Add optional weights parameter to statistics.fmean()

[rhettinger]

BPO	44150
Nosy	@rhettinger, @stevendaprano
PRs	bpo-44150: Support optional weights parameter for fmean() #26175

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-05-21.03:22:49.037>
created_at = <Date 2021-05-16.18:56:52.909>
labels = ['type-feature', 'library', '3.11']
title = 'Add optional weights parameter to statistics.fmean()'
updated_at = <Date 2021-05-21.03:22:49.037>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2021-05-21.03:22:49.037>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = <Date 2021-05-21.03:22:49.037>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2021-05-16.18:56:52.909>
creator = 'rhettinger'
dependencies = []
files = []
hgrepos = []
issue_num = 44150
keywords = ['patch']
message_count = 1.0
messages = ['393751']
nosy_count = 2.0
nosy_names = ['rhettinger', 'steven.daprano']
pr_nums = ['26175']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue44150'
versions = ['Python 3.11']

[rhettinger]

Weighted averages are another basic that we should support.

A professor assigns a grade for a course by weighting quizzes at 20%, homework at 20%, a midterm exam at 30%, and a final exam at 30%:

>>> grades = [85, 92, 83, 91]
>>> weights = [0.20, 0.20, 0.30, 0.30]
>>> fmean(grades, weights)
87.6

The API here matches that for harmonic_mean() and random.choices(), the other places where we support optional weights.

[pjmcdermott]

From manpage:
"Unless explicitly noted, these functions support int, float, Decimal and Fraction." and (for fmean)
"Convert data to floats and compute the arithmetic mean."

However:

Python 3.12.3 (main, Feb  4 2025, 14:48:35) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import statistics
>>> grades = [85, 92, 83, 91]
>>> weights = [0.20, 0.20, 0.30, 0.30]
>>> statistics.fmean(grades, weights)
87.6
>>> from decimal import Decimal
>>> grades = [Decimal("85"), Decimal("92"), Decimal("83"), Decimal("91")]
>>> statistics.fmean(grades, weights)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.12/statistics.py", line 517, in fmean
    num = sumprod(data, weights)
          ^^^^^^^^^^^^^^^^^^^^^^
TypeError: unsupported operand type(s) for *: 'decimal.Decimal' and 'float'

statistics.fmean does not do what is says on the tin