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Replace `dataclass` Mutable Default Values with Call to `field`


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When defining a Python dataclass it is not safe to use mutable datatypes (such as list, dict, or set) as defaults for the attributes. This is because the defined attribute will be shared by all instances of the dataclass type. Using such a mutable default will ultimately result in a ValueError at runtime. This codemod updates attributes of dataclasses.dataclass with mutable defaults to use dataclasses.field instead. The dataclass documentation providesmore details about why using field(default_factory=...) is the recommended pattern.

Our changes look something like this:

-from dataclasses import dataclass
+from dataclasses import field, dataclass

class Person:
name: str = ""
- phones: list = []
- friends: dict = {}
- family: set = set()
+ phones: list = field(default_factory=list)
+ friends: dict = field(default_factory=dict)
+ family: set = field(default_factory=set)

If you have feedback on this codemod, please let us know!


Why is this codemod marked as Merge Without Review?

This change is safe and will prevent runtime ValueError.

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