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NEP: Create NEP 32 for removing the finanical functions.
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| ================================================== | ||
| NEP 32 — Remove the financial functions from NumPy | ||
| ================================================== | ||
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| :Author: Warren Weckesser <[email protected]> | ||
| :Status: Draft | ||
| :Type: Standards Track | ||
| :Created: 2019-08-30 | ||
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| Abstract | ||
| -------- | ||
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| We propose deprecating and ultimately removing the financial functions [1]_ | ||
| from NumPy. The functions will be moved to an independent repository, | ||
| and provided to the community as a separate package with the name | ||
| ``numpy_financial``. | ||
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| Motivation and scope | ||
| -------------------- | ||
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| The NumPy financial functions [1]_ are the 10 functions ``fv``, ``ipmt``, | ||
| ``irr``, ``mirr``, ``nper``, ``npv``, ``pmt``, ``ppmt``, ``pv`` and ``rate``. | ||
| The functions provide elementary financial calculations such as future value, | ||
| 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 | ||
| the financial functions triggered a long thread about these functions [3]_. | ||
| One important point that came up in that thread is that a "real" financial | ||
| library must be able to handle real dates. The NumPy financial functions do | ||
| not work with actual dates or calendars. The preference for a more capable | ||
| 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 | ||
| implementations of some of the financial functions [4]_. It was suggested then | ||
| 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 | ||
| functions from the top-level namespace to ``numpy.financial``. He also | ||
| suggested giving the functions better names. Responses at that time included | ||
| the suggestion to deprecate them and move them from NumPy to a separate | ||
| package. This issue is still open. | ||
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| Later in 2013 [6]_, it was suggested on the mailing list that these functions | ||
| 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. | ||
| * They are not actually useful for "real world" financial calculations, because | ||
| they do not handle real dates and calendars. | ||
| * The definition of "correctness" for some of these functions seems to be a | ||
| matter of convention, and the current NumPy developers do not have the | ||
| background to judge their correctness. | ||
| * There has been little interest among past and present NumPy developers | ||
| 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 | ||
| will have to add the new ``numpy_financial`` package to their dependencies, | ||
| and then modify their code to use the new package. | ||
| * The functions provided, while not "industrial strength", are apparently | ||
| similar to functions provided by spreadsheets and some calculators. Having | ||
| them available in NumPy makes it easier for some developers to migrate their | ||
| software to Python and NumPy. | ||
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| It is clear from comments in the mailing list discussions and in the GitHub | ||
| issues that many current NumPy developers believe the benefits of removing | ||
| the functions outweigh the costs. For example, from [5]_:: | ||
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| The financial functions should probably be part of a separate package | ||
| -- Charles Harris | ||
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| If there's a better package we can point people to we could just deprecate | ||
| them and then remove them entirely... I'd be fine with that too... | ||
| -- Nathaniel Smith | ||
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| +1 to deprecate them. If no other package exists, it can be created if | ||
| someone feels the need for that. | ||
| -- Ralf Gommers | ||
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| I feel pretty strongly that we should deprecate these. If nobody on numpy’s | ||
| core team is interested in maintaining them, then it is purely a drag on | ||
| development for NumPy. | ||
| -- Stephan Hoyer | ||
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| And from the 2013 mailing list discussion, about removing the functions from | ||
| NumPy:: | ||
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| I am +1 as well, I don't think they should have been included in the first | ||
| place. | ||
| -- 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 | ||
| written. If we made an effort (putting up pages with examples of common | ||
| financial calculations and collecting those under a topical web page, | ||
