cdaz5 / python-variables-readme-data-science-alpha

Naming things with Variables

Introduction

"There are only two hard things in Computer Science: cache invalidation and naming things."

-- Phil Karlton

"...But ordinary language is all right."

Ludwig Wittgenstein

Objectives

• Learn about how to use variables to give meaning to data
• Learn how to assign a variable to data
• Learn how to declare a variable
• Learn how to reassign a variable

Declaring and Assigning Variables

So far we have only worked with data -- strings, numbers, and booleans. In this lesson, we'll learn how to use variables to assign names to this data. For example, this is a string from our Working with Data Types Lab.

"art vandelay"

'art vandelay'

Now months later, if we see that string in some code, we may be confused as to what it's about. And when we add more data, this only becomes more difficult. Think of the what we saw in our Data Types Lab: "[e-mail]", "Ceo", "7285553334", "vandelay.com". There's a lot to keep track of.

So let's use a variables to indicate what each of these strings mean.

email = "[e-mail]"

For this, and all of the subsequent code in gray boxes, you should press shift + enter to ensure that the code executes. If you do not do so with the line above for example, then email when we reference email in the lines that follow, Jupyter will throw an error indicating that the variable is undefined. So it is not enough to just type the correct code, we need to run shift + enter on our gray boxes to run this code.

In programming terms, we say that just "declared a variable email and assigned it to the string "[e-mail]"". And to do so, we'll follow the procedure above:

variable = data

Now that we have assigned a variable email to a string, we just type the word email to see the string again.

email

Press shift + enter on the gray box above to see what email equals.

Ok, let's do this with the website too.

website = "vandelay.com"
website

Note that if you introduce a new variable, (declare it), but do not also assign it in the same line, Python will raise an error.

name

----------------------------------------------------------

NameError                Traceback (most recent call last)

<ipython-input-6-9bc0cb2ed6de> in <module>()
----> 1 name


NameError: name 'name' is not defined

So that error tells us that name is not defined. We just fix this by declaring name and assigning the variable in the same line.

name = 'Art Vandalay'
name

So this is assigning and reading a variable. And when we want to see some information again, we can easily find out.

email

Reassigning variables

Now that we have this data, you can imagine using it for some instructions. For example, say you want to write some yourself a memo on who and how to reach out to someone you just met. Here's the message:

"Send an email to Art Vandalay at '[e-mail]' to tell say how nice it was meeting yesterday."

If we construct this message with variables, we can write the following:

"Send an email to " + name + " at " + email +  " to say how nice it was meeting yesterday."

Now you meet someone else, "Liz Kaplan" with the email of "[e-mail]" and want to write a memo with the same instructions, but the only thing that varies are the name and email. So then this is easy enough. First we change set the variables name and email equal to different data.

name = 'Liz Kaplan'
email = '[e-mail]'

So as you can see, we reassign our variable by just setting variable = 'new data'. And our variable is then updated.

name # 'Liz Kaplan'

email # '[e-mail]'

And now, if we copy and run our previous code again, it is automatically updated.

"Send an email to " + name + " at " + email +  " to tell him how nice it was meeting him yesterday."

So in the line above, we are getting to some of the real power of programming. By choosing the correct variable name, we can begin to change the values of name or email and operate on their underlying values in the same ways.

Operating on variables

Just to hammer this point home let's see what we can now do with the name variable.

name

name.upper()

name.title()

So just like we directly call methods on a string, we can also call methods on a variable that points to a string. And, if try to call a method on something that you think is a string, but really is a number, you will see an error.

name = 42

name.upper()

Just like we would recieve that error from calling on number 42 more easily. So now, that we are working with variables, you may run into errors where you think a variable is one thing, but really it is something else. But it's no big deal. We just see what the variable is.

name

And make the change.

name = 'Liz Kaplan'
name

Summary

In this lesson, we got a taste for what makes computer programs so powerful. By using variables, we can write programs that know how to combine data. This can save us time by avoiding boring, repetitive tasks. We declare and assign a variable with the pattern of variable = data. And reassign a variable with the same pattern. To refernece a variable, we simply type the variable's name.

We also saw that one of the things to pay attention to when working with variables is that they are sometimes different from what we expect. So we just type the name of the variable, to see what it really is and make the change.