Python Programming for Biology: Bioinformatics and Beyond

Comparisons and truth

The  question  naturally  arises  as  to  which  expressions  are  deemed  to  be  true  and  which

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false. Naturally, for the special Python logic items named True and False (often referred to

as Boolean values)  the  answer  is  obvious.  Also,  the  logical  states  of  truth  and  falsehood

that result from conditional checks like ‘Is x greater than 5?’ or ‘Is y in this list?’ are also

clear. However, in Python even expressions that do not involve an obvious query can be

assigned a status of conditional truthfulness; the value of an item itself can be forced to be

considered  as  either  True  or  False  inside  an  if  statement.  For  example,  when  tested  in  a

conditional statement the number 1 will be interpreted as being True and the number 0 as

False,  although  their  values  are  otherwise  numeric.  For  the  Python  built-in  data  types

discussed in this chapter the following are deemed to be False in such a context:

None # nothingness

False # False Boolean

0 # 0 integer

0.0 # 0.0 floating point

"" # empty string

() # empty tuple

[] # empty list

{} # empty dictionary

set(()) # empty set

frozenset(()) # empty frozen set

And everything else is deemed to be True in a conditional context.

Python has the standard comparison (or relational) operators:

x > y # x greater than y?

x < y # x less than y?

x >= y # x greater than or equal to y?

x <= y # x less than or equal to y?

x == y # x equal to y?

x != y # x not equal to y?

In  terms  of  syntax,  in  Python  2  you  can  compare  anything  with  anything,  although  if

the  expressions  are  not  of  the  same  data  type  the  result  is  rather  meaningless  in  some

sense. And it is notable that in Python 3 you get an exception (error) if you try to use any

of  the  comparison  operators,  other  than  ==  and  !=,  to  compare  expressions  that  do  not

have a natural comparison, like when comparing numbers with strings. A consequence of

this is that in Python 3 you cannot sort the elements of a list, according to value, where the

items have data types that are not comparable, because they cannot be ascribed as being

greater than or less than one another.

Python has two additional comparison operators:

x is y # rather than x == y

x is not y # rather than x != y

The difference between the two is that == and != compare the values while is and not is

compare whether objects are identical. Everything in Python is an object, and you might

be interested in whether two objects have the same value or are really the same object, a

stronger condition. As an analogy in the real world, two people might have the same name

but  even  if  they  have  the  same  name  it  does  not  mean  they  are  the  same  person.  As  an

example in Python:

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x = [123, 54, 92, 87, 33] # x defined as a list

y = x[:] # y is a copy of x

y == x # True – same value

y is x # False – different object