How to Preserve the Signature of Decorated Functions in Python?

Preserving Signatures of Decorated Functions

Decorators are powerful tools in Python for extending the behavior of existing functions. However, when applied to a function, the resulting decorated function often loses its original documentation and signature. This can be problematic, especially when using generic decorators that perform common tasks like logging or argument conversion.

Common Workarounds

Some common workarounds include:

• Including the signature in the docstring of the decorated function.
• Creating specialized decorators for each specific signature.

Using decorator Module

A more robust solution is to use the decorator module, which provides a decorator function called @decorator.decorator. By applying this decorator to your own decorator function, you can preserve the signature of the original function.

<code class="python">import decorator

@decorator.decorator
def args_as_ints(f, *args, **kwargs):
    args = [int(x) for x in args]
    kwargs = dict((k, int(v)) for k, v in kwargs.items())
    return f(*args, **kwargs)

@args_as_ints
def funny_function(x, y, z=3):
    """Computes x*y + 2*z"""
    return x*y + 2*z

print(funny_function("3", 4.0, z="5"))  # 22
help(funny_function)
# Help on function funny_function in module __main__:
#
# funny_function(x, y, z=3)
#     Computes x*y + 2*z</code>

Python 3.4

In Python 3.4 and later, the functools.wraps() function can be used to preserve signatures.

<code class="python">import functools

def args_as_ints(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        args = [int(x) for x in args]
        kwargs = dict((k, int(v)) for k, v in kwargs.items())
        return func(*args, **kwargs)
    return wrapper

@args_as_ints
def funny_function(x, y, z=3):
    """Computes x*y + 2*z"""
    return x*y + 2*z

print(funny_function("3", 4.0, z="5"))  # 22
help(funny_function)
# Help on function funny_function in module __main__:
#
# funny_function(x, y, z=3)
#     Computes x*y + 2*z</code>

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