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Dynamic function parameters using annotations (Python recipe) by pavel
ActiveState Code (http://code.activestate.com/recipes/578049/)

Python default values for keyword arguments are evaluated only once and not on every function call. For example following function will not work as user may expect:

def printNow(l=[], now=datetime.now()):
    l.append(len(l))

    print('List:', l, ' id:', id(l))
    print('Now:', now)


for i in range(3):
    printNow()
    print()

The "dynamic" decorator solves problem by evaluating callables, that are assigned to parameters using annotations syntax (see PEP 3107).

Python, 30 lines
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from datetime import datetime
from functools import wraps

def dynamic(fn):
    @wraps(fn)
    def wrapper(*args, **kwargs):
        for key, value in fn.__annotations__.items():
            try:
                kwargs[key] = value()
            except TypeError:
                pass
            
        return fn(*args, **kwargs)
    return wrapper


# Example

@dynamic
def printNow(l:list, now:datetime.now):
    l.append(len(l))
    
    print('List:', l, ' id:', id(l))
    print('Now:', now)
    
# Test

for i in range(3):
    printNow()
    print()
Tags: annotations, decorator
Created by pavel on Wed, 22 Feb 2012 (MIT)
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