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NEP 32 — Remove the financial functions from NumPy
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==================================================
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:Author: Warren Weckesser <warren.weckesser@gmail.com>
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:Status: Accepted
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:Type: Standards Track
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:Created: 2019-08-30
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:Resolution: https://mail.python.org/pipermail/numpy-discussion/2019-September/080074.html
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Abstract
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--------
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We propose deprecating and ultimately removing the financial functions [1]_
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from NumPy. The functions will be moved to an independent repository,
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and provided to the community as a separate package with the name
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``numpy_financial``.
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Motivation and scope
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--------------------
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The NumPy financial functions [1]_ are the 10 functions ``fv``, ``ipmt``,
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``irr``, ``mirr``, ``nper``, ``npv``, ``pmt``, ``ppmt``, ``pv`` and ``rate``.
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The functions provide elementary financial calculations such as future value,
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net present value, etc. These functions were added to NumPy in 2008 [2]_.
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In May, 2009, a request by Joe Harrington to add a function called ``xirr`` to
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the financial functions triggered a long thread about these functions [3]_.
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One important point that came up in that thread is that a "real" financial
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library must be able to handle real dates. The NumPy financial functions do
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not work with actual dates or calendars. The preference for a more capable
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library independent of NumPy was expressed several times in that thread.
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In June, 2009, D. L. Goldsmith expressed concerns about the correctness of the
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implementations of some of the financial functions [4]_. It was suggested then
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to move the financial functions out of NumPy to an independent package.
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In a GitHub issue in 2013 [5]_, Nathaniel Smith suggested moving the financial
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functions from the top-level namespace to ``numpy.financial``. He also
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suggested giving the functions better names. Responses at that time included
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the suggestion to deprecate them and move them from NumPy to a separate
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package. This issue is still open.
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Later in 2013 [6]_, it was suggested on the mailing list that these functions
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be removed from NumPy.
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The arguments for the removal of these functions from NumPy:
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* They are too specialized for NumPy.
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* They are not actually useful for "real world" financial calculations, because
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they do not handle real dates and calendars.
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* The definition of "correctness" for some of these functions seems to be a
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matter of convention, and the current NumPy developers do not have the
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background to judge their correctness.
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* There has been little interest among past and present NumPy developers
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in maintaining these functions.
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The main arguments for keeping the functions in NumPy are:
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* Removing these functions will be disruptive for some users. Current users
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will have to add the new ``numpy_financial`` package to their dependencies,
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and then modify their code to use the new package.
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* The functions provided, while not "industrial strength", are apparently
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similar to functions provided by spreadsheets and some calculators. Having
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them available in NumPy makes it easier for some developers to migrate their
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software to Python and NumPy.
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It is clear from comments in the mailing list discussions and in the GitHub
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issues that many current NumPy developers believe the benefits of removing
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the functions outweigh the costs. For example, from [5]_::
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The financial functions should probably be part of a separate package
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-- Charles Harris
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If there's a better package we can point people to we could just deprecate
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them and then remove them entirely... I'd be fine with that too...
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-- Nathaniel Smith
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+1 to deprecate them. If no other package exists, it can be created if
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someone feels the need for that.
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-- Ralf Gommers
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I feel pretty strongly that we should deprecate these. If nobody on numpy’s
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core team is interested in maintaining them, then it is purely a drag on
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development for NumPy.
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-- Stephan Hoyer
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And from the 2013 mailing list discussion, about removing the functions from
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NumPy::
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I am +1 as well, I don't think they should have been included in the first
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place.
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-- David Cournapeau
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But not everyone was in favor of removal::
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The fin routines are tiny and don't require much maintenance once
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written. If we made an effort (putting up pages with examples of common
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financial calculations and collecting those under a topical web page,
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then linking to that page from various places and talking it up), I
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would think they could attract users looking for a free way to play with
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financial scenarios. [...]
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So, I would say we keep them. If ours are not the best, we should bring
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them up to snuff.
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-- Joe Harrington
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For an idea of the maintenance burden of the financial functions, one can
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look for all the GitHub issues [7]_ and pull requests [8]_ that have the tag
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``component: numpy.lib.financial``.
