4.3. 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 must start with a letter
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.

4.3.1. 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 \(n\).

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