| then linking to that page from various places and talking it up), I | ||
| would think they could attract users looking for a free way to play with | ||
| financial scenarios. [...] | ||
| So, I would say we keep them. If ours are not the best, we should bring | ||
| them up to snuff. | ||
| -- Joe Harrington | ||
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| For an idea of the maintenance burden of the financial functions, one can | ||
| look for all the GitHub issues [7]_ and pull requests [8]_ that have the tag | ||
| ``component: numpy.lib.financial``. | ||
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| One method for measuring the effect of removing these functions is to find | ||
| all the packages on GitHub that use them. Such a search can be performed | ||
| with the ``python-api-inspect`` service [9]_. A search for all uses of the | ||
| NumPy financial functions finds just eight repositories. (See the comments | ||
| in [5]_ for the actual SQL query.) | ||
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| Implementation | ||
| -------------- | ||
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| * Create a new Python package, ``numpy_financial``, to be maintained in the | ||
| top-level NumPy github organization. This repository will contain the | ||
| definitions and unit tests for the financial functions. The package will | ||
| be added to PyPI so it can be installed with ``pip``. | ||
| * Deprecate the financial functions in the ``numpy`` namespace, beginning in | ||
| NumPy version 1.18. Remove the financial functions from NumPy version 1.20. | ||
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| Backward compatibility | ||
| ---------------------- | ||
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| The removal of these functions breaks backward compatibility, as explained | ||
| earlier. The effects are mitigated by providing the ``numpy_financial`` | ||
| library. | ||
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| Alternatives | ||
| ------------ | ||
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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).* | ||
| A review of the history makes clear that this is not the preference of many | ||
| NumPy developers. A recurring comment is that the functions simply do not | ||
| belong in NumPy. When that sentiment is combined with the history of bug | ||
| reports and the ongoing questions about the correctness of the functions, the | ||
| conclusion is that the cleanest solution is deprecation and removal. | ||
| * *Move the functions from the ``numpy`` namespace to ``numpy.financial``.* | ||
| This was the initial suggestion in [5]_. Such a change does not address the | ||
| maintenance issues, and doesn't change the misfit that many developers see | ||
| between these functions and NumPy. It causes disruption for the current | ||
| users of these functions without addressing what many developers see as the | ||
| fundamental problem. | ||
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| Discussion | ||
| ---------- | ||
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| Links to past mailing list discussions, and to relevant GitHub issues and pull | ||
| requests, have already been given. | ||
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| References and footnotes | ||
| ------------------------ | ||
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| .. [1] Financial functions, | ||
| https://numpy.org/doc/1.17/reference/routines.financial.html | ||
| .. [2] Numpy-discussion mailing list, "Simple financial functions for NumPy", | ||
| https://mail.python.org/pipermail/numpy-discussion/2008-April/032353.html | ||
| .. [3] Numpy-discussion mailing list, "add xirr to numpy financial functions?", | ||
| https://mail.python.org/pipermail/numpy-discussion/2009-May/042645.html | ||
| .. [4] Numpy-discussion mailing list, "Definitions of pv, fv, nper, pmt, and rate", | ||
| https://mail.python.org/pipermail/numpy-discussion/2009-June/043188.html | ||
| .. [5] Get financial functions out of main namespace, | ||
| https://github.com/numpy/numpy/issues/2880 | ||
| .. [6] Numpy-discussion mailing list, "Deprecation of financial routines", | ||
| https://mail.python.org/pipermail/numpy-discussion/2013-August/067409.html | ||
| .. [7] ``component: numpy.lib.financial`` issues, | ||
| https://github.com/numpy/numpy/issues?utf8=%E2%9C%93&q=is%3Aissue+label%3A%22component%3A+numpy.lib.financial%22+ | ||
| .. [8] ``component: numpy.lib.financial`` pull request, | ||
| https://github.com/numpy/numpy/pulls?utf8=%E2%9C%93&q=is%3Apr+label%3A%22component%3A+numpy.lib.financial%22+ | ||
| .. [9] Quansight-Labs/python-api-inspect, | ||
| https://github.com/Quansight-Labs/python-api-inspect/ | ||
| Copyright | ||
| --------- | ||
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| This document has been placed in the public domain. |