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Internal utility function that sets default value metadata on a type parameter object by adding a has_default method and __default__ attribute.

Purpose

This function is used internally to configure type parameter objects (like TypeVar, ParamSpec, or TypeVarTuple) with default value information. It modifies the type parameter in-place by adding a lambda function to check if a default exists and storing the default value itself. This is part of Python's type system implementation for handling generic type parameters with defaults.

Source Code

def _set_default(type_param, default):
    type_param.has_default = lambda: default is not NoDefault
    type_param.__default__ = default

Parameters

Parameter Details

type_param: A type parameter object (TypeVar, ParamSpec, or TypeVarTuple) that will be modified to include default value information. This object is mutated in-place.

default: The default value to assign to the type parameter. Can be any value or the special sentinel value NoDefault to indicate no default is provided. When NoDefault is passed, has_default() will return False.

Return Value

This function returns None (implicitly). It operates by side-effect, modifying the type_param object in-place by adding the has_default method and __default__ attribute.

Usage Example

# This is an internal function typically used within the typing module implementation
# Example of how it might be used:

class NoDefault:
    pass

def _set_default(type_param, default):
    type_param.has_default = lambda: default is not NoDefault
    type_param.__default__ = default

# Create a mock type parameter object
class MockTypeParam:
    pass

type_param = MockTypeParam()

# Set a default value
_set_default(type_param, int)
print(type_param.has_default())  # True
print(type_param.__default__)    # <class 'int'>

# Set no default
_set_default(type_param, NoDefault)
print(type_param.has_default())  # False
print(type_param.__default__)    # <class 'NoDefault'>

Best Practices

• This is an internal implementation function and should not be called directly by user code
• The function mutates the type_param object in-place, so be aware of side effects
• Always ensure NoDefault sentinel is properly defined before using this function
• The has_default method is implemented as a lambda that captures the default value via closure
• This function is typically used during type parameter initialization in the typing module

Tags

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