Declare a variable with a flexible container type for integers, allowing either list or set operations.

Indicate that the variable may hold either a list or a set of integers, enabling flexible usage.

When you need a variable that can hold either a list or set of integers, e.g., when you might convert between list and set.

Variable declarations in Python, function parameters, class attributes, or module-level constants where a flexible integer container is needed.

items: Union[List[int], Set[int]] = []

variable_name: Union[List[int], Set[int]] = []

items (variable name), Union[List[int], Set[int]] (type annotation), [] (default value)

• List
• Set
• Union
• List[int]
• Set[int]
• []

• List[int] = []
• Set[int] = set()
• Union[List[int]
• Set[int]] = set()

Using mutable default [] as a default argument in functions can cause shared state bugs; use None and initialize inside the function instead.

items: List[int] = [] (list only), items: Set[int] = set() (set only), items: List[Union[int, str]] = [] (list of ints or strings)

Using mutable default [] as a default argument in functions can cause shared state across calls; prefer None and initialize inside the function.

• type annotations
• variable annotations
• union types

The variable starts as an empty list but can later hold a set; type checkers accept both list and set assignments.

Signals to readers and type checkers that the variable is intended to hold either a list or a set of integers, providing flexibility.

items: Union[List[int], Set[int]] = []

items: Collection[int] = []