用 **kwargs 覆盖默认参数的 Pythonic 方法？

Question

With a function f calling another well-known function in Python (eg a matplotlib function), what is the most pythonic/efficient/elegant way to define some default values while still giving the possibility to the user of f to fully customize the called function (typically with **kwargs ), including to overwrite the default keyword arguments defined in f ?

import numpy as np
import matplotlib.pyplot as plt


v = np.linspace(-10.,10.,100)
x,y = np.meshgrid(v, v)
z = -np.hypot(x, y)

def f(ax, n=12, **kwargs):
    ax.contourf(x, y, z, n, cmap=plt.cm.autumn, **kwargs)

fig, ((ax0, ax1), (ax2, ax3)) = plt.subplots(2, 2)
f(ax0) # OK
f(ax1, n=100) # OK
f(ax2, n=100, **{'vmax': -2, 'alpha': 0.2}) # OK
# f(ax3, n=100, **{'cmap': plt.cm.cool}) # ERROR

plt.show()

Here, the last call to f throws:

TypeError: contourf() got multiple values for keyword argument 'cmap'

Answer 1

In your wrapper, you could simply adjust kwargs before passing it to wrapped function:

def f(ax, n=12, **kwargs):
    kwargs.setdefault('cmap', plt.cm.autumn)
    ax.contourf(x, y, z, n, **kwargs)

setdefault will avoid changing the argument if it was passed to your wrapper, but you could just as easily always clobber it if you wanted.

Answer 2

The following minimal example illustrates the options you have when coding this from scratch. Here you can also define a default argument when defining the inner function but NOT specify it when calling it in the wrapper. Note that default variable a is removed from kwargs within the function, if it was already defined as a default argument. Only in run_test_working3 (setting the default value merely with setdefault in the wrapper) default variable a is not removed from kwargs. This might be important if you want to pass kwargs to inner functions within the test function.

kwargs = {"a": 1,
          "b": 2,
          "c": 3}

kwargs2 = {"b": 2,
           "c": 3}
   
def test(required_arg, a="default", **kwargs):
    print(required_arg)
    print(a)
    print(kwargs)

def test2(required_arg, **kwargs):
    print(required_arg)
    print(kwargs)
    

#Set default for a in definition of test but not when calling it in wrapper
# a is removed from kwargs
def run_test_working1(required_arg, **kwargs):
    test(required_arg, **kwargs)

#Set default for a different from definition of test in wrapper
# a is removed from kwargs
def run_test_working2(required_arg, **kwargs):
    kwargs.setdefault("a", "default2")
    test(required_arg, **kwargs)

#Set default value only via setdefault in wrapper
#a is not removed from kwargs
def run_test_working3(required_arg, **kwargs):
    kwargs.setdefault("a", "default2")
    test2(required_arg, **kwargs)


#Provoke TypeError: test() got multiple values for keyword argument 'a'
def run_test_not_working(required_arg, **kwargs):
    test(required_arg, a="default", **kwargs)

print("Demo run_test_working1\n")
run_test_working1("required_arg",**kwargs)
print("\n")
run_test_working1("required_arg",**kwargs2)
print("\n")

print("Demo run_test_working2\n")
run_test_working2("required_arg",**kwargs)
print("\n")
run_test_working2("required_arg",**kwargs2)
print("\n")

print("Demo run_test_working3\n")
run_test_working3("required_arg",**kwargs)
print("\n")
run_test_working3("required_arg",**kwargs2)
print("\n")

print("Demo run_test_not_working\n")
run_test_not_working("required_arg",**kwargs)
print("\n")
test("required_arg")