Fix base case handling in quantiles()

[rhettinger]

Fix an inconvenience in quantiles() by supporting input lists of length one, much like min(), max(), mean() and median() also support datasets of size one.

The principal use case is making statistical summaries of data streams. It is really inconvenient to require a special case for the first data point. Instead, it is much nicer to make updates as new data arrives, starting with the very first datum.

This is what we want:

"Running five number summary for a data stream"
# https://en.wikipedia.org/wiki/Five-number_summary

from statistics import quantiles
import random

stream = (random.expovariate() for i in range(20))
data = []
for x in stream:
    data.append(x)
    print(min(data), quantiles(data), max(data))

Linked PRs

• gh-110150: Fix base case handling in quantiles() #110151