Dynamic default arguments in python functions

Question

I have a need for functions with default arguments that have to be set at function runtime (such as empty lists, values derived from other arguments or data taken from the d

Answer 1

You can replace

baz = baz if baz else blar()

with

baz = baz or blar()

if you're still happy with just testing for falsy values instead of None.

Answer 2

A quick&dirty example implementation of something that might work:

class DynDefault(object):
    def __init__(self, callback):
        self.callback = callback
    def __call__(self):
        return self.callback()

def dyn_default(func):
    def wrapper(*args, **kw):
        args = [arg() for arg in args if isinstance(arg, DynDefault) else arg]
        for k, v in kw.items():
            if isinstance(v, DynDefault):
                kw[k] = v()
        return func(*args, **kw)
    return wrapper

@dyn_default
def foo(bar, baaz=DynDefault(blar)):
    # problem solved

Answer 3

No, that's pretty much it. Usually you test for is None so you can safely pass in falsey values like 0 or "" etc.

def foo(bar, baz=None):
    baz = baz if baz is not None else blar()

The old fashioned way is the two liner. Some people may prefer this

def foo(bar, baz=None):
    if baz is None:
        baz = blar()

Answer 4

You could do something like this:

def getArg():
  try:
      return arg
  except NameError:
      return 0

def foo(x, y=getArg):
    y = y()
    print(y)

foo(1) # Prints 0 (Default)

arg = 7 # Set by argparse?

foo(2)           # Prints 7 (Dynamic global)
foo(3, lambda:9) # Prints 9 (Dynamic passed)