How to check in python that at least one of the default parameters of the function specified

问题

What is the best practice in python to check if at least one of the default parameters of the function is specified?

Let's suppose we have some function:

def some_function(arg_a=None, arg_b=None, arg_c=False)

with some default parameters. In my case, I need to check if either arg_a or arg_b is specified. So I thought of implementing something like this:

def some_function(arg_a=None, arg_b=None, arg_c=False):
    ...
    if arg_a is not None:
        ...
    elif arg_b is not None:
        ...
    else:
        raise ValueError('Expected either arg_a or arg_b args')
    ...
    ...

So, what is more pythonic way to implement such kind of functionality?

回答1:

You could use all to check if they all equal None and raise the ValueError:

if all(v is None for v in {arg_a, arg_b}):
    raise ValueError('Expected either arg_a or arg_b args')

this gets rid of those if-elif clauses and groups all checks in the same place:

f(arg_a=0) # ok    
f(arg_b=0) # ok
f()        # Value Error

Alternatively, with any():

if not any(v is not None for v in {arg_a, arg_b}):
    raise ValueError('Expected either arg_a or arg_b args')

but this is definitely more obfuscated.

In the end, it really depends on what the interpretation of pythonic actually is.

回答2:

Depends on what you expect as values for arg_a and arg_b, but this is generally sufficient.

if not arg_a and not arg_b:
    raise ValueError(...)

Assumes that arg_a and arg_b are not both booleans and cannot have zeros, empty strings/lists/tuples, etc. as parameters.

Depending on your needs, you can be more precise if you need to distinguish between None and 'falsies' such as False, 0, "", [], {}, (), etc.:

if arg_a is None and arg_b is None:
    raise ValueError(...)

回答3:

You may consider using kwargs if you have quite a number of such named arguments with mismatching default values:

def some_function(**kwargs):
    reqd = ['arg_a', 'arg_b']
    if not all(i in kwargs for i in reqd):
        raise ValueError('Expected either {} args'.format(' or '.join(reqd)))

    arg_a = kwargs.get('args_a')
    arg_b = kwargs.get('args_b')
    arg_c = kwargs.get('args_c', False)

回答4:

To check if at least one of the default parameters of the function specified

The solution using locals, dict.values and any functions:

def some_function(arg_a=None, arg_b=None, arg_c=False):
    args = locals()
    if (any(args.values()) == True):
        print('Ok')
    else:
        raise ValueError('At least one default param should be passed!')

some_function(False, 1)            # Ok
some_function('text', False, None) # Ok
some_function(0, False, None)      # Error

https://docs.python.org/2/library/functions.html#any

回答5:

Since you're proposing such a pattern, there's possibly some unifying meaning to arg_a, arg_b and arg_c. If that's so, you could ...

from enum import Enum

class Group(Enum):
    A = auto()
    B = auto()
    C = auto()

def some_function(val: int, t: Group):
    if t == Group.A:
        # do stuff with val
    elif t == Group.B:
        # do other stuff with val
    elif t == Group.C:
        # do other stuff with val

some_function(1, Group.A)

Here the caller is forced to specify both a value, and what the value corresponds to. This is tbh a bit of a hack around the lack of proper enumeration support in python and I've no idea if it's pythonic. It will however be picked up by a type checker.

来源：https://stackoverflow.com/questions/38881888/how-to-check-in-python-that-at-least-one-of-the-default-parameters-of-the-functi

标签

python

function

parameter-passing