functools.partial: don't copy keywoard arguments in partial_call()?

[vstinner]

BPO	27840
Nosy	@rhettinger, @pitrou, @vstinner, @berkerpeksag, @serhiy-storchaka
PRs	Document why functools.partial() must copy kwargs #253
Files	partial_call_kwargs.patch

^{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 2017-02-27.12:36:56.749>
created_at = <Date 2016-08-23.14:34:03.378>
labels = ['performance']
title = "functools.partial: don't copy keywoard arguments in partial_call()?"
updated_at = <Date 2017-02-27.12:36:56.748>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2017-02-27.12:36:56.748>
actor = 'berker.peksag'
assignee = 'none'
closed = True
closed_date = <Date 2017-02-27.12:36:56.749>
closer = 'berker.peksag'
components = []
creation = <Date 2016-08-23.14:34:03.378>
creator = 'vstinner'
dependencies = []
files = ['44199']
hgrepos = []
issue_num = 27840
keywords = ['patch']
message_count = 6.0
messages = ['273456', '273592', '273691', '273697', '288454', '288639']
nosy_count = 5.0
nosy_names = ['rhettinger', 'pitrou', 'vstinner', 'berker.peksag', 'serhiy.storchaka']
pr_nums = ['253']
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue27840'
versions = ['Python 3.6']

[vstinner]

I justed optimized partial_call() for positional arguments in the change c1a698edfa1b to avoid the creation of a temporary tuple when possible.

I noticed that keyword parameters from partial() constructor are always copied. Is it mandatory? Can't we avoid copying them?

Example:
---

import functools
hello = functools.partial(print, "Hello World", end='!\n')
hello()

---

hello keyword arguments are {'end'; '!\n'}.

Attached patch avoids copying keyword arguments when the partial objects is not called with new keyword arguments.

Tests pass, but I don't know if there is a risk that some strange function modify keyword arguments in-place?

[pitrou]

Modifying the keyword arguments dict is quite common so this change would be a heavy compatibility breaker.

A well-known idiom:

  def some_function(..., **kwargs):
      some_option = kwargs.pop('some_option', None)
      # further process kwargs

[rhettinger]

I concur with Antoine.

[vstinner]

Ah yes sorry, I misunderstood the link between kwargs from a C function and
a PY function. The link is simple, it's the same object :-) This issue can
be closed.

[vstinner]

I reopen the issue to propose to add a comment explaining why the dictionary must always be copied:
#253

[berkerpeksag]

PR 253 has been merged.