parachute

A python decorator that helps in splitting code into argument checking and worker functions. It eliminates the need to replicate the worker function signature in the argument checking function definition and enables argument validation before calling into the worker function.

Requirements

Python3

How to use

Move any argument checking code from any function to a dedicated new function and wrap it with the parachute decorator. When the wrapped function gets called with arbitrary arguments, the decorator checks whether a valid call to the worker function can be formed from them, then passes the arguments (as a single dictionary with **expression flattened out) to the argument checking function. The first argument to the decorated function must be a NamedTuple with field names 'func' and 'innerscope' (like the FuncInfo class provided with parachute). In it, func holds the name of the worker function and the boolean value of innerscope specifies whether the decorated function is called from inside or outside func. If called from inside func, the decorator expects a single positional argument which should be a dictionary including the arguments passed to func (but may have additional entries).

What it's not

This is not yet another decorator for checking parameter types

Example

from parachute import parachute, FuncInfo

def worker_function (a, b, c=None, *args, **kwargs):
    start_working_with_the_args()

@parachute
def arg_check(**test_args):
    print (locals())
    # real argument checks should go here instead

Example 1: a valid set of parameters for worker_function

arg_check(FuncInfo(func=worker_function, innerscope=False), 1, 2, 3, 4, d='another', e='yetanother')
# arg_check gets called with a dictionary of bound arguments


{'test_args': {'a': 1, 'b': 2, 'c': 3, 'args': (4,), 'e': 'yetanother', 'd': 'another'}}

Example 2: an invalid set of parameters for worker_function

arg_check(FuncInfo(worker_function, False), 1, d='another', e='yetanother')


Traceback (most recent call last):
  File "<pyshell#38>", line 1, in <module>
    arg_check(worker_function, None, 1, d='another', e='yetanother')
  File "C:\parachute.py", line 30, in wrappee
    arg_dict = getcallargs(func, *args, **kwargs)
  File "C:\Python33\lib\inspect.py", line 1032, in getcallargs
    _missing_arguments(f_name, req, True, arg2value)
  File "C:\Python33\lib\inspect.py", line 964, in _missing_arguments
    "" if missing == 1 else "s", s))
TypeError: worker_function() missing 1 required positional argument: 'b'

Example 3: innerscope call

def worker2 (x, y, z=3, **kwargs):
    # let the decorator determine the set of input arguments from locals() by inspecting worker2's signature
    b = 42 # add something to locals() just to illustrate this
    arg_check(FuncInfo(worker2, innerscope=True), locals())

worker2(3, 7, mode='multiply')


{'test_args': {'x': 3, 'y': 7, 'z': 3, 'mode': 'multiply'}}