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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.



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