如果python中的参数为None，如何打印默认值

Question

[sample.py]

def f(name='Hello Guest'):
    print(name)


def A(name=None):    
    f(name)

A()

Expected Output: 'Hello Guest'

Current Output: None

I'm expecting the answers by without using much more codes like ' name = name if name is not None else some_default_value '

Thanks in advance!

Answer 1

The best way to do this will be to use a shared default:

DEFAULT_NAME = "Hello Guest"

def f(name=DEFAULT_NAME):
    print(name)

def A(name=DEFAULT_NAME):
    f(name)

Answer 2

Does this work for you?

def f(name):
    print(name or 'Hello Guest')

def A(name=None):    
    f(name)

A()
Out: "Hello Guest"

A("Hello World")
Out: "Hello World"

If the name variable is being used multiple times in the function, you could just reassign it in the beginning of the function. name = name or "Hello Guest"

Answer 3

Using inspect.signature to store default is one way to go:

def f(name='Hello Guest'):
    print(name or inspect.signature(f).parameters['name'].default)


def A(name=None):    
    f(name)

A()
# Hello Guest

With some loss of generality, but simpler (shorter and no other lib):

def f(name='Hello Guest'):
    print(name or f.__default__[0])


def A(name=None):    
    f(name)

A()
# Hello Guest

Answer 4

I have been in a similar situation where I can't change the signature or the body of a function I call internally but I want to use the defaults or pass arguments only when they exists(which can get tricky if you plan to pop those keys manually)

This was the cleanest and the most reusable solution I could write.

Simplest solution:

def f(name='Hello Guest'):
    print(name)


def A(**kwargs):
    # If kwargs has any keys which are None in value, will be removed.
    # Keys which don't exists in kwargs won't propagate, hence using default value.   
    f(**{k:v for k, v in kwargs.items() if v})

A()
# This returns "Hello Guest"
A(**{"name": None})
# This returns "Hello Guest"
A(**{"name": "Someone!"})
# This returns "Someone!"

Inspect solution:

Inspect is a great module if you plan to do something complex with function signature, parameters, etc.

from inspect import signature


# This function is untouched
def f(name='Hello Guest'):
    print(name)

# changed the signature so that params can propagate further
def A(**kwargs):    
    t = signature(f, follow_wrapped=True)
    # If kwargs has any key which is None in value,
    # it will be replaced with default values for the function.
    # Keys which don't exists in kwargs won't propagate.
    f(**{k: (v or t.parameters[k].default) for k, v in kwargs.items()})

A()
# This returns "Hello Guest"
A(**{"name": None})
# This returns "Hello Guest"
A(**{"name": "Someone!"})
# This returns "Someone!"

Answer 5

Or

def f(name):
    print(name)


def A(name = 'Hello Guest'):
    f(name)

A()