can verify by using the type() function as we have in previous examples.
>>> type(total_savings)
<class 'list'>
>>> print(total_savings)
['500\n', '200']
Lists on their own are a pretty big topic and we will do a full article only on lists and some of the ways to use them...For this article we will just cover the basics.
To define your own list it is similar to defining a variable, but instead is a list of zero or more values separated by commas and enclosed in square brackets([]). In the code below, you can see we create an empty list by simply adding two empty brackets next to the equal sign.
>>> new_list = []
>>> new_list
[]
>>> new_list = [1, 2, 3, "four", "five"]
>>> new_list
[1, 2, 3, 'four', 'five']
We can reference values in the list using the list name with square brackets and the number of the item we want. Lists start with 0, so in our example, the first value in our list will be provided with the new_list[0] command. By using a colon after the value you are referencing, it will provide you with all the list items until the end of the list.
>>> new_list[0]
1
>>> new_list[0:]
[1, 2, 3, 'four', 'five']
By using two numbers separated by a colon, is referred to as a slice and will provide the values from the start to the end of that slice, not including the last number as we can see below only [1, 2] are returned when we us new_list[0:2]. We can also find the last value of the list by using -1 in the square brackets.
>>> new_list[0:2]
[1, 2]
>>> new_list[-1]
'five'
One of the first ways in which we can manipulate lists is by using the clear() method. As the name implies, all the data in the list is cleared and you are left with an empty List.
>>> new_list.clear()
>>> new_list
[]
The append() method accepts one value between the brackets and adds this value to the end of your list. As you can see we have added a list as the second item in our list. This is called a multidimensional List and we reference values in the second list by adding a second set of square brackets. In our example we are referencing the third item in our second list by using new_list[1][2].
>>> new_list.append("again")
>>> new_list
['again']
>>> new_list.append([1,2,3,4])
>>> new_list
['again', [1, 2, 3, 4]]
>>> new_list[1]
[1, 2, 3, 4]
>>> new_list[1][2]
3
If you want to enter more than one value into the end of a list, instead of having to use append() multiple times, you can simply use extend(). As you can see below, we have used the string "second value" and each character is then added as a separate item into our list.
>>> new_list.extend("second value")
>>> new_list
['again', [1, 2, 3, 4], 's', 'e', 'c', 'o', 'n', 'd', ' ', 'v', 'a', 'l', 'u', 'e']
If you want to know the number of times a specific character, string or number is within your list, you can use the count() method which will provide an integer on the number of times it has found the specific item you have requested.
>>> new_list.count("e")
2
The sort() method, does a sort over your list and changes it to be in that order. As you can see in the example below, we have only used sort for the item at the [1] index of our list, which is another list which is already sorted.
>>> new_list[1].sort()
>>> new_list
['again', [1, 2, 3, 4], 's', 'e', 'c', 'o', 'n', 'd', ' ', 'v', 'a', 'l', 'u', 'e']
The reverse() method will do something similar and will reverse the order of the items in your List:
>>> new_list.reverse()
>>> new_list
['e', 'u', 'l', 'a', 'v', ' ', 'd', 'n', 'o', 'c', 'e', 's', [1, 2, 3, 4], 'again']
The index() method is similar to count() but instead of returned the number of items you are searching for, it will provide you with the index number of the first item it finds, in our case, the letter 'v' at index 4.
>>> new_list.index("v")
4
Lastly we have the insert() method which needs to have two parameters provided to it which are the index to insert an item into the List and the item you want to have added. As you can see in the example below, we are adding the string "test" into the 6 item in our List.
>>> new_list.insert(6, "test")
>>> new_list
['e', 'u', 'l', 'a', 'v', ' ', 'test', 'd', 'n', 'o', 'c', 'e', 's', [1, 2, 3, 4], 'again']
>>> print(total_savings)
['500\n', '200']
If we wanted to get a total of the items in our list we could manually reference the items in the List and then add them together to create a total value. This is a good option, but performing a manual operation could get a little frustrating if we had hundreds of different items in our list.
This is where we can use a Loop to work through our List items
NOTE: If you are not sure about Loops, don't worry, we are going to dedicate our next post to using Loops in Python.
The code below does exactly what we need here. We create a variable called "total" and set the value to 0. We do this so if there are no items in the total_savings list, we will be able to return the value of 0. We then set up what is called a for loop. The line of code "for i in total_savings:" basically says, for every item in the List named total_savings, assign the value of "i" to them. Then perform the code that precedes in the following lines.
The following line is "total += int(i)", which takes our variable named total, and for each item in the List which is assigned to the variable of "i" in the Loop, will be added to the total variable. As you can see we have used the arithmetic operator of +=, which says take the current value of the variable and add the item to the right of the equals sign, or it is short hand in our case for "total = total + int(i)".
>>> total = 0
>>> for i in total_savings:
... total += int(i)
>>> print(total)
700
Lastly, you might have noticed that we are changing the value of "i" to an integer. Because the list is take from a file, it will be a string, which means we need to change it to an integer before we can perform addition on the value.
1 #!/usr/bin/env python
2
3 total = 0
4 f = open("savings.txt", "r")
5 savings_from_file = f.readlines()
6 f.close()
7
8 for i in savings_from_file:
9 total += int(i)
10
11 print("I have saved ${} so far this year".format(total))
We can now run the check_savings.py program again.
python3 check_savings.py
I have saved $820 so far this year
This article has been a lot longer than some of the previous ones so if you've made it this far, thanks for sticking with it. We've created a new program to as the user a question and then place the result into a file. We've introduced Lists and how to work with them, as well as quickly touching on loops. Lastly, we updated the check_savings.py program to now use Lists and Loops to allow it to no longer only read one value from our file.
We'll be back again soon as there is still a lot more information to cover off and we will expand the work we did this week on Loops.