How to pass non-hard-coded parameter to Python decorator?

Question

My goal is to create a trivial unit test decorator, which executes a function and, if it succeeds, do nothing, if it doesn't, print "FAILURE" and all its parameters. I do know about the builtin unittest package. I'm doing this to learn decorators. I'm not taking this any farther than "if actual equals expected, do nothing, else print params".

I found this function which prints out all of a function's parameters:

def dumpArgs(func): '''Decorator to print function call details - parameters names and effective values''' def wrapper(*func_args, **func_kwargs): arg_names = func.__code__.co_varnames[:func.__code__.co_argcount] args = func_args[:len(arg_names)] defaults = func.__defaults__ or () args = args + defaults[len(defaults) - (func.__code__.co_argcount - len(args)):] params = list(zip(arg_names, args)) args = func_args[len(arg_names):] if args: params.append(('args', args)) if func_kwargs: params.append(('kwargs', func_kwargs)) print(func.__name__ + ' (' + ', '.join('%s = %r' % p for p in params) + ' )') return func(*func_args, **func_kwargs) return wrapper @dumpArgs def test(a, b = 4, c = 'blah-blah', *args, **kwargs): pass test(1) test(1, 3) test(1, d = 5) test(1, 2, 3, 4, 5, d = 6, g = 12.9)

Output:

test (a = 1, b = 4, c = 'blah-blah' ) test (a = 1, b = 3, c = 'blah-blah' ) test (a = 1, b = 4, c = 'blah-blah', kwargs = {'d': 5} ) test (a = 1, b = 2, c = 3, args = (4, 5), kwargs = {'g': 12.9, 'd': 6} )

I changed it to this, which prints out the parameters only if the function does not equal 4 (implemented without a decorator param):

def get_all_func_param_name_values(func, *func_args, **func_kwargs): arg_names = func.__code__.co_varnames[:func.__code__.co_argcount] args = func_args[:len(arg_names)] defaults = func.__defaults__ or () args = args + defaults[len(defaults) - (func.__code__.co_argcount - len(args)):] params = list(zip(arg_names, args)) args = func_args[len(arg_names):] if args: params.append(('args', args)) if func_kwargs: params.append(('kwargs', func_kwargs)) return '(' + ', '.join('%s = %r' % p for p in params) + ')' def dumpArgs(func): '''Decorator to print function call details - parameters names and effective values''' def wrapper(*func_args, **func_kwargs): a = func(*func_args, **func_kwargs) if(a != 4): return a print("FAILURE: " + func.__name__ + get_all_func_param_name_values(func, *func_args, **func_kwargs)) return a return wrapper @dumpArgs def getA(a, b = 4, c = 'blah-blah', *args, **kwargs): return a getA(1) getA(1, 3) getA(4, d = 5) getA(1, 2, 3, 4, 5, d = 6, g = 12.9)

Output:

FAILURE: getA(a = 4, b = 4, c = 'blah-blah', kwargs = {'d': 5}) Out[21]: 1

(I don't understand why the 1 is printed in the second line.)

I then changed it to pass in the expected value, 4, as decorator parameter. As described in this answer, it requires that the original decorator be a nested function:

def get_all_func_param_name_values(func, *func_args, **func_kwargs): arg_names = func.__code__.co_varnames[:func.__code__.co_argcount] args = func_args[:len(arg_names)] defaults = func.__defaults__ or () args = args + defaults[len(defaults) - (func.__code__.co_argcount - len(args)):] params = list(zip(arg_names, args)) args = func_args[len(arg_names):] if args: params.append(('args', args)) if func_kwargs: params.append(('kwargs', func_kwargs)) return '(' + ', '.join('%s = %r' % p for p in params) + ')' def dumpArgs(expected_value): def dumpArgs2(func): '''Decorator to print function call details - parameters names and effective values''' def wrapper(*func_args, **func_kwargs): a = func(*func_args, **func_kwargs) if(a == expected_value): return a print("FAILURE: " + func.__name__ + get_all_func_param_name_values(func, *func_args, **func_kwargs)) return a return wrapper return dumpArgs2 @dumpArgs(4) def getA(a, b = 4, c = 'blah-blah', *args, **kwargs): return a getA(1) getA(1, 3) getA(4, d = 5) getA(1, 2, 3, 4, 5, d = 6, g = 12.9)

Output:

FAILURE: getA(a = 1, b = 4, c = 'blah-blah') FAILURE: getA(a = 1, b = 3, c = 'blah-blah') FAILURE: getA(a = 1, b = 2, c = 3, args = (4, 5), kwargs = {'g': 12.9, 'd': 6}) Out[31]: 1

(Again, that 1...)

I'm not clear on how to change this hard-coded 4 to an expected_value parameter, that is passed through at every function call. All the examples I've seen (like this one) have hard-coded parameters.

I currently experimenting with

assert_expected_func_params(4, getA, 1) assert_expected_func_params(4, getA, 1, 3) assert_expected_func_params(4, getA, 4, d = 5) assert_expected_func_params(4, getA, 1, 2, 3, 4, 5, d = 6, g = 12.9)

But it's far from working.

How do I implement a decorator parameter that I can pass in to every function call?

I don't know if you can decorate with a run-time value, since the decorator is evaluated at the time of the function definition, and then not again after that. — TheSoundDefense
– TheSoundDefense, Commented Aug 8, 2014 at 16:27
if you have a function assert_expected_func_params, why do you need a decorator? — Daniel
– Daniel, Commented Aug 8, 2014 at 16:28
You can't do this with @decorator(arg) syntax for the reason @TheSoundDefense provides, but you can do the manual equivalent, i.e. func = decorator(arg)(func). — jonrsharpe
– jonrsharpe, Commented Aug 8, 2014 at 16:29
The reason why the 1 is appearing is because that's the output of your getA function (which just returns whatever param a is). You'd get the same result regardless or not if you applied the decorator. — Michael0x2a
– Michael0x2a, Commented Aug 8, 2014 at 16:30

davidism · Accepted Answer · 2014-08-08 17:26:59Z

Since a decorator wraps the function, you can intercept the input and output of the function when it is called. In this way, you could look for an _expected keyword, strip it out, call the function, then test the return value of the function against the passed in expected value.

from functools import wraps _empty = object() # sentinel value used to control testing def dump_ne(func): @wraps(func) def decorated(*args, **kwargs): # remove the expected value from the actual call kwargs expected = kwargs.pop('_expected', _empty) # call the function with rest of args and kwargs result = func(*args, **kwargs) # only test when _expected was passed in the kwargs # only print when the result didn't equal expected if expected is not _empty and expected != result: print('FAIL: func={}, args={}, kwargs={}'.format(func.__name__, args, kwargs)) return result return decorated @dump_ne def cool(thing): return thing.upper() print(cool('cat')) # prints 'CAT', test isn't run for thing in ('cat', 'ice', 'cucumber'): print(cool(thing, _expected='CUCUMBER')) # dumps info for first 2 calls (cat, ice)

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