# Variables

## Variables

Variables are assigned in Python using the = equals sign also called the assignment operator. The statement:

a = 2


Assigns the integer 2 to the variable a.

>>> a = 2
>>> a
2


Note the assignment operator =(equals), is different from the logical comparison operator == (equivalent to).

>>> a == 2
True


Variable names in Python must conform to the following rules:

• variable names can only contain letters, numbers, and the underscore character _
• variable names can not contain spaces
• variable names can not include punctuation
• variable names are not enclosed in quotes or brackets The following code lines show valid variable names:
constant = 4

new_variable = 'var'

my2rules = ['rule1','rule2']

SQUARES = 4


The following code lines show invalid variable names:

a constant = 4

3newVariables = [1, 2, 3]

&sum = 4 + 4


Let's solve the problem below at the Python REPL using variables.

#### Problem

The Arrhenius relationship states:

n = n_{v}e^{-Q_v/(RT)}

In a system where $n_v = 2.0 \times 10^{-3}$, $Q_v = 5$, $R=3.18$, and $T=293$, calculate $n$.

#### Solution

Use variables to assign a value to each one of the constants in the problem and calculate $n$.

>>> nv = 2.0e-3
>>> Qv = 5
>>> R = 3.18
>>> T = 293
>>> from math import exp
>>> n = nv*exp(-1*Qv/(R*T))
>>> n
0.0019892961379660424