是否有Python等效于C＃null-coalescing运算符？

In C# there’s a null-coalescing operator (written as ??) that allows for easy (short) null checking during assignment:

string s = null;
var other = s ?? "some default value";

Is there a python equivalent?

I know that I can do:

s = None
other = s if s else "some default value"

But is there an even shorter way (where I don’t need to repeat s)?

other = s or "some default value"

Ok, it must be clarified how the or operator works. It is a boolean operator, so it works in a boolean context. If the values are not boolean, they are converted to boolean for the purposes of the operator.

Note that the or operator does not return only True or False. Instead, it returns the first operand if the first operand evaluates to true, and it returns the second operand if the first operand evaluates to false.

In this case, the expression x or y returns x if it is True or evaluates to true when converted to boolean. Otherwise, it returns y. For most cases, this will serve for the very same purpose of C♯’s null-coalescing operator, but keep in mind:

42    or "something"    # returns 42
0     or "something"    # returns "something"
None  or "something"    # returns "something"
False or "something"    # returns "something"
""    or "something"    # returns "something"

If you use your variable s to hold something that is either a reference to the instance of a class or None (as long as your class does not define members __nonzero__() and __len__()), it is secure to use the same semantics as the null-coalescing operator.

In fact, it may even be useful to have this side-effect of Python. Since you know what values evaluates to false, you can use this to trigger the default value without using None specifically (an error object, for example).

In some languages this behavior is referred to as the Elvis operator.

Strictly,

other = s if s is not None else "default value"

Otherwise, s = False will become "default value", which may not be what was intended.

If you want to make this shorter, try:

def notNone(s,d):
    if s is None:
        return d
    else:
        return s

other = notNone(s, "default value")

Here’s a function that will return the first argument that isn’t None:

def coalesce(*arg):
  return reduce(lambda x, y: x if x is not None else y, arg)

# Prints "banana"
print coalesce(None, "banana", "phone", None)

reduce() might needlessly iterate over all the arguments even if the first argument is not None, so you can also use this version:

def coalesce(*arg):
  for el in arg:
    if el is not None:
      return el
  return None

I realize this is answered, but there is another option when you’re dealing with objects.

If you have an object that might be:

{
   name: {
      first: "John",
      last: "Doe"
   }
}

You can use:

obj.get(property_name, value_if_null)

Like:

obj.get("name", {}).get("first", "Name is missing") 

By adding {} as the default value, if “name” is missing, an empty object is returned and passed through to the next get. This is similar to null-safe-navigation in C#, which would be like obj?.name?.first.

In addition to Juliano’s answer about behavior of “or”: it’s “fast”

>>> 1 or 5/0
1

So sometimes it’s might be a useful shortcut for things like

object = getCachedVersion() or getFromDB()

Addionally to @Bothwells answer (which I prefer) for single values, in order to null-checking assingment of function return values, you can use new walrus-operator (since python3.8):

def test():
    return

a = 2 if (x:= test()) is None else x

Thus, test function does not need to be evaluated two times (as in a = 2 if test() is None else test())

In case you need to nest more than one null coalescing operation such as:

model?.data()?.first()

This is not a problem easily solved with or. It also cannot be solved with .get() which requires a dictionary type or similar (and cannot be nested anyway) or getattr() which will throw an exception when NoneType doesn’t have the attribute.

The relevant pip considering adding null coalescing to the language is PEP 505 and the discussion relevant to the document is in the python-ideas thread.

Regarding answers by @Hugh Bothwell, @mortehu and @glglgl.

Setup Dataset for testing

import random

dataset = [random.randint(0,15) if random.random() > .6 else None for i in range(1000)]

Define implementations

def not_none(x, y=None):
    if x is None:
        return y
    return x

def coalesce1(*arg):
  return reduce(lambda x, y: x if x is not None else y, arg)

def coalesce2(*args):
    return next((i for i in args if i is not None), None)

Make test function

def test_func(dataset, func):
    default = 1
    for i in dataset:
        func(i, default)

Results on mac i7 @2.7Ghz using python 2.7

>>> %timeit test_func(dataset, not_none)
1000 loops, best of 3: 224 µs per loop

>>> %timeit test_func(dataset, coalesce1)
1000 loops, best of 3: 471 µs per loop

>>> %timeit test_func(dataset, coalesce2)
1000 loops, best of 3: 782 µs per loop

Clearly the not_none function answers the OP’s question correctly and handles the “falsy” problem. It is also the fastest and easiest to read. If applying the logic in many places, it is clearly the best way to go.

If you have a problem where you want to find the 1st non-null value in a iterable, then @mortehu’s response is the way to go. But it is a solution to a different problem than OP, although it can partially handle that case. It cannot take an iterable AND a default value. The last argument would be the default value returned, but then you wouldn’t be passing in an iterable in that case as well as it isn’t explicit that the last argument is a default to value.

You could then do below, but I’d still use not_null for the single value use case.

def coalesce(*args, **kwargs):
    default = kwargs.get('default')
    return next((a for a in arg if a is not None), default)

For those like me that stumbled here looking for a viable solution to this issue, when the variable might be undefined, the closest i got is:

if 'variablename' in globals() and ((variablename or False) == True):
  print('variable exists and it\'s true')
else:
  print('variable doesn\'t exist, or it\'s false')

Note that a string is needed when checking in globals, but afterwards the actual variable is used when checking for value.

More on variable existence: How do I check if a variable exists?

Python has a get function that its very useful to return a value of an existent key, if the key exist;
if not it will return a default value.

def main():
    names = ['Jack','Maria','Betsy','James','Jack']
    names_repeated = dict()
    default_value = 0

    for find_name in names:
        names_repeated[find_name] = names_repeated.get(find_name, default_value) + 1

if you cannot find the name inside the dictionary, it will return the default_value, if the name exist then it will add any existing value with 1.

hope this can help

The two functions below I have found to be very useful when dealing with many variable testing cases.

def nz(value, none_value, strict=True):
    ''' This function is named after an old VBA function. It returns a default
        value if the passed in value is None. If strict is False it will
        treat an empty string as None as well.

        example:
        x = None
        nz(x,"hello")
        --> "hello"
        nz(x,"")
        --> ""
        y = ""   
        nz(y,"hello")
        --> ""
        nz(y,"hello", False)
        --> "hello" '''

    if value is None and strict:
        return_val = none_value
    elif strict and value is not None:
        return_val = value
    elif not strict and not is_not_null(value):
        return_val = none_value
    else:
        return_val = value
    return return_val 

def is_not_null(value):
    ''' test for None and empty string '''
    return value is not None and len(str(value)) > 0