Variables

Sometimes we want to refer to the same quantity repeatly throughout our code. For example, the sample size $n$ in introductory statistics is used to calculate the average, the variance of the average, the cost of collecting the sample, and the expected number of records in your database. To avoid inconsistencies, we should use variables.

The main benefits of using variables

• Consistent value will be used by avoiding typos and changes at the top will change all following instances of the same value.
• Code would be easier to read if your variable is named sample_size instead of n

How to define a variable

To define a variable, simply use the = and have the variable name on the left and the value you want to assign on the right.

In the following example we would say “we’ve assigned the value 111 to the variable sample_size”

sample_size = 111

Order of operations - assignment happens last

The right-hand-side expression will always be evaluated before the assignment operation, e.g.

x = 3 * 4

If = happened before *, then x could be 3. Fortunately, the assignment operation is always last so x will take on the value 12

Some convenient features in Python

Sometimes we want to have the same default value for some variables (e.g. 0)

Python allows you to define variables in a chain

a = b = 2

Does this work?

a = 2 = b

Restrictions when defining variables

There are restrictions on what your variable name can be.

One of these won’t work

a3 = 1
3a = 2
_a = 3
_ = 4

There are other restrictions, like special keywords, but the key is to try it if you’re not sure.

Something we take for granted

In some programming languages, you have to define the “type” of the variable when you define it. This isn’t true for Python which makes it easier to read but also allows mistakes to happen more easily, e.g. different scripts can assign different values to the same variable name.

a = 1
a = '1'

Always always always spend some time to think about variable names

Naming variables is sometimes considered one of the hardest things to do in programming. Achieving concise and meaningful names is hard. This is especially true in large projectds so you should practice early.

Convention around _

Some people use the _ to prevent information printed in the REPL since algorithms can print a lot of information that swamps your REPL.

The underscore is often used by programmers to indicate something should remain hidden or ignored when reading the code. The following is an example of this. Don’t worry about not following the code (you’ll understand in the next topic!) This is just to show that _ are used commonly in Python but you should just treat it like a letter.

a = "hello"
a.upper()
a.__len__()

Looking up all variable names

The following 2 functions will inform you on the variable names that are defined in your local or global environment by returning a dictionary. Topics like local environment and dictionary will be covered later.

locals()
globals()

Additional References

• LearnPython.org

Something you may not understand yet…

a = 3
b = a
print(a)
print(b)
b = 4
print(a)
print(b)

a = [1, 2]
print(a)
b = a
b[0] = -1
print(a)
print(b)
b = 3
print(a)
print(b)