python - Comparing two lists using the greater than or less than operator

Question

I noticed a piece of code recently directly comparing two lists of integers like so:

a = [10,3,5, ...]
b = [5,4,3, ...,]
if a > b:
     ...

which seemed a bit peculiar, but I imagined it would return True if all of list_a's elements are larger then list_b's and False if each element is equal or list_b's elements are larger then list_a's. So I tested it:

>>> a=[3,3,3,3]
>>> b=[4,4,4,4]
>>> a>b
False
>>> b>a
True

Ok that works. As does:

>>> b = [1,1,1,1]
>>> a = [1,1,1,1]
>>> a>b
False
>>> b>a
False

but when it gets more fuzzy:

>>> a=[1,1,3,1]
>>> b=[1,3,1,1]
>>> a>b
False
>>> b>a
True

or:

>>> a=[1,3,1,1]
>>> b=[1,1,3,3]
>>> a>b
True
>>> b>a
False

the results are a bit stranger. What is python actually doing? It seems that it's returning the result in favour of the first list in which the left most element is greater then the corresponding?

question from:https://stackoverflow.com/questions/13052857/comparing-two-lists-using-the-greater-than-or-less-than-operator

Answer 1

From Comparing Sequences and Other Types in the Python tutorial:

The comparison uses lexicographical ordering: first the first two items are compared, and if they differ this determines the outcome of the comparison; if they are equal, the next two items are compared, and so on, until either sequence is exhausted.

See also the Wikipedia article about lexicographical order.