#33465: Introduce empty __slots__ protocol for SafeString & SafeData
-------------------------------------+-------------------------------------
Reporter: Keryn | Owner: Keryn Knight
Knight |
Type: | Status: assigned
Cleanup/optimization |
Component: Utilities | Version: dev
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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This is a case-by-case proposal ultimately referencing #12826
Because `SafeString` is used ''a lot'' and is otherwise supposed to be
treatable as a untainted `str` we should be able to (AFAIK) update it +
it's inheritance chain to use `__slots__ = ()` whilst still allowing
custom subclasses of either to add additional attributes. By defining
`__slots__` as empty on `SafeString` (**and** `SafeData`) we'd avoid
creation of a `__dict__` on the instance, which mirrors the `str()`
behaviour.
According to pympler, currently in Python `3.10` using the following back
of the napkins strings:
{{{
In [4]: s = "test" # this might be interned, as a short string?
In [5]: s2 = "test" * 100
In [6]: s3 = SafeString("test")
In [7]: s4 = SafeString("test" * 100)
}}}
we get:
{{{
In [8]: asizeof(s) # str
Out[8]: 56
In [9]: asizeof(s2) # str
Out[9]: 456
In [10]: asizeof(s3) # SafeString
Out[10]: 208
In [11]: asizeof(s4) # SafeString
Out[11]: 608
}}}
But if we swap out the implementation to be slots'd, it looks more like:
{{{
In [8]: asizeof(s) # str
Out[8]: 56
In [9]: asizeof(s2) # str
Out[9]: 456
In [10]: asizeof(s3) # SafeString
Out[10]: 104
In [11]: asizeof(s4) # SafeString
Out[11]: 504
}}}
So we're "saving" `104 bytes` per `SafeString` created, by the look of it.
I presume it to be some fun implementation detail of something somewhere
that it is allegedly accounting for more than `56` bytes, which is the
`asizeof({})`
A quick and dirty check over the test suite suggests that for me locally,
running `14951 tests in 512.912s` accounted for `949.0 MB` of SafeStrings,
checked by just incrementing a global integer of bytes (using
`SafeString.__new__` and `--parallel=1`) and piping that to
`filesizeformat`, so y'know, ''room for error''.
After the patch, the same tests accounted for `779.4 MB` of `SafeString`,
"saving" `170 MB` overall.
The only functionality this would preclude -- as far as I know -- is no
longer being able to bind arbitrary values to an instance like so:
{{{
s = SafeString('test')
s.test = 1
}}}
which would raise `AttributeError` if `__slots__` were added, just like
trying to assign attributes to `str()` directly does.
I don't believe this will have any marked performance change, as neither
`SafeString` nor `SafeData` actually have any extra attributes, only
methods.
I have a branch which implements this, and tests pass for me locally.
--
Ticket URL: <https://code.djangoproject.com/ticket/33465>
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