Python Programming for Biology: Bioinformatics and Beyond

Using variables

As  we  have  already  illustrated,  we  can  create  a  named  item,  which  here  we  call  ‘x’  for

simplicity, and assign it to a value:

>>> x = 17

Using the above example we can introduce some more jargon computing terms. On the

left-hand side, before the equals sign, we have a variable. On the right-hand side we have

a literal. The whole line here is a statement and specifies that the variable is set to have

value equal to the literal. As you might expect from the term ‘variable’, the value of x may

be changed by assigning a new value:

>>> print(x)

17

>>> x = 3

>>> print(x)

3

In Python the names we give variable data can contain only the usual 26 letters (upper

and  lower  case),  numbers  and  underscores  (‘_‘),  with  the  additional  restriction  that  they

cannot  begin  with  a  number.  Note  that  names  are  case-sensitive,  so  the  variables  DNA,

Dna  and  dna  are  all  treated  as  distinct.  In  general,  variables  should  have  names  that

indicate  what  their  purpose  and/or  type  are,  in  order  to  make  the  code  more

understandable. For example, if you state freeEnergy = heat-entropy and not x = p-q, you

can  see  at  a  glance  what  is  intended  and  have  your  program  more  easily  understood,

including by yourself at a later date, without any additional comments.

We can use as many different variable names as we like and assign their value based on

other  variables.  For  example,  in  the  following  we  assign  a  value  to  x  and  then  assign  a

value for y based on x:

>>> x = 17

>>> y = x * 13

>>> print(y)

221

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Unlike many computing languages, Python is not a language where you must initially

specify,  and  then  stick  to,  a  given  kind  or  type  of  data  for  a  given  variable.  You  could

initially  allocate  a  numeric  value  to  ‘x’,  without  advance  warning,  and  then  later  on

change ‘x’ to some text. This differs from languages like C and Java, for example, where

you  would  have  to  declare  up  front  what  type  of  data  ‘x’  was  to  contain.  In  Python,  the

type of variable is specified by the type of whatever its value is set to. So if you redefine a

variable  its  type  may  change.  Although  variables  can  change  type,  it  is  usually  best  to

avoid that practice.

>>> x = 4 # x is set to the integer 4

>>> 3*x

12

>>> x = 7.1 # x now set to the floating point number 7.1

>>> 3*x

21.299999999999997

The above example reminds us that floating point calculations are not always precise.

The answer could also depend on the Python implementation and version.