default-factory-kwarg (RUF026)#
Fix is sometimes available.
This rule is unstable and in preview. The --preview flag is required for use.
What it does#
Checks for incorrect usages of default_factory as a keyword argument when
initializing a defaultdict.
Why is this bad?#
The defaultdict constructor accepts a callable as its first argument.
For example, it's common to initialize a defaultdict with int or list
via defaultdict(int) or defaultdict(list), to create a dictionary that
returns 0 or [] respectively when a key is missing.
The default factory must be provided as a positional argument, as all
keyword arguments to defaultdict are interpreted as initial entries in
the dictionary. For example, defaultdict(foo=1, bar=2) will create a
dictionary with {"foo": 1, "bar": 2} as its initial entries.
As such, defaultdict(default_factory=list) will create a dictionary with
{"default_factory": list} as its initial entry, instead of a dictionary
that returns [] when a key is missing. Specifying a default_factory
keyword argument is almost always a mistake, and one that type checkers
can't reliably detect.
Fix safety#
This rule's fix is marked as unsafe, as converting default_factory from a
keyword to a positional argument will change the behavior of the code, even
if the keyword argument was used erroneously.
Examples#
Use instead: