#35603: "string" in F("...") hangs
-------------------------------------+-------------------------------------
Reporter: Tim Graham | Owner: (none)
Type: Bug | Status: new
Component: Database layer | Version: 5.1
(models, ORM) |
Severity: Release blocker | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by Simon Charette):
* stage: Unreviewed => Accepted
Comment:
The issue lies in implementing `__getitem__` without implementing
`__contains__`, per
[https://docs.python.org/3/reference/datamodel.html#object.__contains__
the Python docs]
> For objects that don’t define `__contains__()`, the membership test
first tries iteration via `__iter__()`, **then the old sequence iteration
protocol via `__getitem__()`, see
[https://docs.python.org/3/reference/expressions.html#membership-test-
details this section in the language reference]**.
Basically what `"foo" in F("name")` translates to is the following ''old-
style iteration protocol''
> Lastly, the old-style iteration protocol is tried: if a class defines
`__getitem__()`, `x in y` is `True` if and only if there is a non-negative
integer index `i` such that `x is y[i] or x == y[i]`, and no lower integer
index raises the `IndexError` exception.
{{{#!python
def __contains__(self, value):
index = 0
while True:
try:
item = ref.__getitem__(index)
except IndexError:
return False
if item is value or item == value:
return True
index += 1
}}}
which explains why it hangs indefinitely.
--
Ticket URL: <https://code.djangoproject.com/ticket/35603#comment:1>
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