Python update_wrapper()

The update_wrapper() function from the functools module updates a wrapper function to look like the wrapped function by copying attributes such as __name__, __doc__, and __module__. This is essential for creating decorators that preserve the original function’s metadata, making debugging and introspection much easier.

Syntax

from functools import update_wrapper

update_wrapper(wrapper, wrapped, assigned=WRAPPER_ASSIGNMENTS, updated=WRAPPER_UPDATES)

Parameters:

• wrapper: The function that is acting as a wrapper (e.g., inside a decorator).
• wrapped: The original function being wrapped.
• assigned: A tuple of attribute names to copy (defaults to WRAPPER_ASSIGNMENTS).
• updated: A tuple of attribute names to update (defaults to WRAPPER_UPDATES).

Return value:

The update_wrapper() function returns the wrapper function itself, after updating it with the attributes of wrapped.

Example

This example demonstrates how update_wrapper() preserves the original function’s metadata when creating a decorator:

from functools import update_wrapper

def my_decorator(func):
  def wrapper(*args, **kwargs):
    print(f"Calling {func.__name__}")
    return func(*args, **kwargs)

  # Update the wrapper to look like the original function
  update_wrapper(wrapper, func)
  return wrapper

@my_decorator
def greet(name):
  """A simple greeting function."""
  return f"Hello, {name}!"

# Check that metadata is preserved
print(f"Function name: {greet.__name__}")
print(f"Function docstring: {greet.__doc__}")

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Here is the output:

Function name: greet
Function docstring: A simple greeting function.

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Codebyte Example

This codebyte example shows the difference between using and not using update_wrapper():

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