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One method for measuring the effect of removing these functions is to find
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all the packages on GitHub that use them. Such a search can be performed
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with the ``python-api-inspect`` service [9]_. A search for all uses of the
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NumPy financial functions finds just eight repositories. (See the comments
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in [5]_ for the actual SQL query.)
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Implementation
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--------------
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* Create a new Python package, ``numpy_financial``, to be maintained in the
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top-level NumPy github organization. This repository will contain the
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definitions and unit tests for the financial functions. The package will
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be added to PyPI so it can be installed with ``pip``.
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* Deprecate the financial functions in the ``numpy`` namespace, beginning in
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NumPy version 1.18. Remove the financial functions from NumPy version 1.20.
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Backward compatibility
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----------------------
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The removal of these functions breaks backward compatibility, as explained
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earlier. The effects are mitigated by providing the ``numpy_financial``
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library.
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Alternatives
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------------
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The following alternatives were mentioned in [5]_:
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* *Maintain the functions as they are (i.e. do nothing).*
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A review of the history makes clear that this is not the preference of many
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NumPy developers. A recurring comment is that the functions simply do not
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belong in NumPy. When that sentiment is combined with the history of bug
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reports and the ongoing questions about the correctness of the functions, the
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conclusion is that the cleanest solution is deprecation and removal.
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* *Move the functions from the ``numpy`` namespace to ``numpy.financial``.*
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This was the initial suggestion in [5]_. Such a change does not address the
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maintenance issues, and doesn't change the misfit that many developers see
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between these functions and NumPy. It causes disruption for the current
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users of these functions without addressing what many developers see as the
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fundamental problem.
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Discussion
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----------
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Links to past mailing list discussions, and to relevant GitHub issues and pull
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requests, have already been given. The announcement of this NEP was made on
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the NumPy-Discussion mailing list on 3 September 2019 [10]_, and on the
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PyData mailing list on 8 September 2019 [11]_. The formal proposal to accept
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the NEP was made on 19 September 2019 [12]_; a notification was also sent to
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PyData (same thread as [11]_). There have been no substantive objections.
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References and footnotes
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------------------------
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.. [1] Financial functions,
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https://numpy.org/doc/1.17/reference/routines.financial.html
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.. [2] Numpy-discussion mailing list, "Simple financial functions for NumPy",
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https://mail.python.org/pipermail/numpy-discussion/2008-April/032353.html
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.. [3] Numpy-discussion mailing list, "add xirr to numpy financial functions?",
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https://mail.python.org/pipermail/numpy-discussion/2009-May/042645.html
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.. [4] Numpy-discussion mailing list, "Definitions of pv, fv, nper, pmt, and rate",
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https://mail.python.org/pipermail/numpy-discussion/2009-June/043188.html
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.. [5] Get financial functions out of main namespace,
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https://github.com/numpy/numpy/issues/2880
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.. [6] Numpy-discussion mailing list, "Deprecation of financial routines",
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https://mail.python.org/pipermail/numpy-discussion/2013-August/067409.html
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.. [7] ``component: numpy.lib.financial`` issues,
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https://github.com/numpy/numpy/issues?utf8=%E2%9C%93&q=is%3Aissue+label%3A%22component%3A+numpy.lib.financial%22+
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.. [8] ``component: numpy.lib.financial`` pull requests,
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https://github.com/numpy/numpy/pulls?utf8=%E2%9C%93&q=is%3Apr+label%3A%22component%3A+numpy.lib.financial%22+
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.. [9] Quansight-Labs/python-api-inspect,
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https://github.com/Quansight-Labs/python-api-inspect/
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.. [10] Numpy-discussion mailing list, "NEP 32: Remove the financial functions
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from NumPy"
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https://mail.python.org/pipermail/numpy-discussion/2019-September/079965.html
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.. [11] PyData mailing list (pydata@googlegroups.com), "NumPy proposal to
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remove the financial functions.
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https://mail.google.com/mail/u/0/h/1w0mjgixc4rpe/?&th=16d5c38be45f77c4&q=nep+32&v=c&s=q
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.. [12] Numpy-discussion mailing list, "Proposal to accept NEP 32: Remove the
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financial functions from NumPy"
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https://mail.python.org/pipermail/numpy-discussion/2019-September/080074.html
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Copyright
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---------
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This document has been placed in the public domain.
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