Is there a Python equivalent of the C# null-coalescing operator?

Is there a Python equivalent of the C# null-coalescing operator?

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.


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
        return s

other = notNone(s, default value)

Is there a Python equivalent of the C# null-coalescing operator?

Heres a function that will return the first argument that isnt 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

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