How Does Python Compare Lists with Greater Than and Less Than Operators?

Comparing Lists with the Greater Than or Less Than Operator: Lexicographical Ordering Unsurprises

Upon encountering code comparing lists directly using the greater than or less than operator (> and <), you might assume that such comparisons return True if all elements in the first list are greater than those in the second list, and False otherwise. However, testing reveals unexpected results.

To unravel this mystery, we turn to Python's documentation on Comparing Sequences and Other Types. It states that such comparisons follow lexicographical ordering, not element-by-element comparisons:

• The first two elements of the lists are compared. If they differ, this determines the outcome.
• If the first elements are equal, the next two elements are compared.
• This process continues until one list runs out of elements.

Consider the following example:

a = [3, 3, 3, 3]
b = [4, 4, 4, 4]

Since the first elements (3 and 4) differ, b is considered greater than a. This aligns with our assumption that all elements in b are greater than those in a.

However, the following case illustrates the lexicographical ordering rule more clearly:

a = [1, 1, 3, 1]
b = [1, 3, 1, 1]

Since the first elements (1) are equal, the comparison moves on to the next elements. In this case, the second element of a (1) is less than the second element of b (3). Therefore, despite the fact that a has more elements greater than 1 than b does, b is considered greater than a.

In summary, when comparing lists using the greater than or less than operator, Python employs lexicographical ordering, rather than element-by-element comparisons. This can lead to unexpected results, especially when lists contain elements of different values.

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